MOVEMENT
While we have been examining plants to find out some of the facts about
their other life properties, we have at the same time seen many cases
of movement in their different parts.
For example, we found (Chapter IX.) how the tips of roots move round to
get back their vertical position if they are placed horizontally, and
how the shoots of young plants bend over towards the light when they
are grown in a dark box where it can enter only from one side (Chapter
VIII.). Then, too, as the root tip grows into the soil or between the
crevices of rocks it bends round the stones or other things in its way,
and it is also attracted towards water, thus showing a continual, slow
movement in its growth. The shoot shows a parallel kind of movement in
following the light and placing itself as advantageously as possible
with regard to it.
[Illustration: Fig. 31. Tendrils of the Pea; A young tendrils which
have not yet been touched; B beginning to curl fifteen minutes after
being rubbed with a twig.]
You may see a still faster movement if you carefully examine a twining
tendril. Notice how the young tendrils of a sweet-pea are at first
almost straight, growing out into the air (~see~ fig. 31). Now choose
such a one for the experiment, and another like it which you do not
touch, but keep to compare with the one on which you have experimented.
Gently rub one side of the tendril with a small rough twig, and then
leave it alone. You will see that in about five or ten minutes it
has begun to curve, and in a quarter of an hour may have bent round
completely. Such movement is more rapid than that in the ordinary
growth, and this power of bending so quickly is one of the special
characters of tendrils, and one that is very important in helping them
to do their work for the plant and to seize on any support within reach
as quickly as possible.
[Illustration: Fig. 32. Leaves of Wood-sorrel; A in the day position, B
“asleep” at night.]
Then there are other movements, one of which you must have often
observed in the “sleep” of plants. Many flowers and leaves close up and
bend down at night, taking up their usual position again next day. This
is not the same thing as the opening of buds, for it may occur again
and again in the fully grown parts of plants. For example, you may mark
certain leaves of wood-sorrel or common clover, and watch them close
up at night and re-open in the morning many times. These movements are
not very fast, and you cannot see the plant moving as you can see a
kitten waving its tail, but the difference is only one of degree.
[Illustration: Fig. 33. Leaf of Sensitive Plant in its usual position.]
[Illustration: Fig. 34. Leaf of Sensitive Plant, leaflets at ~a~
beginning to close after being gently touched.]
There are plants, however, which move so quickly that you can see them
close up their leaves at once at the slightest touch. This is the case
in the Sensitive Plant (fig. 33), and if you only tap one of its tiny
leaflets with a straw, that pair of leaflets will immediately fold up,
then the next pair, and the next, till the whole leaf has closed, when
it drops quickly down (~see~ fig. 35), this movement only taking a
moment. If the shock is great, all the leaves on the plant will close
up instantly, and they move so quickly that you can hardly see them
doing it.
[Illustration: Fig. 35. Leaf of Sensitive Plant quite closed, and the
leaf-stalk fallen, after being touched.]
Some foreign plants swing their leaflets round slowly like the arms
of a windmill, but we have not yet found out why they do this. Also
in many flowers we find movement, and in flowers it is generally in
relation to the insects which visit them. For example, some orchids
shut up their big front petal with a sudden snap when an insect alights
on it and shoot the astonished fly towards the middle of the flower.
Parts such as these, which have more power of movement than the rest
of the plant, are called sensitive parts, but though in them we see it
more clearly than in most plants, they only illustrate what is common
to all, and that is ~some power of movement~.
The movements which you have seen so far in plants are different from
those of most animals in one way, and that is in the fact that the
whole plant remains rooted in one place, and only parts of it can move
as the circumstances require, while, though an animal moves its parts
separately, the result of some of those movements is to carry its whole
body about. This may appear to you a great difference between plants
and animals, but it is not quite so great as it seems; nor must we
forget that there are some simple slimy-looking plants which slowly
crawl along the ground, as well as many minute, green plants, which you
could only see with a microscope, which move their whole bodies and
swim about just in the way that tiny animals swim.
SUMMARY OF PART I.
