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Full text of "Botany : or, The modern study of plants"

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THE 

PEOPLE'S 
BOOKS 



BOTANY 



BOTANY 



OR 



THE MODERN STUDY OF PLANTS 
BY MARIE STOPES 

D.Sc. (LONDON), PH.D. (MUNICH), F.L.S. 

AUTHOR OF "THE STUDY OF PLANT LIFE," "ANCIENT PLANTS," ETC. 




LONDON : T. C. & E. C. JACK 

67 LONG ACRE, W.C. 

AND EDINBURGH 



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1 



BIOLOGY 
LIBRARY 

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CONTENTS 

OBiP. FAOX 

I. INTRODUCTION 7 

II. MORPHOLOGY 10 

HI. ANATOMY 23 

IV. CYTOLOGY . . , . . . .32 

V. PHYSIOLOGY 40 

VI. ECOLOGY .60 

VH. PALAEONTOLOGY ..,,.. 58 

. PLANT BREEDING 68 

IX. PATHOLOGY * 74 

X. SYSTEMATIC BOTANY 79 

XI. CONCLUSION 88 

SUGGESTED COURSE OF READING . . .91 

INDEX 93 



BOTANY 

CHAPTER I 

INTRODUCTION 

IN our daily life we have no difficulty in distinguishing 
plants from animals, and we are also seldom in doubt 
as to the difference between a life-containing and an 
inorganic thing. It is true, of course, that at the ex- 
treme limits of the series, among the very simplest 
forms, it is sometimes difficult to separate plants and 
animals ; but in most cases there can be no doubt as to 
which of the two great classes any thing or any creature 
belongs. 

All the life in the world is embraced in one or other of 
the two great classes of Plants and Animals. Out- 
wardly they appear so different from each other, but, 
as we shall see, they have a wonderful unity in the funda- 
mentals of their structure. The science of the study 
of life is called Biology, but in these days, when so much 
detail has been accumulated and stored in books, it 
is no longer possible for one mind to grasp the whole 
subject. It has been divided into the two natural 
divisions of Botany, the study of the plants, and 
Zoology, the study of animals. 

It happens that man is an animal, consequently the 
scientific study of his body should be the work of the 



8 BOTANY 

Soologistc. So much, however, is known about man, 
and so much more knowledge is eagerly wished for, 
that the study of this single animal has become a 
science in itself, of which there are many branches 
human physiology, pathology, &c. This has tended 
to split up the science of " Zoology," and this tendency 
has been further encouraged by the fact that there are 
such extraordinary numbers of some animals, e.g., the 
insects, that their study forms a special science of its 
own called Entomology. 

The science of plant life is much more united, and 
Botany includes all the sides of the study of all plants, 
with the exception, perhaps, of the bacteria which have 
a science of their own. In many ways this unity in 
botany is a great advantage, for none of the branches 
of any science are really independent of each other, and 
it is impossible to study one let us say, for example, 
the physiology of plants without a knowledge of the 
others, and, in this instance, of anatomy and cytology. 

Nevertheless, even in botany, and particularly the 
botany of this century, the various problems in the differ- 
ent branches of the subject have to be attacked in such 
different ways, that it is almost impossible for one man 
to make discoveries in more than one or two restricted 
fields. In each part of the subject the instruments 
used, the language employed, and the methods of at- 
tacking the problems are all so distinct from each other, 
and so elaborate, that they demand an almost life- 
long study. This is parallel to the case of music, 
which is in itself all the harmony of one order of sweet 
sounds, and yet there are but few musicians who have 
complete technical control of more than one or two 
instruments. In the case of science and its branches, 
the worker has not only to attain personal control of 



INTRODUCTION 9 

his tools, but he has to keep in touch with all the work 
and discoveries of the others who are engaged on investi- 
gations akin to his own, and this necessitates an amount 
of reading that rivals the columns of print poured out 
by the daily press. Every country that possesses 
universities and learned societies is rivalling every 
other in the production and publication of additions 
to scientific knowledge. One who is himself adding 
to this must be aware of what all the others are doing, 
lest he repeat work already done, or lest he lose the help 
and inspiration that other work may be to his own. 

We see, then, in the modern science of botany a 
philosophic whole, which is only to be attained by the 
combination of the results of a number of separate lines 
of work, each of which requires special technical study. 
In the following chapters the more important of these 
branches will each be dealt with shortly. In such small 
compass it will not be possible to give very many facts, 
but the text-books are full of them ; it will not be 
possible to go into very abstruse discussions the 
learned Transactions are full of them ; but it will, I 
hope, even in so few words, be possible to illustrate the 
attitude of the workers in each branch of the study, 
and to indicate the field in which they labour. Then 
at the end of the book the reader should be in a position 
to see for himself how it all hangs together and bears 
on the one great problem in biology the evolution of 
life. 



CHAPTER II 

MORPHOLOGY 

THE study of Morphology is the study of the form and 
external appearance of the plant's body. Just as there 
is unity among animals, and we recognise legs, eyes, 
tails, and the various parts of the body in many differ- 
ent guises in the different species of animals, so there 
is a unity of organisation among the higher plants, 
and their bodies are composed of a limited number of 
parts which belong to distinct categories. 

The body of a typical member of the higher plants is 
composed of four elements, viz., Roots, Stems, Leaves, 
and Sporangia. The flowers, which at first sight appear 
so distinct, are in reality composed of modified leaves. 

The extraordinary variety of plant structures and all 
their beautiful and remarkable forms are simply modi- 
fications of these four elements. Each of them has 
its characteristic structure, and its normal functions, 
and in most cases, however the parts are modified, they 
remain recognisable. Some parts may be modified out 
of immediate recognition, as we shall see in a moment, 
but careful study will reveal their true nature. 

If you pull up any common weed, such as a Campion 
or a Poppy, you will notice that the root and the stem 
merge into one another, but that there is a contrast 
between them in colour and form as well as in position. 

The leaves are attached to the stem, and never to the 

10 



MORPHOLOGY 11 

root, and they are typically green expanded surfaces 
of different shapes according to the species. 

The three fundamental elements roots, stems, and 
leaves are all that compose the vegetative plant, 
which, under favourable conditions of nutriment, may 
continue to grow for a long time. Some of the very 
large Monocotyledons, for instance, live the whole of 
their long lives as vegetative plants, and then at the 
end of a lifetime produce a great number of reproductive 
organs and die. 

The fourth set of organs the reproductive are 
known in their simplest terms as Sporangia. The 
" flowers " which we associate with most of our common 
plants are composed of the essential sporangia and 
a number of modified leaves, which form altogether 
structures of extraordinary complexity and variety. 
In many cases the colours, designs, and positions of the 
modified leaves which form the flower have a very 
definite relation to the insects which visit it and do an 
important work in carrying the pollen which is produced 
in the sporangia (pollen sacs) from one flower to another. 
But this will lead us to another aspect of the subject. 
Let us for a moment consider the four essential elements 
of the plant's body. 

The Roots generally ramify in the soil and live alto- 
gether underground ; this is, however, a physiological 
rather than a morphological character. Morphologi- 
cally the principal difference between roots and stems 
is that, though the roots and the leaves both spring 
from the stems, the roots themselves do not bear leaves. 
Some plants have underground stems, which are often 
extremely like roots in their external appearance, 
but on them one can generally find traces of the re- 
duced leaves in the form of small brown scales, which 



12 BOTANY 

show that the root-like organ is really a stem. In 
their internal anatomy the two organs differ essentially, 
as we shall see in the next chapter, and there are cases 
of modified leaves and stems which have departed so 
far from the normal that the external morphology 
gives no clue to their real nature, and then the anatomy 
alone can determine to which category each belongs. 

The typical root is a colourless or brown series of 
circular or flattened branches. It is never broad and 
expanded like leaves, though in some cases, e.g., epi- 
phytic orchids, it may be green. The main root is the 
continuation of the original primary root of the seedling, 
which has subdivided indefinitely with its growth, and 
this is often supplemented by further roots which arise 
adventitiously on the stem wherever they are needed, 
either in the soil, in the air, or in water. A sprig of 
Mint or Ivy left in a jar of water will often show the 
white tufts of adventitious roots springing out of the 
base of the stem. The great prop roots of the Mangroves 
and some of the tropical species of Ficus are woody 
and covered with bark, so that it is hard to find any 
external feature other than their position by which 
to distinguish them from the stem-trunks. 

The Stems which support the leaves and connect 
them with those sources of food supply, the roots, are 
generally upright, cylindrical, and branched in the air. 
They have, however, an infinite variety of form, and 
range from the sturdy Oak to the slender climbing 
Convolvulus, from the great pudding-like Cactus and 
swollen masses of the Potato to the slender threads of 
the water Ranunculus ; and from the root-like Solomon's 
seal running underground, to the contracted stem of 
the serial Orchid perched aloft on the branches of other 
plants, so that it never comes down to earth. Normal, 



MORPHOLOGY 



18 



serial stems are generally green when they are young, 
and as they age they put on a coating of thick bark 
and cork outside their woody growth. There are stems, 




FIG. 1. Part of a twig of Ruscus (the Butcher's Broom) showing the leaf-like 
modified branches I, which are attached to normal stems. Beneath each is 
seen the scale-like real leaf, s2, in whose axils the branches arise. Similar 
Bcales, si, subtend ordinary branches. 

however, which never have the appearance of true 
stems, but which simulate leaves. Perhaps the best 
known example of this in the British flora is the Butcher's 
Broom (Ruscus). A branch of this plant appears to 



14 BOTANY 

be covered with simple oval dark-green leaves just 
like any other ordinary shrub. But if you examine 
these " leaves " closely you will see that they have just 
beneath each of them a small scale-like structure. 
This is the true leaf, and the big apparent leaf is a 
flattened branch coming in the axil of the reduced leaf. 
The stem nature of these apparent leaves becomes 
obvious at the time of flowering. Then a little flower 
or tuft of flowers arises in the middle of its surface. 
Text-figure 1 shows a sketch of a Ruscus branch with 
its false leaves that are really stems. 

The Leaves are of all the organs the most subject to 
variation, and their modifications are endless. The 
normal foliage leaf is flat and expanded, its outline may 
be quite simple or deeply cut and elaborately shaped. 
Commonly there is a leaf stalk which attaches it to the 
stem. Foliage leaves are green because they contain 
the green substance which is such an essential factor 
for the nutrition of plants (see Chapter V.). Leaves 
are modified, however, to serve innumerable purposes, 
and, according to the functions they perform, so do they 
become changed sometimes almost out of recognition. 
They may be rendered f unctionless and useless by the 
position in which they find themselves, as, for instance, 
when the stem bearing them runs underground. They 
are then reduced to the merest remnant of scales, 
brown or colourless, and thin of texture. Sometimes 
in the underground position they take on a new function 
that of storage. Where they cannot produce food 
they adapt themselves to store what the other air leaves 
have produced, and this we see in the bulbs of Tulips 
and Lilies and Onions. The fleshy part of the " bulb " 
is composed of the modified leaves filled with the stored 
food. In many trees we find modified leaves on the 



MORPHOLOGY 

same branches that bear normal ones. 



15 



For example, 

the hard brown scales which surround and protect the 
delicate foliage leaves in the bud are themselves simply 
leaves which have been modified for this purpose. In 
some buds, for example the Horse Chestnut, you can 




FIG. 2. A spiny cactus, showing the rounded fleshy stem which is green, and 
performs the food assimilation instead of the leaves. The true leaves are 
modified into hard spines. 

find a gradual transition from the outermost brown hard 
scales to the inner ones, which are soft and green. 

In some plants the leaves are all modified and 
hard, and the stem does the work of assimilating. 
For instance, in the Cactus the leaves are all reduced to 
needle-like spines, but the stem is soft and fleshy and 
;;reen- coloured, and manufactures all the food. The 



16 BOTANY 

rounded fleshy mass of the stem exposes much less 
surface for evaporation than would the laminae of 
ordinary leaves, and the plant is thus able to inhabit 
very arid regions. 

A great contrast to the Cactus, with its pudding-like 
stem, is the delicate Creeper that is not strong enough 
to stand alone. Here the leaves, instead of being re- 
duced, have additional work to do, for when a plant 
economises in the tissue it puts into its stem, and has 
a slender axis requiring support, it may call on its leaves 
to assist it in attaching itself. The Sweet-pea does 
this, and at the ends of its compound leaves several of 
the leaflets are reduced and modified into tendrils, 
which are sensitive and motile and cling to any support. 
The well-known creeper, the Ampelopsis, is another 
example of this, in which case the whole of one leaf 
in each pair is modified to form several tendrils, each 
ending in an adhesive disc. 

One of the strangest modifications of leaves is that 
in connection with the capture of insect prey. The 
Sundew (Drosera) with its red leaves covered with 
sparkling tentacles, the sickly yellow leaves of the 
Pinguicula, and the strange and elaborate Pitcher 
plants of all sorts have modified and elaborated their 
leaves to produce traps for the insects they capture 
and use as food. 

Though the leaves naturally are supported by the 
stem, there are not wanting cases where the leaves 
have become the support of the whole plant, as, for 
instance, the great Stag's-horn fern, which is attached to 
tree trunks, and, with its large shield-like leaves, forms 
a bracket which catches fragments of soil and holds 
the water, forming a kind of flower-pot in which the 
roots ramify. Even more specialised " flower-pots " are 



MORPHOLOGY 17 

known in the tropical, rock-inhabiting Discidia. In 
this plant one leaf of a pair forms a bag, much like that 
of a Pitcher plant, in which the adventitious roots from 
each node are contained. 

Such extreme modifications are unusual, but every 
normal plant has various kinds of leaves, and we must 
now turn to the modified leaves which unite to form, 
with all their infinite varieties, what we call the 
flower. 

The essential parts of the flower are the sexual cells, 
but, like the individual tissue cells, these are very minute, 
and so, for their protection and assistance, a number 
of leaves have become particularly modified on a given 
plan which, in its essentials, is common to most flowers. 

The outer leaves of a flower are protective, and these 
are generally green or brown and of strong texture. 
In most of the higher plants they have a definite number, 
often three, four, or five. Within them the next set 
of leaves is generally more brilliantly coloured and of 
more delicate texture. To this special series of leaves 
the name corolla is given, and the individual leaves 
are called the petals. Their work is entirely different 
from that of ordinary leaves, and, while it is partly 
protective, their use is largely to make the flower 
attractive to the insects which come (or .used to come 
in the past) to carry the pollen which effects cross 
pollination. We next come to the more important 
" leaves," which are reduced in general to small stalks, 
bearing the male sporangia, called the pollen sacs. The 
Sporangia belong to a distinct category of organ, and 
though they arise on the modified (and in some families 
on the normal) leaves, they are distinct from them in 
just the same sense that the leaf is distinct from the 
stem that bears it. Indeed the distinction is more 

B 



18 BOTANY 

fundamental when one goes back to the origin of things, 
for the simplest kinds of plants have only two kinds 
of cells, the vegetative and the sporangiate. 

These reduced leaves of the flower and their spore 
sacs are called stamens ; the pollen grains, or spores 
which they produce, contain the male nuclei. The re- 
duced stalk-like " leaves " of the stamens have a 
great tendency in many flowers to enlarge and become 
petal-like. The large flowers of the Rhododendron 
commonly show many intermediate stages between 
ordinary petal leaves, through half reduced petals 
with one or more anthers, to the normal stamens. The 
"doubling'' of Buttercups, Cherries, and such flowers 
is due to the greater part or all of the stamens becoming 
petaloid. When the doubling is complete the flower 
cannot produce any pollen of its own, and must either 
be pollinated from the single flowers or remain sterile. 

