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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
>&•
1
BIOLOGY
LIBRARY
G
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 wrhat 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 wras 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 wrork 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 flowrers 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
Printed by BALLANTYNE, HANSON &* Co.
Edinburgh &* London.
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SCIENCE
1. Introduction to Science . . .{** J^C. D. Whetham, M.A.,
2. Embryology— The Beginnings of Life By Prof. Gerald Leighton, M.D.
3- Biology-The Science of Life .
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6. Bacteriology . . -f B* W. E. Carnegie Dickson, M.D.,
l_ L>.oC.
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9. Darwin ....... {By?°z' W- Garstang, M.A., D.Sc.,
»io. Heredity ....... By J. A. S. Watson, B.Sc.
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Handbook ..... \ F.R.C.S.E.
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