We have now done a number of experiments with plants, and found out
many facts about their way of life, and I think you will agree that
we have collected enough evidence to prove the statement made at the
beginning of Chapter II.--that on the whole plants show the same “signs
of life” as do animals.
We have seen that like animals they ~breathe in~ a part of the air, and
that they ~breathe out~ with the air the added carbonic acid gas, which
is the characteristic “waste product” of the out-breathing of animals.
They practically “~eat~” when they take in substances as food into
their bodies, even though they have no gaping mouths which can open and
close. We noticed, too, the interesting parallel between young plants
and young animals, where both (the plants in the food in the seed, the
animals in their mothers’ milk) are supplied with ready-made food at
first, and as they get older have to find what food they require for
themselves. As regards their feeding, the plants do more work than the
animals, for they manufacture the starchy food for themselves out of
simpler elements, while the animals require their starch to be ready
made.
Then the fact that plants ~grow~, increasing in size and forming new
structures, has been known to you ever since you were a baby yourself.
Although we noticed here an important difference between the kind of
growth in plants and animals, yet the growth itself is alike in the two
cases, for both plants and animals build up their living bodies out
of simpler substances which they take in as food and change till the
not-living food becomes part of themselves and is living.
~Movement~ is not nearly so great in plants as it is in animals, and
most plants are firmly fastened in the ground. Yet there are some
plants in which we can see very rapid movements of some of the parts,
while many simple little plants living in water can swim actively about
like animals. All plants show some form of movement, though it is
generally slow.
As a result, we find that all the signs of life we noted in animals,
~viz.~ ~breathing~, ~eating~, ~growing~, and ~moving~, are to be found
in plants, and we must look on them as being just as much alive as
animals. We can see that their mode of life and the work they do are
distinctly different from those of the animals, but they are no less
vital, and important for the world as a whole.
[Illustration: PLATE II.
A WHOLE PLANT, TO SHOW ALL THE PARTS
A POPPY]
PART II.
THE PARTS OF A PLANT’S BODY AND THEIR USES
CHAPTER XI.
ROOTS
If you have a garden of your own, or have even watched another person
gardening, you must have found out that it is not always an easy thing
to get rid of the weeds, and that when one tries to “pull them up by
the roots,” they often resist it very strongly indeed. If you have
never done this, try to pull up a large grass tuft or a hedge mustard,
or any fairly big common plant, and you will find that often when it
does not look very strong it may be extremely difficult to get it
completely out of the soil, and even when it comes out you may find
that you have not got it quite whole, for the finer branches of the
root will generally break off. Now this shows us one of the uses of its
roots to a plant; they keep it firmly in the soil, and prevent the wind
from blowing it away, and people or animals from overturning it too
easily.
To see the form of a complete root it is wise to choose a fairly small
plant, let us say a daisy, wallflower, candy-tuft, or young holly; then
loosen the earth all round it and pull it very gently from the soil.
Shake off the mud and then wash it clean and spread it out on a sheet
of white paper so that you can examine it properly. Notice that there
is a central chief root, with many side branches which have again finer
and finer branchlets (~see~ fig. 36). At the tip of the very finest
you should see a number of delicate hairs, the root hairs, but it is
very possible that you will have torn these off with the soil. To see
them best, look at some of your seedlings which have grown in moist
air, where they are very well developed (~see~ fig. 8). In any of
these plants you will notice that the main root seems to be a downward
continuation of the main stem, and that the side roots come off all
round it, just as was the case in your bean seedlings (~see~ figs. 36
and 7). Such a root is called a ~tap root~.
[Illustration: Fig. 36. Root of a young Holly: ~l~, level of soil;
~s~, stem; ~c~, chief root with many side branches and finely divided
rootlets.]
Now dig up a small grass plant and compare its root with these, and
you will see that there is no main root, but very many roots coming
off in a tuft from the base of the stem, just as was the case in your
corn seedlings (~see~ fig. 37). The difference between these roots and
tap roots is not of much importance as regards the actual work they do
but is one of difference in form; the finer branches in both are very
similar and have the same work to do.
[Illustration: Fig. 37. Grass plant, showing the many finely divided
roots.]