We have spoken of the production of the male nuclei 
in the pollen, and this, of course, presupposes the ex- 
istence of a female cell with which it can fuse. These 
female cells are produced in " ovules," which are con- 
tained in one or more cases or carpels lying in the centre 
of the flower. These structures are exceedingly complex, 
and the details of their morphology require much study, 
and are still the subject of investigation and discus- 
sion. There is, however, no doubt that the closed cases 
or carpels which contain the ovules represent a leaf 
in which the edges have rolled over and joined up to 
form a little bag-like structure. This may be entirely 
closed, or may tend later to split open again, as it does 
in the Larkspur, for example, when the seeds are ripe. 
The unfertilised seeds or ovules containing the egg- 
cell develop on the inner edges of the carpel leaves, 
and are thus protected by the closed bag they form. 



MORPHOLOGY 19 

In many details the ovules correspond to sporangia, 
but they are not simply sporangia, and they have added 
to them several coats and inner tissues which no simple 
sporangium has. The egg-cell, however, is the funda- 
mentally important feature in them, and it is with this 
cell that the male nucleus fuses, and it is for the sake 
of bringing these two cells together, and protecting the 
young embryo formed after their fusion, that all the 
complexity of the flower has been developed. How 
complex it is, and how ancient its history, one can only 
realise after studying the fossil types which have gradu- 
ally led up to it. 

Some of the fossil seeds from the Coal Measure period 
are even more complex than those of the present day. 

We have now noticed shortly all the organs of a plant. 
It is likely that a reader will immediately think of fruits 
and seeds which appear such distinctly characteristic 
structures. They are, however, but modifications of 
the parts we have already mentioned. The seeds are 
but the ovules enlarged with the growing embryos, 
in their tissues storehouses of food, and with the 
outer ovular coats hardened. The fruit, whether fleshy, 
winged, or plumed, is a further growth and modifica- 
tion of the carpel leaves or of several carpel leaves 
fused together, or of the carpels with some of the other 
flower-parts adhering to it and ripening with it, instead 
of being shed as soon as the flowering was done. The 
only new thing in the fruits and seeds is the embryo, 
and that begins a new cycle and belongs to a new 
generation. It is composed, however, of the funda- 
mental vegetative organs a root, a stem, and the first 
leaves. These organs are produced in miniature in 
the seed, and then they lie there for a long resting 
period in most plants. 



20 BOTANY 

The germination of the seed is the waking of these 
same organs to life and further growth. In the growth 
and development which follows the germination of the 
seedling there are many features of considerable morpho- 
logical interest. The young plant often tends to repeat 
in its own life history some of the stages through which 
its species passed as a whole in its evolution. Thus 
we find in the development of plants with divided, 
complex leaves that the first three or four leaves of the 
seedling are simpler, and it is only as it grows that it 
attains the elaborate adult foliage. Plants, too, which 
have specialised stems or elaborate structures to re- 
place ordinary foliage, will generally have a much 
simpler and more normal structure when they are very 
young. The study of seedlings is, therefore, a very 
useful factor in attempting to elucidate some of the 
morphological problems. 

So far we have considered only the body of the higher 
plants, in which, though there is infinite variety of detail, 
there is a uniformity of plan throughout. Among many 
of the lower plants we find the vegetative body com- 
posed of the same set of organs root, stem, and leaf 
as in the case of the higher plants. Further comparison 
is rendered more difficult by the fact that the alterna- 
tion of generations, common to nearly all plants, is 
in them expressed in terms of two distinct individuals, 
and a small green plant (known as the Prothallus) 
bears the sexual cells of the large, leafy fern. The 
prothallial plant is produced from the spores of a simple 
kind which are often borne, not on flowers, but on the 
ordinary foliage of the vegetative plant. All our 
common ferns have this character, and the brown 
marks on the leaves are clusters of small sporangia, 
while the little prothallial plant they produce is gener- 



MORPHOLOGY 21 

ally entirely neglected and overlooked owing to its 
minute size. The mosses also have an alternation of 
generations, but in their case the reverse is true, and 
what we know as the moss plant is the prothallial 
generation, which has elaborated itself so that it has 
much the appearance of a leafy plant, though it is so 
different in its origin from the leafy plants of other 
groups. 

In the algae we find the plant body represented by 
simpler structures. The whole algal body is often 
called a thallus, and this has regions which correspond 
more or less closely to root, stem, and leaves in the 
more elaborate and larger of the seaweeds. In most 
algae, however, there is little differentiation among 
the cells, and in the simple hair-like forms so common 
in the fresh water ponds and streams, there are only 
green vegetative cells and reproductive cells with no 
modification into true " organs." 

In the fungi we get also a very simple plant body, 
generally like that of the thread-like algae. Sometimes 
many of these filamentous cells intertwine to form 
quite large and apparently complex bodies, the toad- 
stools for instance, but the plants have not truly differ- 
entiated organs. 

It is interesting to notice how a number of the higher 
plants have degenerated and lost the differentiation 
of their parts. For example, the Dodder (Cuscuta), 
which grows with such deadly success on the Clover 
and Furze, appears to have lost all differentiation of 
stem, root, and leaves, and has become a mere tangle 
of fine pinkish fibres, which attach themselves to the 
stems of other plants and draw all nourishment from 
them. Its flowering, however, it must do for itself, 
and the parts of its flowers, which appear in relatively 



22 BOTANY 

large clusters on the thin stems, are quite normal. One 
of the most interesting cases of a reduced structure 
is the plant body of the giant-flowered Rafflesia. This 
has the largest flower in the world, and it appears to 
have no vegetative body at all ! That is because it is 
so completely parasitic that it gets the whole of its 
nourishment from a host on which it preys, so that 
it can afford to reduce its own vegetative body to the 
minimum, viz., a series of white fungus-like threads 
which are enclosed in the body of the host. In this 
plant roots, stems, and leaves are all gone except for the 
modified leaves of the flower. 



CHAPTER III 

ANATOMY 

WHILE the morphologist deals mainly with the external 
form of the organs of the plant's body, the anatomist 
inquires into the internal structure of those same organs, 
and investigates the arrangement of the tissues of which 
they are composed. 

The plant body, like that of the animal, is built up 
of a number of different tissues, each of which has its 
function to perform in the economy of the whole 
organism. In the animals there are bones, muscles, 
nerve fibres, fat, and so on ; in plants there are wood, 
ground tissue or parenchyma, strengthening tissue or 
sclerenchyma, and so on. The physiological functions 
performed by each of these sets of tissues is generally the 
same throughout the whole animal and plant kingdom. 
Thus the bones, for example, whatever their shape or 
arrangement, form the support of the body, and to 
them the muscles are attached ; the nerves, whatever 
their plan of distribution, are the channels through 
which stimuli and nervous messages are passed. In 
plants, whatever its structure, the wood serves as the 
channel for the conduction of water ; and the scleren- 
chyma, wherever it may be placed, is there for the purpose 
of strengthening or protecting the organ in which it 
develops. Hence, though it is neither wise nor possible 
to divorce entirely the study of anatomy from that of 



24 BOTANY 

physiology, the main work of the anatomist deals with 
the tissues themselves, and concerns itself with their 
individual characters and the comparative study of 
their development in the different orders of organisms. 

The plant body is composed of jive principal kinds 
of tissue. These are the Epidermis, or skin, with its 
hairs and other minor developments ; the Parenchyma, 
forming the general ground tissue of the plant, with a 
number of minor modifications ; the Sclerenchyma, or 
thick-walled strengthening tissue ; and the vascular 
tissue, which is of two kinds, viz., the Wood, which is 
thick-walled, and conducts water and also helps to 
strengthen the plant, and the Bast or Phloem, which 
forms the channel for the passage of the elaborated 
food-stuffs. For the higher plants, although there is 
much specific variety, there is a characteristic plan for 
the arrangement of these tissues in each of the organs 
root, stem, and leaf. 

In roots there is no true epidermis, but the outer cells 
of the young root are extended to form long hairs with 
thin absorbent walls. The parenchymatous ground 
tissue forms the main mass of the root, and the vascular 
tissue is a compact, central strand. In most roots 
there is no pith, and the wood forms a solid mass in 
the centre with groups of the phloem outside it. This 
cylinder is shut off from the surrounding ground tissue 
by a specialised sheath, which is generally much better 
developed in roots and in the lower plants, such aa 
ferns and lycopods, than it is in the other organs of the 
higher plants, though it is sometimes clearly marked 
in their stems. 

Stems have an epidermis while they are young, and 
this protective layer is replaced by an ever increasing 
secondary coat of cork as they increase in size. The 



ANATOMY 25 

ground tissue parenchyma may be modified into several 
kinds of cells fcr different purposes, and in young stems, 
which are green, the outer layers of the parenchyma 
usually contain minute green grains, the chlorophyll 
granules which play such an important part in the manu- 
facturing of food. Often mixed with the parenchyma, 
in regular strands 'or groups, are thick-walled scleren- 
chyma cells, and their position in the stem is almost 
always that which is mechanically most advantageous. 
In stems there is generally a pith of soft parenchyma 
cells, and round that the vascular tissues are arranged 
in groups, each group composed of a strand of wood 
and a strand of bast. As the stem grows these separate 
strands of vascular tissue are joined to form a ring by 
secondary formations of wood and bast. Instead, 
therefore, of the central, solid strand of Vascular tissue, 
as in the root, the stem is characterised by a hollow 
cylinder which is formed round a central pith. In some 
few stems of the higher plants, outside this cylinder an 
endodermis sheath like that in the root can be seen, and 
this is a fact which is of much theoretical importance. 

There are many views as to the real meaning and 
origin of the woody cylinder, and the one which seems 
to be best supported by facts considers the hollow 
vascular cylinder to be the descendant of a solid strand 
not unlike that in the root, the central cells of which 
lost their character as wood cells and became simple 
parenchyma. The stems, which are preserved for us 
as fossils, seem to support this view, though at first 
sight it may sound rather far-fetched to say that the 
cells of the parenchyma on one side of the vascular 
strands have a different value from those on the other 
side of the same strands. 

Probably one of the most powerful influences in the 



26 BOTANY 

development of the wood on these lines was the mechani- 
cal advantage which was thereby gained, for, with the 
same number of thick-walled wood cells, a stronger 
column is produced when it is in the form of a cylinder 
than when it is solid. The wood cells in the stem have 
not only to conduct the water current to the leaves, 




FIG. 3. Transverse section of part of a stem of Aristolochia, showing the 
different kinds of ground tissue and vascular cells. The four largest cells 
in the centre are wood vessels, and the narrow layer of cells just behind 
them, is the cambium layer which gives rise to the new tissue year by year. 

but have also to play a large part in making the stem 
strong enough to stand upright. 

As the stem gets older the ring of secondary wood 
and bast increases greatly, and in perennial plants solid 
rings of wood are added year by year which soon dwarf 
the original primary groups of wood, and they cease 
to function after a time. In trees and woody shrubs 
the formation of the secondary zones of wood increases 
largely, and they become the principal feature in the 
trunk. 



ANATOMY 27 

The formation of rings of secondary wood takes 
place also in roots, so that when they are very old, 
and the inner tissues are crushed, it is not easy to dis- 
guish them from stems. 

The primary structures, however, are easily dis- 
tinguished, and when there is any doubt from the ex- 
ternal morphology alone as to whether any organ is a 
root or a stem, a section showing the internal tissues 
will establish its nature. 

The leaf, with its flat expanded surface, differs from 
the stem and root in having a bilateral and not a radial 
symmetry. In a typical dicotyledonous leaf the single 
vascular strand which runs out from the stem into its 
petiole branches in one plane to form a complete net- 
work like a fan. Each finer branch of the vascular 
strand in this is like the one from which it arose, and 
is composed of a single group of wood cells and a group 
of bast cells side by side. Between the meshes of this 
fan, webbing the whole together, is the soft-celled 
parenchyma. In most cases the upper layers are more 
closely packed and composed of more regular cells 
than those on the lower side, and generally all of them 
contain numerous green granules of chlorophyll. En- 
closing and protecting this web of tissue on both sides 
is an epidermis. In many cases, particularly in the 
tough leaves of plants which grow in hard conditions, 
there are strengthening bands and props of scleren- 
chymatous tissue arranged to great mechanical ad- 
vantage. 

To the theoretically minded anatomist, and him who 
concerns himself with the phylogeny of plant structures, 
the greatest interest lies in the woody tissue. Not only 
is this easier to recognise and stain in living plants, but 
it is better preserved in the fossils than the softer cells, 



28 BOTANY 

and has more character ; while the other tissues seem 
to group themselves round it. It is to the plant's 
body what the bony skeleton and the arterial system 
combined are to the animal. It is thus not surprising 
that most work on plant anatomy treats principally 
of the woody cylinder. 

What we have considered so far has been the vascular 
arrangement in the highest and most important family 
of plants, the flowering plants. In the lower families, 
both living and extinct, there are many other types 
of arrangement. The study of anatomy, therefore, 
bears on systematic botany, for the constant internal 
characters of the organs form reliable criteria for the 
separation of the different groups. 

The outstanding features in the anatomy of the other 
principal groups of plants is as follows : 

The Gymnosperms (the pine-tree group) have a general 
structure similar to that of the Dicotyledons. Their 
wood differs, however, in the character of its uniform 
cells and in the pitting of their walls a point we have 
not yet considered. They have a hollow primary 
cylinder with secondary zones of wood, quite similar 
to those in the flowering plants. 

The Ferns, as they are now represented by the living 
species, are very different in their stem anatomy from 
these higher plants. In the first place, the primary 
organisation of their stems shows great variation in 
type in the different species. Yet the majority agree 
in having a number of separate strands, each organised 
like that of the root of the higher plants in so far as it 
has the wood in the centre with the bast surrounding 
it, and that each such strand is shut off from the sur- 
rounding parenchyma by a specially organised sheath 
the epidermis. In a few ferns a single hollow cylinder 



ANATOMY 29 

is arranged on this plan, but in most there are several 
strands, and in many ferns the number of anastomos- 
ing strands is very large. In none of the living ferns 
are these primary strands united by any secondary 
growth of woody tissue. In the Lycopods the arrange- 
ment, though with individual peculiarities, is much like 
that in the ferns. So long as only living forms were 
studied, it was thought that the formation of secondary 
wood was a character only developed in the Gymno- 
sperms and the flowering plants. Since the anatomy 
of the fossils has been studied, however, the remark- 
able fact has come to light that in the early and extinct 
forms of the ferns and the Lycopods, and even of the 
Equisetaceae, secondary woody tissue was developed 
in considerable quantities, and apparently on the same 
plan as is now found in the Gymnosperms. Their 
primary structures were like those of their living re- 
presentatives, and quite unlike the higher plants. It 
is almost universally true that the primary structures 
of the plant are the truest guides to its affinity. The 
development in time past of the secondary wood in 
the Lycopods and other extinct Pteridophytes was at 
a time when they were among the largest tree-like forms 
of plants then extant. To support their mighty shafts and 
to supply their crown of leaves with water it was necessary 
to have additional woody tissue, which was developed 
in the most straightforward and simplest way in radial 
rows of cells. That Lycopods to-day do not develop 
such wood is doubtless due to the fact that they do not 
grow to such a size as to require it. But, when we ask 
why we do not now find them growing to such a size, 
we have left the province of anatomy and entered the 
philosophical field in which uncertainty still reigns. In 
the families below the ferns there is little that greatly 



30 BOTANY 

concerns the vascular anatomist. The Mosses have but 
little differentiation into true tissues, though the well- 
known genus, Polytrichum, has something corresponding 
to wood and phloem cells. 

The Algae have no differentiation into true tissues, 
and only some of the largest of them, the Laminarias, 
show anything approaching the vascular cells of the 
vascular plants. In them there are zones of elongated 
cells with sieve-like plates between which distinctly 
resemble some of the bast cells in higher plants. The 
thread-like algae and the fungi are simply composed 
of slightly differentiated cells which are fundamentally 
parenchymatous. For anatomical interest, then, we 
must return to the Pteridophytes and the higher plants. 

From a study of the present-day ferns and the many 
fossil genera of Pteridophytes and that extinct group, 
the Pteridospermae, it appears that a great many varieties 
of arrangement of the woody tissues have been attempted 
by plants. Many of these were much more complex than 
the simple hollow cylinder which is now found in the 
most successful and highest types. It appears almost 
as though the present simple type of structure were 
the result of reduction from something more cumber- 
some. The remnant of the endodermis, for example, 
which is found in some Dicotyledon stems to-day, is 
one of the clues that suggest this. Further, while 
it is out of the question in the present state of our 
knowledge to fill in the gaps in a direct series of descent, 
it is yet possible among the fossils of different families 
to show a conceivably parallel series in which the simple 
hollow cylinder of wood is connected with foims which 
had a solid central mass of wood, and, again, with others 
in which the pith was beginning to be formed in the 
middle of it. In the anatomy of all plants the rela- 



ANATOMY 31 

tion of the leaf strands to the vascular tissue of the 
main stem is a very important factor. In the modern 
higher plants the primary vascular strand passing up 
the stem passes directly out to the leaf stalk, so that 
the leaf strands and those from the stem are the same 
and form one system. In some of the lower plants, 
and in many of the fossils, this does not appear to be 
so, and it is possible that in the early forms the stem 
had a system of vascular strands of its own which helped 
to complicate matters for those who theorise. 



CHAPTER IV 

CYTOLOGY 

THE anatomist, as we have seen, deals with the cells 
of the plant as they are grouped in tissues. To him 
the tissues (which are themselves composed of numer- 
ous cells) are the units with which he works. The 
cytologist deals with the ultimate unit of the plant 
body the individual cell. 

The body of a plant, like that of an animal, is ulti- 
mately composed of innumerable minute cells, which in 
the plant are each enclosed in a cell wall, and, together, 
they form a kind of honeycomb. The differences be- 
tween the cells of the various tissues are principally, 
differences in the nature of their cell walls. Within, 
the fundamental living cell is extraordinarily uniform 
tliroughout the whole plant world. And even more 
remarkable is the likeness between the cells of plants 
and animals. In their fundamental and essential 
features, particularly at that critical time of division 
and reproduction, the likeness between the plant and 
the animal cell amounts almost to an identity. This 
branch of botany and this branch of zoology still remain 
under the old heading of biological science, for it is 
impossible to go deeply into cytological work without 
using both plants and animals as illustrations of funda- 
mental facts. 

The typical cell consists of a mass of protoplasm, 
with a central kernel the nucleus ; in plants this is 



CYTOLOGY 33 

almost always enclosed in a cell-wall of definite shape. 
Individual cells are very seldom large enough to be seen 
with the naked eye, though egg cells are in some families 
large enough to be recognised, and in some cases fibres 
and hairs several millimetres or more in length are com- 
posed of a single cell. In general, however, the study 
even of the grosser features of cells can only be under- 
taken through the microscope. To see the finer details 
an exceedingly high power of magnification is required. 
To separate an individual living cell from the rest in a 
tissue is not easy, and yet for examination under high 
magnification the specimen must be exceedingly thin ; 
even two of the smallest cells lying on the top of 
each other are too opaque for microscopic examination, 
consequently mechanical means are employed to cut 
thin sections of the tissues. The material to be ex- 
amined is killed and " fixed " by some chemical solu- 
tion which quickly penetrates to the finest ultimate 
structures in the cells, so that they remain as nearly 
as possible exactly as they were when alive. Many 
hundreds of sections may be cut from an object that 
is being studied, and the course of the life processes is 
reconstructed from them. Thus it happens that the 
motions and behaviour of the nuclei, for instance, 
though described as if from observations made on a 
living specimen, are seldom based on actual observa- 
tions, and our knowledge of them is reconstructed 
from innumerable fixed sections. 

The first glance at a parenchyma cell shows that the 
mass of protoplasm within its wall is finely granular, 
and that in it there is a darker mass, also granular, 
which is often found in a somewhat central position, 
and is called the nucleus. The nucleus is the most 
vital part of the cell, and its elaborate behaviour has 

c 



34 BOTANY 

attracted much study. What the wood is among 
tissues to the anatomist, that the nucleus is to the 
cytologist the principal object of his research. Before 
we turn our attention to the nucleus, however, it is well 
to notice that in the protoplasm are a number of other 




in. 



FIG. 4. A single cell from typical vegetative tissue, cw, the cell wall, w, the 
walls of the adjacent cells, showing how they fit into each other to make 
a honeycomb-like mass. The cell is filled with granular protoplasm, in 
which lie c, the chromatophores, and ?i, the nucleus. A membrane, m, 
surrounds the nucleus, which Is of denser composition than the protoplasm, 
and has several granular masses of a proteid nature in it. 

granules which vary according to the nature of the cell. 
The commonest of these are starch grains, proteid 
granules, oil drops, and, in cells from the leaf or the 
outer part of a young stem, green chlorophyll granules. 
All these materials are not a fundamental part of the 
protoplasm, but are a result of its activities. 

In the figure we see a sketch of a typical resting cell. 
Such is the mature and permanent condition of many 



CYTOLOGY 35 

cells. On the other hand, such a cell may continue to 
add to the material laid down in its cell- wall, and may 
do this to such an extent that the wall attains a great 
thickness and the cell may become what is called sclerised. 
Sometimes the cell elongates meanwhile, and a long, 
thick-walled fibre is formed. By the modifications of 
the cell- wall also, the much elongated and complex vessels 
of the vascular tissues are characterised. Several cells 
fuse together, end to end, for their formation, and the 
walls are thickened and sculptured in many different 
ways. When such modifications have taken place the 
protoplasm and nuclei of the cells die, and no further 
development is possible. The cells which retain the 
power to divide and form new tissue, whether it be in 
the wood-forming cambium, in the stem-growing tip, 
or in the sexual organs, such cells remain soft-walled 
and undifferentiated. In all such cases of division and 
the formation of new cells the prime mover is the nucleus. 
While it is at rest the structure of the nucleus appears 
comparatively simple. It is composed of a granular 
mass with one or two large and more definite bodies 
within it, the nucleoli, and between it and the cell 
protoplasm is a fine wall, the nuclear membrane. But 
when the impulse to divide has stirred in it its structure 
changes, and the granular substance kaleidoscopically 
becomes a long thread coiled many times on itself. In 
the meantime the nucleoli disappear, then the thread 
breaks up into short segments of equal length termed 
chromosomes. By this time faint striations are seen 
radiating from two poles in the nucleus, and the little rod- 
like lengths of the original thread arrange themselves on 
the equator of the striations. They gradually split and 
move apart from one another, equal numbers going 
to each pole. When they have reached this a line of 



36 BOTANY 

thickening appears along the equator of the thread-like 
striations, and these gradually fuse together and separate 
by a wall the two groups of bent rods that went to the 




FIQ. 5. _A series of simplified diagrams to show some of the most important 
stages of the process (called mitosis) through which a nucleus passes in its 
division to form new cells. In 1 the chromosomes are in a tangled skein, 
in 4 they are separately seen as curved, horse-shoe like loops at each end 
of the nuclear spindle. 6 shows the two nuclei of the daughter cells 
settling down to the normal, and the wall nearly completed between the 
results of the division. In rapidly growing tissue (such as root tips) these 
cells will quickly grow to the size of 1, and then go through the procesg 
again. 

two poles. At the poles these rods intertwine and unite to 
form a long tangled thread once again, and this reverts 
to the condition it was in in the original nucleus, and 
VTQ see a granular mass with nucleoli at each pole. 



CYTOLOGY 37 

The fine polar striations have disappeared, and their 
thickenings alone remain, and form the cell- wall, dividing 
the two newly formed cells from each other. 

This, in a few words, is a simple account of the typical 
process in this exceedingly complicated phenomenon. 
Among the different tissue regions of various plants 
considerable range of detail is found. It is a mere 
outline of the marvellous process that is undergone 
every time a cell is added to the body of the plant. One 
of the most extraordinary and apparently one of the 
most important features in this process is the fact that 
the number of curved rods which range themselves 
on the equator of the spindle is always constant for a 
given species. For instance, there are twenty-four in 
the Lily, fourteen in the Evening Primrose, and so on. 
Though between each spindle formation the rods appear 
to be completely lost, first in the long tangled thread 
and then in the granular mass of the nucleus, each time 
the process is repeated they appear in the same number, 
and as they are ranged on the equator they split, so 
that an equal number go to each pole and thus to each 
of the newly formed nuclei resulting from the division. 
The number of these rods varies in different species, but 
it is seldom very large ; in some parasitic animals it is as 
low as four. They are called technically chromosomes. 

One of the not least remarkable features of this whole 
process is the fact that the stages described and illus- 
trated above are found, not only universally in plants, 
but also in animals. In their ultimate structure plants 
and animals approximate closely, though in the kinds 
of tissues formed by the aggregates of their cells, and 
in their external features, they differ widely. 

Mention was made in the previous chapter of the 
fusion which takes place between the male and female 



38 BOTANY 

nuclei.| This is the act of fertilisation when the two 
nuclei melt into one another and become as one, though 
the distinct chromosomes retain their individuality. 
The stimulus which results starts the active produc- 
tion of new cells by the repeated division of the original 
fertilised egg cell, and ultimately tissues differentiate. 
But as the number of those rods (chromosomes) in the 
dividing nucleus is fixed, it would appear that the intro- 
duction of the male nucleus should perpetually double 
the number, and thus disturb the regular specific 
character. This would take place were it not for what 
is called the reduction division, which occurs in both 
the sexes in the generation of cells immediately pre- 
ceding the actual male and female nucleus. By this 
means there is but half the normal vegetative number 
in the two fusing cells, the egg cell and the male cell, 
and so when they fuse the number of chromosomes is 
doubled and thus brought back to the number normal 
in vegetative cells for the particular species. 

There are innumerable interesting details connected 
with the reproductive cells, and indeed the work of 
cytologists is principally with such problems. The 
extreme delicacy of manipulation and the accuracy 
of observation which are required make the study pre- 
eminently one for specialists, and also account for the 
diversity of opinion which now prevails regarding many 
fundamental questions. As each new individual of a 
new generation arises from the divisions of the fused 
egg and male cell, it is certain that its characters, which 
are largely inherited, must have been transmitted in 
the minute structure of those two cells. The male 
cell is generally much smaller than the female, even 
though that is itself microscopic, and as the male enters 
the female it is said to lose all its outer protoplasm 



CYTOLOGY 39 

and enters the female nucleus simply as a naked nucleus. 
It is, therefore, supposed that in the nucleus alone all 
the inherited characters are carried. As the definite 
rods (the chromosomes) appear always to be so constant 
in the nucleus they have been suspected of being the 
actual bearers of the inherited qualities. There is, 
however, such a small number of them in comparison 
with the number of characters to be carried that they 
cannot be the ultimate units, and many theoretical, 
ultra-microscopic structures have been imagined to 
do the work. Finality has not yet been reached, 
though it lies within the province of cytology to discover 
the nature of the structures that carry the transmitted 
characters, and that are consequently of such excep- 
tional interest to us, for in man the problem is ultimately 
the same as in the plants ; and in the ultimate units 
composing the chromosomes, it would appear, lies the 
basis of our mental as well as physical characteristics. 
Some evidence goes to show that the cytoplasm is also 
the bearer of inheritable characters, but its importance 
in this respect has not yet been demonstrated so fully 
as that of the chromosomes. Plant cytology is of 
supreme importance in dealing with these questions, 
because the nature of plants makes them such suitable 
material for experiment. 



CHAPTER V 

PHYSIOLOGY 

THE life processes and reactions of a living entity form 
the special study of Physiology, whether it be of plants 
or animals. These life processes and reactions among 
plants are not nearly so obvious as they are in animals, 
but many of them are strikingly similar in the two 
classes of creatures. The most fundamental differ- 
ence between plants and animals is in their methods 
of feeding. The plant is constructive, and works up 
for itself the simplest elements into food, while animals 
are ultimately destructive and, in using these same 
elements, destroy their combination, and leave them 
in a form which is useless for food until they are once 
more worked up by plants. All the carbohydrates, 
the starches, and sugars, and all the nitrogen compounds, 
the proteids, are ultimately provided for the whole 
animal world by the plant world. The study of nutri- 
tion, then, is one of the important branches of physio- 
logical work, but it is not by any means the only one. 
The breathing, drinking, and moving of plants must 
also be studied, and their appreciation of and reaction 
to light, heat, and gravitation. The sum total of all 
these reactions and responses results in what we call, 
simply, growth. And this " growth " is expressed by 
the stretching, enlargement, or alteration in shape of 
the organs and their increase in numbers according to 
certain rules and rhythms, which are also studied by 

40 



PHYSIOLOGY 41 

the physiologist. Finally, the ultimate result of all 
the growth and reactions is the reproduction of the in- 
dividual ; and the details of this culmination are also 
within the province of physiology. 

Though the physiologist looks on the plant from quite 
a different point of view from the anatomist, the mor- 
phologist, or the cytologist, he must, nevertheless, 
take into consideration the results of their work, for 
there is no use in trying to make observations on the 
work of a machine unless you know how it is put to- 
gether, and what it is intended to do. The physiologist 
must also have a considerable knowledge of organic 
chemistry, for the processes that go on in the organs 
of plants in the course of their breathing, feeding, &c., 
are in reality complex chemical reactions, the key to 
the comprehension of winch is a knowledge of the simpler 
reactions which can be made to take place in test- 
tubes and retorts. Indeed, a laboratory for the ad- 
vanced study of plant physiology appears outwardly 
very much like a chemical laboratory, with its glass 
tubes and reagents and complicated pieces of apparatus. 