If you leave the plants you have pulled up lying in the air for an hour
or two, you will find that they will wither, the leaves becoming quite
limp and the whole plant drooping. Now place them with their roots only
in water, and you will soon find that they are beginning to revive.
They will revive fully and live a long time if their roots are kept in
water. This reminds us of the second very important use of its roots
to a plant, which we have already found out (~see~ Chapter IV.), and
shows us again that the roots absorb water and keep the whole plant
supplied with it. Of course you know that cut flowers can drink up
water with their stems, but that is only for a short time, and is not
quite natural. The special part of the rootlet, which does the actual
absorption, is the part near the tip which is covered with root hairs.
You have already seen these root hairs in the course of your work
(~see~ pp. 13 and 15).
There are then =two chief duties of roots=, ~to absorb water from the
soil for the whole plant, and to hold it firmly in the ground~. The
fine fibres of the root, which are so much divided and run in the soil,
serve both these purposes, as they expose a large area to contact with
the soil, and so can absorb much from it, as well as getting a good
hold of it.
As well as these two chief functions, =there are many other pieces of
work which roots may do=, and according to the special work they take
up, so they become modified and look different from usual roots.
[Illustration: Fig. 38. Tap root of Carrot, swollen with stored food.]
One thing they often do is to act as ~storehouses of food~. For
example, examine the root of a carrot. The part we commonly call the
carrot and which we eat, you will see is really the main axis of the
tap root, and has the little side roots attached in the usual way.
The unusual thickness of the main root is due to the large quantities
of food which it stores. Just in the same way radishes and many other
plants have their main roots very thick and packed with food, while
dahlias have their side roots thickened in a similar way (~see~ fig.
39). Such modified roots, which look quite different from ordinary
ones, are called ~Storage roots~, and if you examine many of them you
will find them packed with starch (~see~ p. 11 for iodine test).
[Illustration: Fig. 39. Dahlia, with storage roots packed with food.]
Although it is general for the roots to hold the plant firmly in the
ground, ~in some cases they grow out of the stem in the air~ and help
to hold it up against a tree or wall, or some support, as in the case
of ivy. If you pull off a branch of ivy which is climbing up a tree
you will find that all along the back of the stem there are tufts of
short thick rootlets which often come away holding a piece of the bark
of the supporting tree. These roots, you will see, do not come out in
the usual way from the main root or base of the stem, but come out all
along the stem itself (~see~ fig. 40). Such special roots are called
“Adventitious,” and they grow from the stem wherever they are needed.
[Illustration: Fig. 40. Adventitious roots growing out from the stem of
Ivy between the leaf stalks.]
~Adventitious roots~ may also come out from a wounded plant which has
had its true roots cut away. For example, take a piece of Forget-me-not
stem without any roots, and slit it at the base, and put it in a glass
of fresh water. After a week or so you should see little white roots
growing out from the stem into the water, and if you let them get
strong you may then plant the sprig and get a new forget-me-not plant
from it. In this and all “cuttings” adventitious roots growing out
from the stem do the usual work of roots. There are many other kinds
of adventitious roots, but we must only mention the orchids, some of
which have long tufts of roots which grow out irregularly from the stem
and hang in the air. These are special ~air-roots~, and grow on many
orchids, but also on some other plants which live attached to trees and
absorb the water out of the air instead of from the soil (see fig. 41).
[Illustration: Fig. 41. Tufts of air roots of an Orchid.]
[Illustration: Fig. 42. Supporting or stilt roots growing out from the
base of a small Palm in a pot.]
There are many other curious roots, particularly in plants which grow
in tropical countries, ~e.g.~, the ~stilt roots~ which come out from
the base of the stems of many palms and make tripod-like supports
(~see~ fig. 42), and others which grow from the high branches to the
ground like pillars, and prop up the heavy trunks. However, we do not
need to go so far to find very many different kinds of roots, and if
you examine carefully those of the plants growing in our woods and
lanes, you will find what a number of extra pieces of work they can
do, in addition to their two chief duties of drawing in water from the
soil, and holding the plant in its position in the earth.