Speaking as a physiologist, the leaf is the most im- 
portant part of a plant. The leaf is the actual factory 
of the food of the world. In the leaf the carbon is ex- 
tracted from the carbonic acid gas in the atmosphere, 
and is worked up with the hydrogen and oxygen in 
water to form soluble sugars, and is deposited tem- 
porarily in the leaf as starch grains, which are carried 
away as sugars and deposited ultimately in roots, 
stem, or other places of storage. The atmospheric 
carbon dioxide enters the leaf through the pores or 
stomata in its epidermis, and the water which is in 
every living cell is supplied from the soil by the roots. 
The process of turning these simple elements into the 



42 BOTANY 

organic compound, starch, is called carbon assimila- 
tion, and it is only possible for it to take place in 
those parts of the plant where the cells are green, or 
rather, it only takes place in the cells which contain in 
their protoplasm small green bodies called chloroplasts. 
These chloroplasts, by reason of their colouring matter, 
are able to use and convert the energy of the sunshine 
to supply the chemical energy necessary to cause the 
combination of the elements that form the starch. 
In the darkness the leaves are like a factory in which 
the engines have been stopped and nothing can be 
done. It is only in the light, with a supply of the atmos- 
pheric gases and of water, and with the green bodies in 
a healthy condition, that the manufacture of food can 
go on. Some plants, or parts of plants, do not appear 
green, but are red or some other colour, as in the case 
of the red seaweeds for instance. This does not neces- 
sarily mean that they are not producing their food, for 
sometimes coloured sap or other granules mask the 
chlorophyll in the cells, but without interfering with 
their activities. On the other hand, some coloured 
plants, such as the brilliant toadstools, for example, 
are not able to make any food at all, for they are funda- 
mentally devoid of the chlorophyll grains. Such plants 
can only get their food by stealing it from some living 
green plant, or by using what is left in the protoplasm 
of dead ones. Such chlorophyll-less plants correspond 
to animals in their nutrition in that they have not the 
power to work up the simple elements for themselves. 

Important in nutrition as are the carbohydrates, the 
manufacture of which we have just indicated, they are 
not alone enough for the nutrition of protoplasm, 
whether of plant or animal. Some nitrogen and a few 
mineral salts among which iron, phosphorus, potassium, 



PHYSIOLOGY 43 

and sodium are very important must be worked into 
the complex molecules which form the basis of life. 
These mineral salts the plant gets in weak solution 
from the water in the soil. Curiously enough, although 
all the solid carbon it requires it can obtain direct from 
the atmosphere, in which there is such a small percentage 
of gas containing it, the nitrogen necessary can only 
be utilised when it is in compounds in solution in water ; 
and all the abundance of gaseous nitrogen in the air 
is useless to an ordinary plant. Hence of the manures 
that must be added to soil that is exhausted by the 
growth of many generations of plants upon it, those 
containing nitrogen are of great importance. A few 
plants are able, with the help of certain bacteria, to 
obtain nitrogen from the air, and these are to the farmer 
of the greatest assistance. Clover, Peas, Lupins, and 
indeed the whole family of Leguminacese, as well as a 
few trees, have on their roots small swellings which are 
produced in connection with, and inhabited by, bacteria. 
There are also in the soil other bacteria which do part 
of the business of turning the free nitrogen in the atmos- 
phere which permeates the soil into chemical compounds, 
which are then further worked up by other bacteria 
till the clover and other plants with bacterial nodules 
are able to utilise the resulting mineral solutions. 
Simple experiments can be made to illustrate the need 
of plants for the solutions of nitrates, iron, &c,, by 
growing series of seedlings in glass jars, some in dis- 
tilled water, which is devoid of any minerals, others 
in distilled water with all the necessary salts in solution, 
and others in solutions with one or more of the im- 
portant salts missing. For instruction in this and the 
other experiments that can be made to prove the 
general facts of nutrition and assimilation stated above, 



44 BOTANY 

reference should be made to a text-book of plant 
physiology. 

The water, which is so important to the plant because 
it holds the necessary food-minerals in solution, is also 
essential in another way. The living protoplasm must 
not only be permeated by water, it must have sufficient 
in it to keep the cells firm and taut. A plant immedi- 
ately " droops " when the water contents of the cells 
is reduced, and instead of the stems being brisk and 
upright and the leaves spread out to the light, the 
stems and petioles fall and the leaves crumple up. So 
that, in addition to the food content of the water, the 
plants need the water itself, and may be as truly said 
to drink it as that we do so. Many of the organs and 
tissues of the plant have their part to play in keep- 
ing the water current going. By the chemical process 
of osmosis the soft-walled root hairs draw in the water 
from the soil ; from these cells it passes from cell to 
cell of the root till it reaches the long, specialised wood 
vessels (which we noted as of so much anatomical 
interest), and up these it passes into the corresponding 
cells and vessels in the wood of the stem, thence, by 
similar cells in smaller bundles in the leaf stalks, it 
passes out to the expanded lamina of the leaf itself. 
There, in the cells of the leaf laboratory, it is chiefly 
of use, but as each single drop of water contains only 
a minute amount of nitrates, &c., in solution, any 
given water drop is soon exhausted, and must then be 
replaced. Before it can be replaced, however, it must be 
got rid of, for the cells are each bounded by cell walls, 
and have, therefore, a limited capacity. The walls of 
these cells are delicate and permeable, and they are 
loosely packed in the tissue of the leaf, so that there 
are many air spaces between them, and this air is in 



PHYSIOLOGY 45 

continual circulation because it is in direct continuity 
with the general atmosphere through the pores in the 
epidermis. This air, circulating round the thin-walled 
cells, tends to dry them, and thus removes the water 
from them almost as fast as it reaches them through 
the other tissues from the roots. Hence a stream of water 
vapour is constantly being given off from the leaves. 
The circulation of water from the soil through the 
roots and stems and from the leaves once more into 
the atmosphere is technically called the transpiration 
current. When all goes well with a plant in this circu- 
lation of water the roots supply as fast as the leaves 
give off, and the cells are provided with all they want, 
but in a drought, when the soil is parched, or if the con- 
nection with the roots is severed, the leaves give off 
more than they are receiving, and the plant wilts and 
will ultimately die of lack of water. The amount 
of water that is kept in circulation by a large tree is 
enormous, as is brought home to one by the bleeding 
of a trunk that has been cut off in the spring, when 
the sap is flowing fast to supply the call of the young 
leaves. 

One more relation to the atmosphere must not be 
forgotten, and that is the breathing of plants. It is 
a widespread error to imagine that plants do not breathe 
at all, or else to confuse the carbon assimilation with 
breathing. The process of breathing is really one for 
the oxidation of the tissues, and in both plants and 
animals oxygen is taken in for this purpose ; some of it 
is used, and the waste product resulting is carbonic 
acid gas. In the lungs of animals this process goes on 
simply, but in the leaves of plants, where it also goes 
on, it is masked by the other process of feeding, in which 
carbonic acid gas is taken in as food and split up, and 



46 BOTANY 

oxygen left as a waste product. Nevertheless, in every 
leaf the two processes are going on simultaneously in 
the same cells at the same time during the day. At 
night, when it is dark, the carbon assimilation ceases 
and the process of breathing is not masked, and, con- 
sequently, the only gas given off by the leaves is car- 
bonic acid. It is this fact that has led to the old wives' 
tales that plants are healthy in daytime but poisonous 
at night. 

Breathing, eating, and drinking are the most vital 
functions in a plant's life, for if any one of these gets 
seriously out of order the individual must die. Growth 
may be arrested, reproduction may be delayed, but in 
most plants breathing and feeding dare not be inter- 
rupted for long. In the cases of hibernating animals 
and hibernating plants, such as our trees when the 
leaves are off them, there is plenty of food stored in 
the tissue cells to carry on the passive life of a sleeping 
organism. 

The plant's responses to the many other stimuli 
which it is capable of perceiving to a greater or less 
degree, are generally found to assist it in the main 
functions of its life. For instance, take the case of 
the plant's sensitiveness to light heliotropism, as it is 
called by the professional physiologist. That stems 
and leaves grow out towards light the geraniums in 
any cottage window demonstrate. The simple mechani- 
cal explanation of this bending towards light is that 
the light actually tends to retard the growth of indi- 
vidual cells, thus those on the shady side of the leaf 
stalk grow more quickly, and the whole stalk is con- 
sequently curved towards the light, carrying the leaf 
blade with it. This growing towards light is an in- 
herent character in these parts of plants. It cannot 



PHYSIOLOGY 47 

be said in any way that the plant knows that its leaves 
require the light, and yet the result in the plant's whole 
economy is that the tendency to grow towards the light 
places the leaves so that the necessary light falls on 
them, and they are thus able to perform their function 
of food-making for the benefit of the whole individual. 

Another influence which helps to direct growth is 
the attraction or repulsion of gravitation. The plant, 
in some way which has not yet been fully explained, is 
able to perceive whether it is growing in the direction 
of the force of gravitation or at an angle to it. The 
minute starch grains in the tips of organs fall to one 
side or the other of the cells as the position is changed, 
and it seems probable that they act somehow like the 
" statoliths " in the invertebrate animals. Not one 
organ alone, but various parts of the plant, react in- 
dependently when the position of the whole is changed. 
This sensitiveness is called geotropism, and is the main 
cause of the roots growing down into the earth and of 
the stems growing upright in the air. Such plants as 
climb or creep are affected by other influences which 
to a greater or less extent counteract the rectangular 
response which is normal in most. An illustration of 
the strength of the effect of gravitation may be well 
seen in a tall herb which has been " laid " by the 
wind or broken under foot in an empty flower-bed. 
It will begin to "raise its head" in a few hours, 
and the end of the shoot will grow upright. That this 
return to the upright position is not due to heliotropism 
or the growth towards light is shown in the case of a 
plant in an empty flower-bed, for there the prostrate 
leaves would not be overshadowed by other vegetation. 
The fact that the different organs respond differently 
to gravitation, and roots are positively geo tropic, 



48 BOTANY 

while the stems are negatively geotropic, is of the greatest 
importance to an ordinary plant, for the function of 
the roots is to grow into the soil to hold the plant and 
to absorb water from the moist earth, while the function 
of the stems is to grow out into the air and carry the 
food-producing leaves into the light and air, where 
they get the essentials for their manufactures. 

Another physical factor to which plants are sensitive 
is the temperature. Heat and cold have a great in- 
fluence on the growth and activity of all the parts. 
Roots which are chilled cannot absorb water, and it 
will be remembered how essential that is for the well- 
being of the individual. On the whole, most vegetation 
responds favourably to a comfortable warmth. But 
the range of temperature is not very great, and ex- 
cessive heat is bad for, and finally kills, most plants, 
except those strange little algse which inhabit hot 
springs. 

The fact that both cold and heat are bad and a nice 
medium warmth is the most favourable temperature 
for the general life, illustrates one of the interesting 
results of a scientific study of plant physiology. A 
similar, though not nearly so easily noticeable, series 
of processes is observed in relation to light. Dark- 
ness stops the food-forming activities of leaves (as well 
as affecting the tissues in other ways) and light en- 
courages it. But this light must not be too strong or 
it is again harmful. True, in England, our plants do 
not generally get any opportunity of experiencing this, 
for the light intensity on these islands is not high ; 
still experiments can be made with artificial light, and 
it is found that when the light becomes very intense 
it destroys instead of assisting the life functions. It 
ia found, therefore, that there is a minimum quantity 



PHYSIOLOGY 49 

say, of heat or light that is endirable ; that there is 
also a maximum quantity of light or heat beyond 
which the life suffers or dies ; and that somewhere in 
between them is the best and most suitable quantity, 
which is called the optimum. This scale of maximum, 
optimum, and minimum quantities of light, heat, or 
whatever it is, differs for nearly every plant, and for 
the different organs in some cases. So that the most 
favourable, the optimum of heat for example, for one 
species may be too near the maximum of another to 
let it thrive at all where the first is most flourishing. 
The study of these " limiting factors," as they are 
called, is now one of the great branches of physiological 
work. 

Each plant's relation to light, heat, air supply, water, 
and a number of the other physical factors in its environ- 
ment can be expressed in series of mathematical curves 
or diagrams. 



CHAPTER VI 

ECOLOGY 

AFTER having outlined the departments of study in 
which the plant is considered individually its relation 
to physical factors, its organs, and the cells which com- 
pose them we must now turn to a wider field where 
the plant is merely an individual in a community, 
and consider its environment and its neighbours. This 
study of the plant in its home has been called ecology, 
from the Greek word for home. Just as sociology, as 
a branch of the study of human animals, is a compara- 
tively new " subject," so ecology is a very recent branch 
of botany. 

In a general way the communities which plants form 
have been recognised for long we speak in common 
parlance of " woods " and " heaths," of " marshes " 
and of " moors " but a detailed study of the relations 
of such groups of plants and their surroundings and of 
the laws that form such communities and hold them 
together was first started by Professor Warming, who 
is still living. The systematic study of ecology was, 
indeed, only taken up in England in the last ten years. 

When we speak of " woodland plants " we bracket 
in our minds many individuals of very different types 
not only the tall, woody trees, but the Bracken fern 
and Bramble bushes growing under them, and also tho 
short-lived Blue Bells and Wood Anemones of the spring ; 
and when we speak of the " moors " we think not only 

50 



ECOLOGY 51 

of the Heather and the Cotton Grass, but of the Sphagnum 
Moss as well. Such groups of quite dissimilar plants 
growing together form the communities, or " forma- 
tions," as they are sometimes technically called, and, 
in a way, they correspond to a city among men where 
there is room for a certain number of tanners and 
bakers and printers and postmen, but where, if the com- 
munity is to succeed, the types must not all be adapted 
to the same trade nor exactly the same environment. 
The interaction of the individuals on each other is as 
important a part of the environment as are the merely 
physical conditions. Indeed, among plants as well as 
among animals, they largely determine the physical 
conditions. For example, the ground immediately 
under a tall, spreading tree is often quite dry even in 
the heaviest rain ; it is then futile to measure the rain- - 
fall for the district and to assume that in that district 
all plants that require that rainfall would be happy in 
it. So in any community, because the plants are 
growing together, it does not at all follow that they 
require the same conditions for life ; but that they fit 
into each others needs, and together help to adapt to 
their requirements the natural physical environment. 

In speaking of a plant community or formation, 
however, one does not only consider the plants that 
form it, for to some extent we have, subconsciously in 
our minds, the thought of the physical nature of the 
locality in which the plants are growing. For instance, 
" a marsh " almost postulates the conception of a flat, 
low-lying, water-logged piece of ground, while " a 
heath " conveys the idea not only of a mixture of Heather 
and dry grasses but of a stretch of comparatively high 
land of a dry and often sandy nature. 

If we take such communities as units and imagine a 



52 BOTANY 

map of England or of the world in which the different 
areas covered by heath, moorland, woodland, marsh, 
and so on were coloured in different colours, then we 
can recognise at once that though the extent of the 
different patches would not entirely coincide with the 
different physical characters of the ground, yet there 
would be a distinct tendency for them to coincide, 
except where cultivation has seriously interfered. 

Rich, warm soil, with a sufficiency of water which is 
well drained off, yields most of the " normal " plants, 
while difficulties of any kind, such as the want of water 
on a high sandy soil, or the extreme scarcity of water 
combined with a troublesome shifty soil in the sand- 
dunes, tend to produce plants with organs specialised 
to meet the peculiarity of the environment. Such 
specialised plants are among the most interesting and 
curious, for one organ is often elaborately developed, 
apparently out of all proportion to the others, as in 
the case of the little tufted plants, where there may be 
a root many feet long to provide a visible plant only 
an inch high above ground. 

As a general rule, the strange modifications and elabo- 
rate devices in plant organs have taken place in relation 
to the water supply. Hence the study of those which 
live under desert and other drought conditions has 
been one of the most attractive and obvious fields 
of ecological work. The Cactus, with its leaves all 
turned into spines, and the fleshy-leaved Stonecrop, 
the plant with dry, rolled-up leaves or those thickly 
covered with woolly hairs, each finds these peculiarities 
an aid to retaining the scanty water which would not 
suffice to supply ordinary broad soft leaves, from which 
water evaporates rapidly. The Cactus and the leaves 
of the fleshy-leaved Stonecrop, by becoming cylindrical 



ECOLOGY 58 

or spherical, much reduce the area of surface which can 
evaporate in proportion to their contents ; the rolled- 
up leaves not only save the exposure of both surfaces 
at once, but, in general, their pores are only on the side 
which is rolled inmost, and so evaporation, or transpira- 
tion, takes place into the nearly closed cavity made by 
the rolled leaf instead of into the open air ; while the 
woolly covering of hairs prevents the air currents 
sweeping over an unprotected surface and tending to 
dry it, for the felt of hairs helps to keep the air stagnant 
over the pores and thus to reduce the amount of tran- 
spiration. Reference must be made to the numerous 
instances of such adaptations described in nearly every 
book on botany. 

An interesting point to notice is the tendency that 
several swamp and salt marsh plants^ show to develop 
some of the characteristics of desert vegetation. This 
is to be explained by the fact that the water, which is 
present in abundance in a physical sense in swamps or 
salt marshes, is wanting in a physiological sense, because 
water that is heavily charged with humic acid or with 
mineral salts is of very little use to the plant. As we 
mentioned in the chapter on physiology the roots 
absorb the water in the soil by a process of osmosis. 
Now, in this chemical process the majority of the com- 
pounds dissolved in the water enter with it, but if the 
solution is too strong then more salts enter in solution 
than the cells can use up, and the cells get clogged and 
poisoned. Hence the entry of the water must be re- 
stricted, and hence the surface transpiration must not 
be too great, and the plant is as badly off for water as 
if it were living in a region where there is very little in 
the soil. 

In considering plant communities we have not only 



54 BOTANY 

a host of such facts to notice, but also the relation of 
the various kinds of plants to each other. For instance, 
many of our typical spring woodland flowers only grow 
in the woodland community because at the time when 
they are most in need of light the tall trees above them 
have not yet got their leaves, and the light comes 
sufficiently between the bare branches. In the eternal 
shade of a wood composed of evergreens we do not 
find the same carpet of flowers as in the light, deciduous 
forests. 

One other illustration of this must suffice the Creep- 
ing Willow and many other plants of the sand-dune 
would never have been able to grow on the shifting 
sand at all if it had not been for the sand-binding grass, 
the Psamma, which forges ahead into the bare places, 
and makes a substratum firm enough for the other plants 
to inhabit. 

It will be realised, consequently, that the various 
species are not only adapted to different features in the 
environment, but that the peculiarities of one species often 
prove to be most useful to another by preparing and 
changing the physical features of available soil. The 
morphologist and the anatomist look on the peculi- 
arities of the individual as adaptations for its own 
purposes, but the ecologist takes a broader view than 
that and sees the various types interacting and inter- 
dependent. 

Further even than this the ecologist must go and 
see the plants actually affecting the physiography and 
even the geography of some districts. A good illustra- 
tion of this is seen in that very sand-grass just mentioned. 
The loose sand thrown up by the sea is blown by the 
wind to and fro and piled up in mounds only to be 
scattered again as the wind changes, but once the creep- 



ECOLOGY 55 

ing rhizomes of the sand-grass get a hold on it their 
power is greater than that of the wind, and by means 
of the long ramifying roots and the branching rhizomes 
the sand is held together long enough for other plants 
to come in and to establish themselves one by one till 
the surface of the sand is covered. In this way acres 
of dry land may be accumulated, and its character 
changed from that of the bare sand of the shore to the 
dry pasture land of the low heaths. 

The series of different kinds of plants playing " follow 
my leader " into the fresh water ponds is another good 
illustration of the power of the unaided plants to change 
the nature of a given spot. Into the open water of a 
mere or pond, with its minute flora of microscopic 
algae, push out the underground rhizomes of the Phrag- 
mites reed and the Bulrushes. They send up tall shafts 
with leaves and flowers, and in the autumn these die 
down, and the half rotting and fibrous remains are 
tangled together with the roots and rhizomes, and all 
tends to catch any further fragments or detritus that 
is drifting in the water. Gradually, by this means, 
the reeds collect a soil which tends to make the edge 
of the pond shallower, so that the Bog-Bean and other 
shallow water plants can come in and help in the work 
till so much soil is accumulated that the water is quite 
shallow, and rushes and Queen of the Meadow and King 
Cups grow on little marshy mounds with water all round 
them. These close up, and grasses and sedges and Butter- 
cups grow in between, and the land is almost firm and 
established enough to be called meadowland. Behind 
the grassy strip creeps down the forest, and the trees, 
keeping their distance behind the zone of grass, advance 
with its advancing edge till in time the opposite shores 
meet and the forest closes over the space once occupied 



56 BOTANY 

by the pond. When this has happened we see that 
the one community of plants, viz., the woodland, has 
ousted the other, the community of water plants. It 
is not only individuals that struggle against each other, 
but whole communities that usurp each other's place. 
Here, indeed, we can hardly say that there is a struggle 
between the land and the water plants and those of 
the shallow shore, because by their natural growth and 
accumulation the former merely follow on where the 
latter have, by their own growth, rendered the place 
no longer suitable for themselves, but well adapted for 
those which need a built-up soil. 

Recently it has been recognised that there are definite 
laws which govern the series of communities that 
inhabit a region, and a trained ecologist, seeing one set 
of plants growing under certain conditions, can predict 
accurately what type of community will follow it 
always supposing that there is no great physical change, 
such as would be caused by the sweeping away of the 
land by a great flood or its disturbance by a landslide. 

When such a case as this occurs, and we have bare 
fresh land exposed, it is of interest to watch the way 
it is colonised. The general law that is followed is a 
series of changes, first from an entirely bare space to 
one with a few species scattered at fairly regular wide 
intervals over the surface, then by more species, the 
individuals growing closer together, but each still with 
space to develop completely. At this stage there are 
generally a very considerable number of species in 
proportion to the actual number of individuals. Then 
the species really adapted to the soil and the conditions 
begin to take a firm hold, and they grow more crowded 
together and oust the others, till at the end, when the 
vegetation for the spot is firmly established, there are 



ECOLOGY 57 

great numbers of individuals which completely cover 
the ground, but there are comparatively few species. 

In every case the plants of a spot depend to an enor- 
mous extent on the soil. Many species are exceedingly 
sensitive to very small traces of such compounds as 
lime, silicates, salt, &c. Some can only live when 
supplied with lime or chalk, which to others is well- 
nigh a poison. It is well known that the Orchids and 
other plants which grow on the chalk downs cannot 
live on the quartz sand of an old dune. 

In a country so much cultivated as England, however, 
it is often difficult to see the direct influence of the soil 
on the communities of plants growing on it, for hardly 
any of the fields which form so great a part of the land 
have not been subjected many times to manuring and 
planting and to the weeding out of the original in- 
habitants, either entirely or in part. 

The seashores, with their salt-marshes and sand- 
dunes, and the freshwater ponds, where the land plants 
are encroaching, are, perhaps, the best illustrations of 
natural communities which are available for ecological 
study in these islands. 



CHAPTER VII 

PALAEONTOLOGY 

WE have merely hinted at an outline of the branches 
of study in the modern plants, but that outline suggests 
the great extent of detail that must be offered to the 
student by the thousands of living plants that have 
already been named. The palseobotanist is faced by a 
still vaster problem, for in the last thirty million years 
or so, during which the world has been a comfortably 
habitable place, the races of plants have never remained 
the same, for each is altering, evolving, or " devolving " 
(if the word may be used in a new sense) all the time. 
Even at the present time it must be actually true, 
though we so seldom observe its slow progress, that no 
species is fixed and stationary for long together. Every- 
thing is either evolving or dying out. A student of 
fossil botany, therefore, has not only to consider all 
the plants of any one given epoch, as has the modern 
botanist, but he is concerned with series of vegetations 
which differ more or less from each other according to 
the length of time that separates them from each other. 

Of these it is probably not a wildly extravagant 
estimate to say that twenty-nine thirtieths are extinct 
species. If they* are extinct, that means that they are 
no longer alive how then can they be studied ? 

If you walk along a shore to-day at high tide you 
will find many fragments of land plants in the debris, 
not only orange peel and banana skins brought by man, 

58 



PALAEONTOLOGY 59 

but leaves and branches and bits of wood brought 
down by the rivers and drifted out to sea. Often a 
slight change of current or a higher tide will cover these 
scraps with sand or silt, and if they are well covered 
they are perserved from decay between the layers of 
fine silt or mud. This is one of the ways fossils are 
formed. There have been seashores with sand and mud 
washed up by the waves ever since there have been 
habitable lands, and from all the epochs of early time, 
with all their different kinds of plants, there have been 
fragments here and there preserved on the old sea- 
shores or in the deposits that once formed the bottoms 
of lakes or broad rivers. Buried with the mud or sand 
of these shores and lake bottoms, deposited now here 
and now there as the physical geography changed, are 
remnants of the vegetation that was living in the various 
epochs. Sometimes the local currents favoured the de- 
position of many plants in one place, and at others 
there are almost no remains of the local vegetation. 
From the fragments in the rocks palaebotany pieces 
together the ancient plants, and in some fortunate 
cases can discover, not only w r hat they looked like 
externally, but also the very details of their internal 
anatomy. 

The aim of palseobotany is to restore the whole series 
of plants that have lived upon the earth. If that were 
done completely then there would be no need for the 
further theorising about past evolution ; we should 
have before us clear evidence of the actual series of 
forms through which our recent plants have evolved. 
But this state of affairs is excessively remote, for at 
present we have only rescued from the preserving 
strata of the rocks fragments of the extinct genera. 

These fragments, all of which are called fossils, are 



60 BOTANY 

preserved in three main ways. The first and best 
known are impressions. These we see when we split 
open a slab of shale or limestone, and a fragment of a 
fern leaf, or a branch with its foliage, lies pressed between 
the layers of the rock. Sometimes these impressions 
look quite black against the stone, and this is due to 
the carbonisation of the vegetable matter of the tissues. 
In such an impression we have the external form of 
the plant retained as if it were a pressed specimen, 
but all its internal cells are decomposed. 

The second form of fossil is the cast. Here, as in 
the previous kind, it is generally the external features 
of the plant that are preserved. The cast is formed 
by the enclosure of the parts in some generally fine- 
grained, detrital matter. This retains the plant until 
its characters are imprinted on it, so that when the 
vegetable tissue decays the rock still holds its features, 
as plaster of Paris holds the engraving of a medal. Both 
casts and moulds of plants are formed, and sometimes, 
too, we find casts of the internal features of hollow stems. 

The third and most useful form of fossil is the true 
petrifaction. In this case there is often no sign of the 
external features of the preserved plant. A mass of 
silica, or of carbonate of lime, or of dolomite, entirely 
encloses, permeates, and petrifies the inner tissue cells 
and the wood of stems or leaves or seeds. Thin sections 
of these stony masses can be cut in the same way as 
sections are cut of minerals or fossil corals. Then, 
through the microscope, we can see the cells just as 
they can be studied in sections of living plants. From 
series of such sections we can restore not only the 
internal anatomy of plants that have been extinct, per- 
haps, for millions of years, but even points in their 
cytology are discoverable. Such fossils can sometimes 



PALAEONTOLOGY 61 

be associated with impressions which show the external 
form of the plant till we have a fair idea what it was 
like both inside and out. From these data we can 
do something to deduce the ecological condition under 
which it grew. This again leads us on to consider 
such data as indicators of the climates of the departed 
continents. Hence we see that the field that is opened 
up by fossil botany is a very extensive one. 

This branch of the science is, indeed, only in its 
infancy, but it has obtained some results of great interest. 
One or two of them we should now consider. 

Without recapitulating the elements of geology, it 
is well, perhaps, to point out that the epochs of the 
world's history, since the deposition of the sedimentary 
rocks began, have been found to be characterised by 
different series of dominant animals first, the lower 
invertebrates, then the simple vertebrates, such as 
fishes, then the higher in the scale, up to the mammals, 
and, lastly, in very recent times (speaking geologically) 
man himself. The history of the plant world seems 
to be expressed in a similar series, and, on the whole, 
there is a wonderful agreement in result between the 
study of the plant and animal fossils. 

If we begin our study of the botany of the past at 
the end nearest the present, then the first really im- 
portant point to notice is that in comparatively recent 
times in England, in the middle and lower Tertiary 
rocks, for instance, there must have been a rather 
different climate from the present, for we find remains 
of Palms and other semi-tropical plants in these isles. 

We do not have to go very far back in the history of 
the whole earth to come to the time when none of the 
higher plants were living at all. All the members of 
the huge and important group of Angiosperms are of 



62 BOTANY 

comparatively recent origin, for not one really undoubted 
specimen of this now dominant family has been found 
in rocks older than the base of Cretaceous times. One 
or two very rare and doubtful fossils, which may be 
Angiosperms, are known as far back as the Lias. We 
have then to picture in all the earlier epochs a vegeta- 
tion in which not only all the living species are absent, 
but one in which the leading families now dominating 
nearly every locality in the present earth were not at 
all represented. There were not only no trees of the 
nature of Oaks, Beeches, or Poplars, no Daisies, or Lilies, 
or Roses, no Palms, but not even grass. In the times 
preceding the earliest Cretaceous, when the advent of 
these modern families changed the face of the vegeta- 
tion, the most highly evolved family appears to have 
been one which is now extinct, but was not unlike in 
external appearance the rare family of Cycads still 
living. In several ways these curious plants may be 
taken as a parallel in the vegetable kingdom of the 
strange Duck-billed Platypus in the animal world. 

While the extinct members of this cycad-like group 
took the highest place in the scale of evolution of the then 
existing plants, several members of the lower families 
were abundant and bore a more familiar aspect. Pine- 
trees, very similar to those now living, must have been 
numerous then, as well as members more or less closely 
allied to the present Monkey-puzzle (Araucaria). There 
were also numerous ferns which differed externally but 
little from many living genera, and there must have 
been club-mosses, though we know but little about them 
at that epoch. There were also large and small equi- 
setums, very similar in habit to those now living. 

Going back to the earlier times, the plants get increas- 
ingly unlike the modern types until we get back to the 



PALAEONTOLOGY 63 

true Palaeozoic epoch. From the point of view of the 
fossil botanist this epoch is unique because it includes 
the period of the Coal Measures. During this period in 
Europe there was not only a remarkable tendency to 
produce coal in a number of successive layers, but the 
plants which provided the necessary vegetable matter 
for the coal layers were fortunately preserved in large 
numbers. All the different varieties of fossils casts, 
impressions, and very wonderful petrifactions are 
abundant in deposits of this age. We have, conse- 
quently, a more complete knowledge of the flora of the 
Coal Measures than we have of any other epoch, ex- 
cepting that of the present day. All the genera and 
species from these beds are not only extinct but are 
fundamentally different from forms now living. Many 
great volumes have been written on the plants of the 
Coal Measures, but we must only glance at one or two 
of the more interesting of them. Those highest in the 
scale were probably the fossils well known as Cordaites. 
They were tall trees with solid woody shafts and long, 
sword-like leaves, and they bore seeds in cones which 
were more complex than those of the living family 
which is least remote from them, the Monkey-puzzles. 
But the majority of the large tree-like forms of these 
times were much more remote from any living trees 
than were the Cordaites. The two genera, Catamites 
auJ. Lepidodendron, were large trees with very numerous 
different species. Their shafts were sometimes as 
much as three or four feet in diameter, and many speci- 
mens have been recorded that show that they reached 
the height of tall forest trees. The bulk of the stem 
was composed of softer tissue than is usual now in any 
self-supporting tree, but there was a quantity of the 
regularly developed secondary wood which is now only 



64 BOTANY 

found in plants of the Gymnosperm and higher families. 
The early trees, however, belonged to a much lowlier 
family, to the Lycopodiacese, which ranks below the 
ferns and is now represented by the Club-moss or Lyco- 
podium, and the delicate moss-like Selaginella, which 
is so often cultivated in greenhouses. It is improbable 
that any living form is actually descended from these 
giant tree forms of the coal forests, though sometimes 
the modern genera are spoken of as the degenerate 
representatives of the old stock. A truer statement of 
the case would be that the family, as a whole, reached 
its acme of success in these early times, and that the 
dominant position in the forests having been won from 
them by the higher plants as these evolved, the only 
representatives of the group for winch there remained 
room in the scheme of things are the small green herbs. 
Using the words in the accepted sense, which implies 
advance, it is impossible to say that the modern lycopods 
are more evolved than the fossil ones. Both in the 
structure of their wood and in their complexity of fructi- 
fications, as well as in their large size, the fossil trees 
represent more highly organised organisms than do 
the simple modern herbs. One remarkable genus of 
the fossils (Lepidocarpon) had large fructifications 
which almost amounted to seeds, while to-day the true 
lycopods have only simple spores. It appears that 
not only do individuals have a lifetime of waxing and 
waning, but so do families as a whole, for it is certainly 
true that in the time of the Coal Measures one of the 
most numerous, successful, and dominant types w r as the 
Lycopod family, which now is represented by few and 
small species. 

A history almost parallel to this belongs to the other 
great pteridophytio tree group of the Coal Measures 



PALAEONTOLOGY 65 

the Calamites. Tlieir modern representatives are the 
Equisetums or Mares' tails, which are often very numer- 
ous in the places where they grow at all, and which are 
represented by species adapted to life in dry ground 
and others that inhabit shallow water. The English 
species do not exceed a few feet in height, but there 
are some foreign ones that grow in groves together 
and thus help to support each other's slender shafts to 
a height of twenty or more feet. These plants must 
represent on a somewhat smaller scale much of the 
external appearance that was probably presented by 
their sturdier and more complex ancestors. 

One other family from the coal flora must be men- 
tioned and this is one that has now no relative still 
living. Its existence would never have been suspected 
had we not had detailed knowledge of the fossils. This 
group was recently discovered, or rather recognised, 
and named by Professor Oliver and Dr. Scott the 
Pteridospermse. Its name indicates the nature of the 
group, for it means Pteridophytes, that is fern-like 
plants bearing seeds. Among modern plants seeds 
are only borne by the higher families the Gymno- 
sperms and the Angiosperms, ferns and all the tribes 
below them having nothing more advanced than spores. 
Hence this ancient group which connects the fern-like 
plants with those which bear seeds is a most important 
link in the chain of evolution of the vegetable world. 
There are many side issues of interest connected with 
the recent discoveries of these fossil forms, and one of 
these is the stress it has laid anew on the dictum which 
all know and all ignore, viz., that appearances are 
deceitful. One of the most generally accepted tenets 
about the flora of the past in Coal Measure times had 
been that it was the " Age of Ferns," because there 

E 



66 BOTANY 

were such large number of fern leaves among the 
fossils representing the epoch. The impressions of 
these fern leaves were sometimes remarkably perfect, 
and showed the form of the divided fronds which in 
externals so much resemble modern forms. The first 
clue to the discovery that these plants were not what 
they seemed resulted from the study of the specimens 
which have their internal cells petrified. Under micro- 
scopic examination their internal anatomy \vas found 
to be much more highly organised than that of modern 
ferns. The discovery from petrified remains that these 
plants bore seeds of complex structure was followed 
by the recognition in impressions that several other 
species supposed to be ferns also had seeds attached 
to their fern-like leaves. There are now grounds for 
supposing that a large proportion of the " ferns " of 
the Coal Measures belonged to the higher seed-bearing 
group of the Pteridosperms. This extinct group 
bridges one of the great gaps in the series of modern 
plants. Among those which are still living to-day there 
are almost none which indicate the connection between 
ferns and seed-bearing plants. Clear-minded botanists 
some time ago had seen some obscure points of structure 
that hinted to them that some such connection must 
at one time have existed, but the exact form which 
it took, and the time of its existence, were matters 
purely of the imagination. The Pteridosperms and all 
that they reveal are matters of fact. 

It must not be supposed that these are the only 
trophies of the study of modern palaeobotany. Every 
fossil plant that is discovered helps to fill in the blank 
spaces in the great genealogical tree, and many of them 
show quite as interesting or unexpected features as do 
the fossils just described. 



PALEONTOLOGY 67 

When we turn to the rocks that represent still older 
periods of the earth's history we do not find nearly as 
much as we should like in the way of fossils. That 
there must have been plants, and land plants too, in 
Cambrian and Silurian times, and probably earlier, is 
generally agreed, but their nature has not yet been 
revealed. That the Palaeozoic forests with their highly 
complex Gymnosperms and great variety of vascular 
plants are very far from primitive is obvious. Alas, 
that the plants recorded from the earliest times should 
as yet reveal very little indeed about the origin of 
things. 

It is indeed doubtful whether human knowledge will 
ever get down to the roots of life. In the meantime, for 
our reconstruction of the ramifications of the branches of 
the tree of vegetable life, there is no source of facts to 
be compared to the fossils. 



CHAPTER VIII 

PLANT BREEDING 

SATISFACTORILY to define a species is one of the most 
difficult questions in botany, yet if one leaves aside 
for the moment the more abstruse considerations, it is 
possible for the present to get a tolerable idea of what 
we mean by a species. For instance, if we talk of 
" Blackberries," we do not indicate a narrowly defined 
species, for there are so many varieties of Rvbus that 
some consider that there are really a number of species 
more or less closely related passing under the same name, 
while others look on the forms as all one species in a 
scientific sense, which has a number of sub-species or 
varieties. But if, on the other hand, we speak of the 
common little Daisy of our lawns we are more nearly 
indicating a true scientific species, for there is much 
less variability in its forms, and there is not such a plexus 
from which to disentangle our ideas of what a species is. 
Even when we take a comparatively well-marked 
species, like the Daisy or the red Field Poppy, which 
cannot be mistaken for any other species, we find on 
comparing several individuals that there are slight 
differences in the shape of the leaves or in the hairs on 
the stems, or in the brilliance of colour in the petals. 
When plants which have arisen from a pure line of 
ancestry show such differences, it is considered that 
they are purely individual and that they depend on 
trifling differences in the plant's environment. On 

68 



PLANT BREEDING 69 

the other hand, plants which show a great amount of 
variation between the individuals growing together, 
are generally suspected of being the results of cross- 
breeding, or hybrids, as they are called, because by 
experiment it has been shown that the results of cross- 
breeding from slightly different stocks is to induce a 
great amount of variability in the offspring. 

Now, in the vegetation which is untouched by man 
indeed in the past vegetation that had been flourishing 
before ever man appeared there have been innumerable 
opportunities for cross-breeding, both between closely 
allied species and those remote in characters, because 
most flowers are open to the face of heaven, and there 
are the wind and innumerable insects to act as distri- 
buting agents for the pollen. Many flowers are so 
wonderfully adapted that the chance of unexpected 
pollen reaching the stigmas is very slight, while in all 
cases the mixture of two very remote races is prevented 
by the inability of pollen to develop in alien tissue. 
Yet that still leaves enormous possibilities for the for- 
mation of natural hybrids. A pretty example of natural 
hybrids with a good deal of variation is the case of 
Primroses and Cowslips, with the varieties of the hybrid 
Oxlips which have resulted from their interbreeding. 

Scientists have not yet decided how much the vari- 
ability in what appear as pure races is due to the im- 
mediate environment of the individual, and how much is 
the effect of interbreeding in the distant past of the stock, 
but, be that as it may, the fact remains that there is 
this variability, and that it is in the highest degree 
important to the farmer and fruit grower. Fruit or 
flower growers, for instance, cross the pollen from one 
plant on to the stigma of another that has some quality 
they want to breed. From the great variety of offspring 
in a successful cross they select the ones that approxi^ 



70 BOTANY 

mate most closely to the type they desire. After 
many generations of such breeding, forms have been 
obtained which differ materially from either of the 
original parents. The most notable gardener at the 
present time who has undertaken this work on a large 
scale and has obtained many useful or beautiful varieties, 
is Luther Burbank, who has extensive experimental 
gardens in California, and whose varieties of fruit are 
grown all over the world. 

But though it is the most practically useful branch 
of the subject, the mere production of economic varieties 
is by no means the most interesting branch of the study 
of breeding in plants. The gardeners' results, as a rule, 
have been obtained by more or less haphazard crossing, 
and from them alone there are few indications of the 
great laws that underlie the production of the new 
forms and their bearing on evolution and heredity. 

The great work of Charles Darwin, who established 
the theories of evolution and the flux of species on in- 
numerable minute observations, is so universally recog- 
nised, and has had so many more or less popular 
exponents, that there is no need to enlarge on " Dar- 
winism " in these pages. 

All the problems of heredity and the means of trans- 
mission of characters are of supreme importance to 
the evolution theory, and, since Darv/in, the next 
most important contribution to the knowledge of 
heredity was made by the Austrian monk, Mendel. 
He found that an extremely simple numerical law 
governed the appearance of the different characters in 
the second generation of the results of cross-breeding, 
and that, if we note any one given pair of characters, 
they appear in the second generation in the proportion 
of one of one kind, one of the other, and two of the 
mixed character. This can be expressed in algebraic 



PLANT BREEDING 71 

form as follows : where A is one of the characters and 
B the other the result in the second generation of the 
offspring is that, however many there are, they are 
in the proportion, 1A+ 2AB+ IB. 

But this is not at once apparent to the uninitiated, 
for in the pairs of characters we find that one is stronger 
than the other and masks it. For instance, if one pair 
of characters is the smoothness and the hairiness of the 
leaf, then if the hairiness is the strongest character, 
the dominant, as it is technically called, it hides the 
other, and of the offspring we get one smooth, one 
hairy, and two smooth-hairy, which appear hairy, thus 
giving as an apparent result one smooth and three 
hairy. The existence of the smoothness in the hairy 
ones comes out when they are bred again, and from the 
two mixed parents, which looked hairy, one offspring 
h smooth, one hairy, and again two mixed. 

Of course in any given individual there are the results 
of an enormous number of pairs of characters, and the 
more highly organised the organism the greater the 
complexity of the characters, so that the extreme 
arithmetical simplicity of Mendel's law is all the more 
surprising, and it stands out like a solid rock in a sea 
of uncertainty. 

Nevertheless, the meaning of Mendel's work and the 
value it has, both for theoretical and practical purposes, 
was very long in receiving recognition. Mendel himself 
died (in 1884) before scientists had awakened to the 
realisatiqikpf his discoveries, and it is indeed only in 
the last aecade that there has been any considerable 
recognition accorded him. 

Like all really great theories or formulated laws, that 
of Mendel has stimulated other workers to experiment, 
some with the object of proving and others disproving 
it, and the advantage of this is that innumerable new 



72 BOTANY 

facts are in the meantime accumulated which might 
never have been sought for otherwise. Sometimes the 
results of the experiments have seemed at first very start- 
ling and difficult to explain. For instance, in the course 
of Mendelian work, one experimenter had two races 
of Stocks, one with white flowers and one with cream 
flowers. These were crossed in the usual way, and all 
outside pollen carefully kept from them. The result- 
ing offspring were not white, nor cream, but a brilliant 
reddish-purple. At first sight this would look as if 
something was wrong with the laws the experiment 
set out to test, but in reality it indicated the inter- 
play of other pairs of characters which affected the ones 
that were for the moment under investigation. Work 
such as this leads on through an endless chain of ex- 
periment, hypothesis, theory, and again, and all the 
way along, experiment. 

Experimental work on these lines is, of course, done 
also by zoologists, but for many of the problems plants 
afford more convenient working material. Care at 
the time of pollination and in the collecting of seeds 
are the main things in plant breeding. There are few 
of the complicated pieces of apparatus required for 
such work as are necessary for experimental physiology, 
and, consequently, for a botanist cut off from the big 
institutions experimental breeding offers one of the 
most profitable fields of research. In modern experi- 
ments often thousands of specimens are grown all of 
one kind, and their pedigrees are kept for generation 
after generation. 

Modern research in experimental breeding of plants 
received an enormous stimulus and a new direction 
from the work of Hugo de Vries, whose book on " The 
Mutation Theory " appeared so recently as 1901. The 
essential difference between the work and theories of 



PLANT BREEDING 73 

de Vries and the modern school of experimenters, 
stimulated by him either to support or controvert his 
views and the original Darwinian conceptions, is the 
introduction of the conception of the mutant. The 
mutant is a new variety or species which arises suddenly 
and not from a gradual series of inherited modifications, 
and which breeds true. The best known example of 
a species which has given rise to such mutations is the 
Evening Primrose (Oenothera). Of the various species 
of this plant literally tens of thousands of carefully 
selected specimens have been bred by botanists all 
over the world, and the several old established species 
have yielded nearly a dozen of new, suddenly produced 
forms, all of which ultimately " breed true," that is, 
have offspring which, coming from seed, entirely 
resemble the parents. 

The mutants of the Evening Primrose are not start- 
lingly different from their original stock, but they are 
constantly and recognisably different. Their produc- 
tion at all is of great importance to the theories of 
evolution, for since their recognition it has been possible 
definitely to experiment and test this theory and the 
many others which arise out it. 

At present the majority of plant breeders and muta- 
tionists deal only with external characters, but a few 
workers have begun to correlate these external changes 
with the minute details of the cytology. It will be 
remembered that in the chapter on cytology the im- 
portance of the nucleus was emphasised, and we know 
that all the characters that a plant inherits, whatever 
they are, must have lain in one stage in one of the two 
fusing gametes. A great field of experimental and 
theoretic work lies in the future in the correlation of 
the internal and external features in hybrids and in 
the so-called mutants. 



CHAPTER IX 

PATHOLOGY 

EVERY living organism is liable to have the balance of 
its delicate mechanism disturbed by some cause or 
another, and plants, no less than animals, suffer from 
a variety of such causes which destroy utterly, or merely 
locally affect their lives. The diseases of plants have 
not yet been studied so elaborately as those of animals, 
and " doctors " generally confine their attention to 
the higher vertebrates, but, nevertheless, there is a 
great mass of facts which have been -accumulated about 
the various parasites and diseases which attack the 
vegetable world. 

Accidents, like broken limbs or wounds caused by 
stones or sharp instruments, happen to plants as they 
do to animals. In such a case, if the individual to 
whom the accident happens is normally healthy, the 
tissues respond and attempt to heal the gap or to mend 
the fracture. In the case of trees such wounds arise 
oftenest by the felling of a trunk or by the snapping of 
a branch in a gale. The broken surface exposes inner 
tissues to the atmosphere, laden, even in the woods, 
with germs and microbes of disease, and the first essential 
is that the broken surface shall be covered. The plant 
makes an effort to do this by the growth of " callus." 
In the neighbourhood of the wound the cells are stimu- 
lated to divide and grow rapidly, and they attempt to 
form a healing tissue across the surface of the wound. 

74 



PATHOLOGY 75 

Also of the nature of an accident are the various 
forms of poisoning that may happen to healthy plants. 
They may be poisoned by gases in the atmosphere, or 
they may be poisoned by minerals in the soil. In the 
cases of slow poisoning the growth of the tissues may be 
arrested or altered and truly pathological conditions 
set in, in which abnormal cell growths take place. On 
the other hand, where the poison is stronger, the plants 
simply die, as, for instance, when the paths are sprinkled 
with weed-destroying compounds. These enter the 
roots in the osmotic process of root absorption, and 
travel through the cells of the tissues. 

Accidents may happen to the healthiest individuals ; 
the pathologist is more concerned with the diseased 
ones and with those where the tissues are abnormal. 
One of the most fatal diseases that can overtake a 
plant is Chlorosis, or the lack of colouring matter. This 
disease, in its essentials, is very similar to anaemia in 
human beings, and as the plants depend on their colour- 
ing matter for the manufacture of their own food, an 
extreme case cannot survive at all. Chlorosis is an 
obscure disease, but in some cases it certainly appears to 
be caused by a lack of iron, and without iron the human 
blood is not red nor plant granules green. Generally 
the seedlings attacked by the disease die out very early, 
but sometimes sickly whitish-leaved specimens struggle 
along for a little while. The disease is often local, and 
in compound leaves one leaflet here and there may be 
entirely colourless. This character is best seen in the 
gardeners' u variegated " varieties, where the leaves 
are mottled or striped with cream-coloured patches 
and bands. The green parts there do enough work to 
carry on the life of the individual, while the colourless 
parts are non-producers. If this is not carried too far 



76 BOTANY 

the plants can be quite healthy, but if gardeners tried 
to breed an entirely white race, it would die of mal- 
nutrition. 

All the innumerable questions of nutrition come very 
near the borders of the study of pathology, for an ill- 
nourished individual, even if it lives, is much more 
liable to disease than a healthy one. 

The great sources of infected disease for the plant 
world, as for the animal, are the fungi and bacteria. 
The higher plants are attacked by innumerable small 
parasitic forms of fungi, some of which finally kill the 
host. The study of the fungal diseases of plants is an 
enormous one, for there are thousands of species of 
infecting fungi, and in some cases they have most 
complex life histories and pass through cycles of two 
or three generations which inhabit different hosts. In 
the study of human and animal disease many instances 
are well known now of the parasite inhabiting several 
hosts, for instance, man in one generation and the pig 
or the mosquito in another. So it is with plants, and 
the disease which, works havoc with the grain crops 
goes into a new generation that inhabits the Barberry. 
Often the w r ork of connecting the different generations 
of the same disease is rendered excessively difficult by 
the elusive and unexpected nature of the cycles ; and it 
is only by the most careful breeding of the fungus pro- 
ducing the disease and by experiment that the actual 
data can be separated and the life history of the disease 
established. We are, once more, back in the highly 
equipped laboratory and studying details under the 
microscope. 

The economic importance of plant pathology is self- 
evident, for the crops we eat are often attacked by 
disease, much of which modern science has learned to 



PATHOLOGY 77 

subdue. Still epidemics arise, and "rust," "smut," 
and " scab " are still known to the farmers. Potato 
rot and peach curl, spoiled fruit and wasted turnips are 
due to parasitic fungi. The pathological effect on the 
host plant varies with the kind of disease. In some 
eases its life is drained away with almost no outward 
sign, in others the presence of the fungus acts as an 
irritant, and abnormal swellings or discoloured lumps 
are produced by the stimulated tissue cells. These 
correspond to some extent to the tumours and swellings 
that occur in the tissues of animals. 

Such swellings are also produced by animals in the 
plant tissue. These are often harmless enough, and 
merely locally disfigure the leaf or branch without 
materially affecting the whole individual. Such are 
most " galls " which are formed by insects depositing 
their eggs in the plant tissue, whose larvae develop there 
and with their growth stimulate an abnormal develop- 
ment of the plant cells. These pathological tissues are 
often extremely interesting, and are developed with 
characteristic regularity according to the insect stimu- 
lating them. Zones of woody and fibrous coloured cells 
are developed pathologically in the soft tissues of leaves 
for instance, which would normally be incapable of 
forming any such cells. 

A third type of attacking organism with which the 
ordinary plant has to contend is the higher parasite of 
which the Dodder (Cuscuta) is a well-known illustration. 
This pest belongs to one of the highest orders of the 
flowering plants, but by reason of its parasitism it has 
degenerated to a mere colourless thread which sucks 
all its nourishment from its host. It does its own 
flowering, however, and produces seeds which shortly 
after their germination begin their course of aggression. 



78 BOTANY 

This parasite specially attacks clover, but heather, 
gorse, and other hard forms are not exempted from it. 
And while it does not produce pathological growths in 
its host, it simply sucks out its nourishment until it is 
destroyed and great patches of the host plant are killed. 

Hardly to be considered actual disease, there are 
still other abnormal phases of growth of which mention 
should be made here, and they are the growing together 
of series of stems, or several leaves and stems or other 
parts to form a broad irregular structure. This is called 
u fasciation," and the tendency to produce it seems to 
be inherited. The hypertrophy of some organs and 
numerous other irregular departures of growth may 
affect plants as well as animals. Many of these are of 
special interest to the morphologist, for these " sports " 
have sometimes given the clue to the explanations 
desired regarding the interpretation of normal structures. 

In every phase of this work, as in all other branches 
of modern science, large numbers of data have to be 
collected, tabulated, and correlated, and the resulting 
deductions tested by experiment. When the import- 
ance of agriculture and forestry are fully recognised, 
we may expect to see plant doctors and health inspectors 
augmenting the comparatively small number who to- 
day concern themselves with plant diseases. 



CHAPTER X 

SYSTEMATIC BOTANY 

IN the early days of the science nearly every botanist's 
energies were devoted to that branch of it which we 
now call systematic botany. This is very natural, for the 
first stage in the attack on a mass of unknown things 
is to arrange and name them for ready reference. Lin- 
naeus was the first to bring some order out of the chaos, 
and to give all plants known to him names on a uniform 
system. He instituted the present binominal nomen- 
clature, in which every species has a generic name 
(corresponding to a surname) and a specific name 
(corresponding to a baptismal name) in the form of an 
adjective, either in Latin or latinised modern language. 
In making the genera and arranging them in families 
attention is only paid to the floral organs, and plants 
are classified according to the number and position 
of the parts that make their flowers, cones, or spore- 
bearing organs. In a genus itself, however, the differ- 
ent species are of ten t distinguished by some vegetative 
characters, such as the hairiness or shape of the leaves 
or the habit of the stems. 

Species when named had to be described so that other 
workers should not give the same plant another name, 
and, as it has always been very difficult to describe in 
words the minute details of any object, these descrip- 
tions were found to be very much mor^ serviceable 
when accompanied with a drawing or figure of the 

79 



80 BOTANY 

described new form. Thus the descriptive floras were 
the most important part of the literature of the earlier 
botanists. These and the dried herbaria were, and are, 
to the botanist what the card index is to the librarian 
in a huge library. By now most of the species in the 
inhabited countries are known, but there still remain 
very many unrecorded species to reward any traveller 
and careful observer. There are named and described 
close on a quarter of a million of living species of plants 
altogether, including the lower and often nearly in- 
visible forms, and of this vast number about one hundred 
and thirty thousand belong to the highest group of all 
the Angiosperms. This fact acquires a further interest 
when we remember that this group has evolved in such 
comparatively recent geological times. 

Botany has often been classed with stamp collecting 
in the older days when the only object of many who 
went under the name of botanist was to collect and name 
all the plants of their district, and when the naming of 
a new species was the ultimate crown of success. It 
is true that there have been many such in the rank and 
file of the adherents of the science, but one of the re- 
markable things about the great systematic botanists 
of the old school is the insight they obtained into the 
relations of the innumerable species they described. 
They not merely labelled and arranged the chaos, they 
classified the genera into families and cohorts which 
indicate the scheme of evolution, if not in all its details, 
at least in its main outlines. 

The living plants may be divided into five main 
classes according to the complexity and structure of 
their reproductive organs. This is paralleled in the 
main by their vegetative structure, so that in general 
one can recognise a Seaweed, a Moss, a Bracken fern, 



SYSTEMATIC BOTANY 81 

a Pine tree, and a Rose as belonging to different grades ; 
and that, for instance, a Toadstool, a Liverwort, a Harts- 
tongue fern, a Yew tree, and a Lily form a similar series. 

These series of plants each represent the five principal 
groups into which systematists have divided the families. 
The scientific names of these groups are the Thallo- 
phyta, the Bryophyta, the Pteridophyta, the Gymno- 
sperms and the Angiosperms. In addition to these, 
there are one or two important kinds of plants which 
existed in past time, but which have since become ex- 
tinct. Of these the Pteridospermae, mentioned already 
in the chapter on Palaeontology, lie between the pterido- 
phytes and the gymnosperms. 

Each of the five main groups are divided into a number 
of divisions, sometimes called phyla, each of which is 
composed of several families. 

The Thallophyta have the largest number of species 
after the Angiosperms, and number about eighty 
thousand species all told. They are all comparatively 
simple in structure and have no differentiation into 
true leaves, stems, and roots, and have no woody or 
true vascular tissue. They have only spores and no 
seeds, but some of them have an alternation of genera- 
tions. In this case, in one generation reproduction is 
by simple spores, and in the second it is by means of 
a spore resulting from the fusion of two sexual cells. 
This is not at all regular, however, and in many cases 
it depends on the nutrition and other conditions, which 
method of reproduction results. A large number of 
the Thallophyta never produce other than the simplest 
spores. A great proportion of these forms are very 
small and simple and live in the protecting medium of 
water. Such are all the small green algae- of the ponds 
and streams, all the seaweeds, red, green, and brown, 

F 



82 BOTANY 

and a number of fungi. The Thallophyta include also 
the large fungi, the toadstools, and all the parasitic 
and disease-producing forms mentioned in the pre- 
ceding chapter. 

The Bryophyta form a much smaller group, reported 
to have about sixteen thousand species. Some of these 
appear, as do the mosses, to have true leaves, but their 
apparent leaves are not really homologous with those 
of the higher plants. They have some differentiation 
of conducting cells in the tissue, but no true wood or 
vessels. They have a definite alternation of genera- 
tions, but the spore-producing generation grows on to 
the a leafy " sexual generation, and is generally, but 
wrongly, called its "fruit capsule." To this group 
belong all the Mosses and Liverworts, and between them 
and the rest of the cohorts there is one of the greatest 
gaps in the whole plant world. We have no clue to 
the course of their evolution, and no definite idea as 
to their relation to the other groups. It is evident, 
however, from their structure that they are less highly 
organised than the succeeding group of the Pteridophyta. 
This group, which makes so much more general impres- 
sion on the landscape than does the preceding one, 
does not include so many as five thousand species altc 
gether. All its members have a well-marked differ- 
entiation into leaves and stems, some with large leaves 
like the Bracken fern and some with small leaves like 
the Club-moss. All are provided with well-differenti- 
ated wood and phloem, which are arranged in bundl % 
in the stem, but none of the living forms have those 
zones of secondarily formed wood which is character- 
istic of the present higher plants and of the fossil pterido- 
phytes. All the members, also, have a well-marked 
alternation of generations, but it differs from that of 



SYSTEMATIC BOTANY 88 

the bryophytes, for the leafy plant which is conspicuous 
is the spore-producing generation, while the sexual 
generation is a very small and inconspicuous little 
structure, as simple as an alga except for its sexual 
organs. To this cohort belong all the ferns, all the 
Equisetums or Horsetails, and the Club-mosses and 
Selaginellas. These three types of pteridophytcs are 
separated into different phyla, for they differ in a 
number of important respects, and their fossil repre- 
sentatives add some further families to the group, but 
they all agree in the essentials enumerated for the group 
as a whole. In modern plants we have again a great 
gap, and then come the Gymnosperms. This gap is 
bridged by the fossil Pteridosperms. The gymnosperms 
have all a well-marked differentiation into roots, stems, 
and leaves, and all have differentiated wood and phloem. 
Most of them grow to a considerable size, and have 
strong, woody trunks with zones of secondary wood. 
They all have complex fructifications with seeds, and in 
most cases these are borne on special leaves or branches, 
which often form a cone. The male cells are produced 
in pollen which is borne by small separate cones. To 
this group belong the Pine and Fir trees, the Yews, Cedars, 
Larches, and the Spruce, as well as the sub-tropical and 
comparatively rare Cycads. Of these there are not 
more than a total of about five hundred species, though 
in many districts, owing to their large size and their 
numbers in the forests, they appear to be the most 
important plants of the districts, as in the spruce forests 
of Canada or the pine belt of the continental mountains. 
The last and greatest group, the Angiosperms, with 
over a hundred and thirty thousand species, contains 
nearly all the plants that yield crops of economic im- 
portance to man, or that decorate his gardens, or that 



84 BOTANY 

feed his sheep or cattle. Nearly all have highly differ- 
entiated organs, with wood and vessels more differ- 
entiated than in the other groups. The majority of 
them have zones of secondary thickening, and all have 
the reproductive organs on special leaves, generally 
arranged together in " flowers," most of which are 
brightly coloured and ornamental. To many collectors 
tins group alone constitutes the " flora " of a district, 
and the number of families it comprises is in proportion 
to the huge number of species it includes. When this 
group is further examined, there are found to be two 
well-marked divisions of it called the Monocotyledons 
and the Dicotyledons. The first has embryos with only 
one cotyledon or " seed leaf," the second has embryoa 
with two. In the first group the leaves are generally 
long and narrow and have parallel veins, while the 
stems do not have secondary wood ; in the second 
group the veins are reticulate, and the ring of primary 
bundles augmented by secondary thickening. To the 
former belong the Grasses, Palms, Lilies, and Orchids, 
and to the latter all the leafy trees like the Oak, Beech, 
and Maple, the majority of crops such as the Cabbage, 
Peas, and Strawberries, and flow r ers such as the Rose, 
Daisy, and Clematis. The families in both the two 
groups are separated principally according to the numbers 
of the parts in the flowers, and the relative positions 
of these parts which, on the whole, seem to bring to- 
gether the species which are truly like each other. 
Speaking generally, one may say that there is a pre- 
ponderance of four or five, or multiples of these numbers, 
in the flower parts of the Dicotyledons, with an almost 
universal appearance of three or its multiples in the 
flower of the Monocotyledons. The details of the 
classification of the families will be found in any flora, 



SYSTEMATIC BOTANY 85 

where the species are all described and where keys 
are provided so that any unknown plant can be identified 
and named. 

With nearly a quarter of a million described forms to 
deal with the value of such keys will be recognised. Let 
us take an imaginary instance to illustrate the course 
of procedure with a new species. Let us imagine that 
in the English woods a plant very like a violet is found, 
but that, instead of the plain purple petal of the ordinary 
woodland species, it has a white fringed edge with red 
spots on its veins. Its flower would therefore resemble 
in some degree an orchid, and the finder would at once 
examine it to see whether it is a new violet or an orchid. 
We will imagine its leaves, however, to be similar to 
those of the ordinary violet except for a red streak 
down the main nerves. They would thus have net- 
work veins, which would at once separate the plant 
from the Monocotyledonous orchids. This, too, would 
be indicated by the five petals and the structure of the 
ovary. Let us imagine that the flower differs in no 
particular from the ordinary violet except in the points 
mentioned. Reference to an English flora would soon 
show that it is at any rate a new species for this country, 
but it may have been an " escape " from some garden 
to which it has been brought from some foreign country. 
The next thing to do is to look at the leading continental 
and American and other floras in the family of Violaceae 
for the different parts of the world. These can all be 
seen at the British Museum. If such a plant is not 
described in any of them, it still does not prove that 
it is an unknown and therefore a new species. New 
plants are described in such numbers that they are 
not all incorporated in the current floras, and it might 
well be that it had been published in the transactions 



86 BOTANY 

of some learned society, and riot yet reproduced in the 
published general floras. To discover this, application 
would have to be made to some specialist at Kew or 
the British Museum. If the plant is unknown to them 
it is almost certain to be really a new species. The 
discoverer is then at liberty, indeed it is his duty, to 
describe and publish figures of it, and with this original 
description it must be named. Now, as we saw at the 
beginning, this imaginary flower is so like the violets 
that it must not be put in the genus Viola. The species 
name should be selected to give some indication of the 
nature of the plant. The red- veined leaves and the red 
spots along the petal nerves are very characteristic, 
and so a good name would be rubrinervis. In the 
future the violet would be known as Viola rubrinervis 
Smith, after the Mr. Smith we can imagine having 
discovered and described this new flower. In giving the 
species a name one most important point must be 
observed, and that is that no other Viola from any part 
of the world has that same species name. The con- 
fusion this would cause is obvious, and so one of the 
strictest rules followed by all systematists is that no 
new plant shall have a name already appropriated by 
another in the same genus, and if, unwitting, an author 
gives such a name, it shall immediately be superseded 
and renamed. To assist botanists in this there is a 
monumental work called the Index Kewensis, in which 
all the specific names ever given to plants are recorded 
with all their synonyms. 

New species may merely swell the numbers of new 
forms known to systematists, or they may be import- 
ant clues in the incomplete scheme of evolution. Some- 
times in the latter sense some of the numerically smaller 
families are of the greatest interest. For instance, 



SYSTEMATIC BOTANY 87 

the plant known as Ginkgo Uloba has no fellow-species 
in its genus, but is a single species composing a genus, 
and that genus by itself composes a family, and there 
are good grounds for putting that family in a phylum 
by itself. Thus, one single species by itself can form a 
whole phylum of plants, while in other cases there 
may be a thousand species or more in a phylum. In 
such a case that single species is obviously of greater 
interest and importance than one of the thousand. In 
the case of the Ginkgo just mentioned the reproductive 
organs have some unusual features, of which the most 
striking are the motile sperms, which swim like in- 
fusoria in a drop of water and are found in none of the 
higher families of plants but Ginkgo and the Cycads, 
and are similar to those in the ferns. The genus is 
interesting also in being the only representative left 
alive of a once large and widespread group. To the 
philosophical systematist, therefore, all his species are 
not of the same value, but all must be registered with 
equal care. The correct registering of the known plants 
of the world is the first duty of systematists a know- 
ledge of their inter-relations and phylogeny the greatest 
result of their work. 



CHAPTER XI 

CONCLUSION 

WE have now surveyed, not in the details of fact but 
in the outline of fundamental principles, the field of 
modern botany. We see that it is no narrow and re- 
stricted subject, dry as the herbarium plants which 
used long ago to symbolise it. It is full of living interest, 
ramifying in many directions ; it comprises branches 
technically distinct and requiring considerable know- 
ledge and dexterity to pursue, all of which are com- 
bined and held together by the main philosophical 
principles that underlie the whole. 

The really essential study in modern botany may be 
summed up in the phrase that it attempts to discover 
how plants live and how they came to be alive. Each 
branch of the subject described in the preceding chapters 
bears on these two problems. The systematist de- 
scribes and arranges the plants now living, and, in con- 
junction with the palaeobotanist, those also of the past. 
When they are in order it is seen how they grade 
themselves, and the question arises whether this series, 
from simple to complex, represents the order in which 
they appeared on the earth, and whether the systematist 'a 
classification corresponds to a more or less complete 
genealogical tree. The palseobotanist partly answers 
this question in the affirmative, but at the same time 
still further amplifies it, and discovers new questions 

88 



CONCLUSION 89 

with the unknown forms which he unearths. On the 
other side of the systematist stands the experimentalist, 
with his hybrids, varieties, and mutations, and offers a 
warning against holding any species as an immutable 
thing. A reminder that all the binomially named 
species in our text-books and floras are established 
only in a relative sense, for, since man's history began, 
new forms have arisen and taken their place in the 
ranks of those which "breed true," and therefore should 
be considered true species. From these branches of 
botany we get, if not cut and dried ideas on evolution, 
at least suggestive and stimulating ones. The morph- 
ologist, anatomist, and physiologist are chiefly concerned 
with the question of how plants live to-day, and the 
manner in which their mechanisms are adapted to the 
conditions in which they find themselves, and the way 
the delicate machine is balanced and adjusted. These 
living individuals the ecologist sees in communities, 
with inter-relations between the different members and 
adaptations to their conditions of environment. The 
results from all these studies again reflects light on the 
problems of the palseobotanist, for the plants of the 
past were also individuals, breathing, assimilating, with 
organs differing only in details from those of modern 
plants ; and they also lived in communities. This works 
out like a sum in algebra with an unknown factor, for 
of the fossils there are only the anatomical and mor- 
phological features left, while of living plants these are 
available combined with experimental work on their 
physiological and ecological bearings. The relation 
between these being discovered in modern plants we 
can draw the conclusions about the conditions of the 
past communities. Here not many details have yet 
accumulated, but the work promises well, and it opens 



90 BOTANY 

the door to knowledge of past continents that have 
vanished with their floras. 

With the actual origin of plant life botanists would 
gladly deal had they any data. That is hid in the en- 
tirely impenetrable past however, and we return to the 
study of the present flora as it is represented in the 
simplest Thallophytic forms which still multitudinously 
inhabit the earth. It is probable that there we see 
the comparatively unchanged descendants from the 
simple forms which were among those which early in- 
habited the waters. Still, to-day there are some which 
have such a mixture of the characters of both plants 
and animals that it is almost impossible to say to which 
group they belong. Here we see, as we noticed in the 
cytological study of the most complex process, in the 
highest plants and animals an extraordinary unity 
between the two great branches of the tree of life. 



SUGGESTED COURSE OF READING 

TEXT BOOKS 

STOPES, M. G.The Study of Plant Life. 2nd ed. Blackio, 1910. 

A simply written general text-book of botany for beginners. 
SCOTT, D. H. An Introduction to Structural Botany. 
Part I." Flowering Plants." Black, 1909. 
Part II." Flowerless Plants." Black, 1907. 
A detailed account, including the internal structure, of a sample 
type from each of the important plant groups, suitable for 
those beginning the serious study of botany. 
STBASBURQER, E. Text-book of Botany. Translated from the 

German. Macmillan, 1908. 
A comprehensive text-book of university standard, 

GENERAL BOOKS 

BATESON, W. Method and Scope of Genetics. Inaugural lecture. 

Cambridge, 1908. 

A semi-popular lecture on the subject of plant-breeding, &c. 
BATESON, W. MendeVs Principles of Heredity. Cambridge Press, 1909. 
An advanced, well-illustrated book, dealing with heredity in 

both plants and animals. 

BOWER, F. 0. The Origin of a Land Flora. Macmillan & Co. 1908. 
An advanced book, nevertheless written in a popular way, well 

illustrated. 
CLEMENTS, W. Research Methods in Ecology. U.S.A., 1905. 

A treatise on ecology in which many new suggestions are made. 
CONNOLD, E. T. Plant Galls of Great Britain. Adlard, 1909. 
Profusely illustrated account of insect-caused deformities. 
TJie Encyclopaedia Britannica, articles on the various branches of 
botany. See first the article "Botany," in which reference is 
made to the others. Cambridge University Press. 1 1th ed. ,1911. 
KEENER, A. and OLIVER, F. W.The Natural History of Plants. 

Vols. i. and ii. Blackie, 1894. 

Still the beet and most delightful general account of plant 
biology. Well illustrated. 

91 



92 BOTANY 

MASSEE, G. Diseases of Cultivated Plants and Trees. Duckworth, 

1910. 
A well-illustrated, technical account of plant diseases. 

SACHS, J. VON. -History of Botany (1530-1860). English edition. 

Oxford Press, 1890. 

A very delightful book on the early history of botany. 
SCHIMPER, A. F. W. Plant Geography on a Physiological Basis. 

English translation. Oxford, Clarendon Press, 1903. 
A finely illustrated account of the biology, ecology, and distri- 
bution of plants. 

SCOTT, D. H. The. Evolution of Plants. Williams & Norgate, 1911. 
A popular account, primarily dealing with evidence from the 

fossils. 
SCOTT, D. H. Studies in Fossil Botany. Vol. i. 2nd ed. Black, 

1908. Vol. ii. 2nd ed. 1909. 
An advanced text-book, giving a detailed account of fossil 

plant anatomy. 
SEWAED, A. C. Links with the Past in the Plant World. Cambridge 

University Press, 1911. 
An essay on some plant families, principally gymnosperms and 

their ancestors. 
STOPES, M. C. Ancient Plants. Blackie, 1910. 

A simple general account of fossil plants. 
TANSLEY, A. O. British Vegetation. 

Types of British Vegetation. Cambridge University Press, 1911. Tho 
combined work of the English Ecologists, and the first attempt 
to present the native flora ecologically. Well illustrated. 
VINES, S. H. Lectures on the Physiology of Plants. Cambridge 

Press, 1886. 
A rather advanced text-book very pleasantly written. 

Da VRIES, H. Plant- Breeding, Comments on the Experiments of 

Nilsson and Burbank. 1907. 
A profusely illustrated book, simply -written. 

WARMING, E. Ecology of Plants. English translation. Oxford, 1909. 
Tho original exposition of the subject, presented in English in 
a very readable form. 



INDEX 



ADVENTITIOUS roots, 12 

Alga, 21, 30, 81 

Ampelopais, 16 

Anatomy, 23 

Angiosperms, 61, 65, 80, 83 

Annual rings, 26 

Araucaria, 62 

BACTERIA, nitrogen-obtaining, 

in root nodules, 43 ; causing 

disease in plants, 76 
Bast, 24 
Biology, 7 

Breathing of plants, 45 
Breeding of plants, 68 
Bryophyta, 81, 82 
Bulbs, 14 

Burbank, Luther, 70 
Butcher's Broom, 13 
CACTUS, 15, 52 
Catamite*! 63, 65 
Callus formation, 74 
Carbohydrates, in nutrition, 41 
Carbon assimilation, 42 
Carpel, 17, 19 . 
Casts, fossil, 60 
Cell, structure, 32 
Chlorophyll, 25, 42 
Chloroplasts, 42 
Chlorosis, 75 
Chromosomes, 35, 36 
Classification of plants, 79 
Club-moss, 64, 82, 83 
Coal measures, fossil plants of, 

63 

Communities of plants, 50 
Cordaites, 63 
Corolla, 17 
Creepers, 16 
Cross-breeding, 69 
Cuscuta, 21, 77 
Cycads, 62, 83, 87 
Cytology, 32 



DARWIN, Charles, 70 

Dicotyledons, 28, 30, 84 

Discidia, 17 

Diseases of plants, 74 

Dodder, 21, 77 

Drosera, 16 

ECOLOGY, 50 

Embryo, 19 

Epidermis, 24 

Equisetaceaa, 29 

Equisetum, 65, 83 

FASCIATION, 78 

Ferns, 20, 28 ; fossil, 65 

Ficus, 12 

Flower, 10, 17, 69, 84 

Fossil plants, 29, 58 

Fruit, 19 

Fungi, 21 ; causing disease in 

plants, 76 
GALLS, 77 
Geological systems, fossil plants 

in, 61 

Geotropism, 47 
Ginkgo biloba, 87 
Gravitation, influence on plants, 

47 

Growth of plants, 40 
Gymnosperms, 28, 65, 81, 83 
HEATH, 61 
Heliotropism, 46 
Heredity, 70 
Horse chestnut, 15 
Hybrids, 69 

Hypertrophy of plant organs, 78 
IDENTIFICATION of plants, 85 
Impressions, fossil, 60 
Index Kewensis, 86 
LAMINAKIAS, 30 
Larkspur, 18 

Leaf, 10, 13, 14, 27, 41, 44, 52, 84 
Leguminacese, root nodules in, 

43 



98 



94 



INDEX 



Lepidodendron, 63 

Light, influence on plants, 46 

Liverworts, 82 

Lycopodiaceae, 29, 64= 

Lycopodium, 64 

MANGROVE, 12 

Marsh, 51 

Mendel, 70 ; Mendel's law, 70 

Mineral salts, in nutrition, 42 

Monkey-puzzle, 62 

Monocotyledons, 11, 84 

Moor plants, 51 

Morphology, 10 

Mosses, 21," 30, 82 

Mutants, 73 

NITROGEN, in nutrition, 42 

Nucleolus, 35 

Nucleus, 32 ; mitosis of, 36 

Nutrition of plants, 40, 76 

Oenothera, 73 

Origin of plant life, 90 

Ovule, 18, 19 

PAL^OBOTANY, 58 

Palaeontology, 68 

Pathology, 74 

Parenchyma, 24, 33 

Peach curl, 77 

Petals, see flower 

Petrifactions, 60 

Phloem, 24 

Physical conditions, influence 

on plant growth, 52, 59 ; 

influence of plants on, 51 
Physiology, 40 
Pinguicula, 16 
Pitcher plant, 16 
Plant breeding, 68 
Poisoning of plants, 75 
Pollen, 17, 18, 69 
Polytrichum, 30 
Pond plants, 55 
Potato rot, 77 
Primrose, Evening, 73 
Prop (aerial) roots, 12 
Prothallus, 20 



Protoplasm, 32 

Psamma, 54 

Pteridophyta, 29, 30, 64, 65 ; 81, 82 

Pteridospermae, 30, 65, 66, 81, 83 

EAFFLESIA, 22 

Reproduction of plants, 18, 68 

Rhododendron, 18 

Root, 10, 11, 24 

Rubus, 68 

JRuscus, 13 

Rushes, 55 

SALT-MARSH plants, 63 

Sand plants, 54 

Scales, modified leaves, 15 

Sclerenchyma, 24 

Seashore plants, 54 

Sedges, 55 

Seed, 18, 19 

Selanginella, 64, 83 

Soil, influence on plant growth, 

52 

Sporangia, 10, 17 
Stamen, 18, 19 
Stem, 10, 11, 12; modifications 

of, 13 ; anatomy of, 24 
Stonecrop, 52 
Sundew, 16 
Swamp plants, 53 
Sweet-pea, 16 

Swellings,abnormal,inplants,77 
Systematic botany, 79 "N 
TEMPERATURE, influence on 

plants, 48 
Thallophyta, 81 
Thallus, 21 

Transpiration current, 45 
VASCULAR tissue, 24 
Viola, 86 

de Vries, Hugo, 72 
WATER, circulation in plants, 

44 ; modification of plants 

due to water supply, 52 ; 

water plants, 55 
Wood, 24, 44 
Woodland plants, 50, 54 
Wounds of plants, 74 



I/I 2 



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THE FIRST SIXTY VOLUMES 

The volumes now (February 1912) issued arc marked with 

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