CHAPTER XII.
STEMS
Examine the stem of a sunflower; it is tall and straight and grows
upright in the air, bearing leaves which stand out from it.
In a young holly, and many other plants, we find growing out from the
central stem smaller side branches which bear the leaves. As we have
found already (Chapter VI.), the leaves are the active parts of the
plant and do the food-building, so that the stem is chiefly useful as a
support, which keeps them in a good position as regards the light and
air. In general, we do not see much of the stem because it is largely
hidden by the covering of leaves, so that if you want to study stems
you should go to the woods in the winter when there are no leaves on
the trees, and you can see the form of the branches themselves.
In big trees, such as the oak and beech, the stems are very important,
and the chief stem or ~trunk~ becomes very thick as it gets old. It is
made of hard wood which is tough and strong, for such high trees have
to bear great strain from the winds, as well as the weight of all the
leaves. If you go into the woods when it is very windy, and watch the
thick wooden boughs swaying, boughs which you could not move, you will
see how much force the wind may sometimes have. The branches need all
their strength in the summer to support the curtain of leaves which
catches the wind. In a big tree we find the few chief branches thick
and strong, but there are many hundred smaller ones, some of them
dividing to quite delicate branchlets which bear the leaves, so that
the whole tree body is very much complicated (~see~ fig. 43).
[Illustration: Fig. 43. Much-branched stem of the Oak.]
Each kind of tree has a way of branching which is characteristic of its
species, so that even without leaves or flowers a woodman can tell what
a tree is. This one can learn by practice in the woods, but to begin
with it is rather difficult. Without going into detail, however, we may
notice great family differences, such as exist between a larch or a
Christmas-tree and an oak. In the first two there is one straight main
trunk, with side branches at very regular intervals (~see~ fig. 44),
and in the oak the main thick trunk soon bears several large branches
nearly equalling the main stem; these divide again and again in a
rather irregular fashion (~see~ fig. 43).
[Illustration: Fig. 44. The Larch, showing its strong central trunk and
more delicate side branches.]
In many of the smaller plants the stems are not strong enough to stand
up against the wind, and they simply lie along the ground or support
themselves by growing among other plants, such, for example, as the
common Stellaria, where the stem is very delicate indeed (~see~ fig.
45). Then again, if you pull up a large water-lily, you will notice
how soft and limp the long leaf-stalks are. They cannot support
themselves at all in the air, though they were upright in the water.
This is because the stalks get their support from the water which
allows them to float up, so that the plant does not build a strong
stem. You will find that plants are very economical in their use of
strengthening material, and never waste it where it is not wanted. If
you remember this, and then study all the stems you can, and note when
and where they are strengthened, you will find what good and economical
architects plants are.
[Illustration: Fig. 45. Delicate stem of the Stellaria, partly lying on
the ground.]
As well as supporting the leaves, the stems have another very
important duty, something like that of the roots. Just as the roots
absorb the water from the soil and carry it up, passing it on to the
stems, so the stems carry it on to the place where it is finally used,
that is, to the leaves. In both stems and roots there are channels or
“water-pipes” which carry water about, as well as other special “pipes”
which carry the manufactured food.
So that =the two chief duties of stems= are to act as supports for the
leaves and flowers, and to carry the food materials and water between
the roots and leaves.
Just as we found in the case of the roots, =there are many extra duties
which the stems may take over=, and as a result, we find great variety
in the appearance of stems. For example, in some plants the stem does
not grow up into the air at all, but ~creeps along just below the
surface of the ground~. This you may see if you dig up a Solomon’s Seal
or an iris, when you will find that the stem looks very like a thick
root running horizontally in the ground. That it is really a stem you
can tell from the fact that the leaves grow out from it, and you can
see the scars of old ones as well as the present leaves, and also some
little brown scaly leaves, and a large number of adventitious roots.
The stem is rather swollen with food materials which are stored up in
it, and it is not coloured green like many of those growing in the
air. Such a stem, creeping under the earth, and only sending its green
leaves into the air, has a special name, and is called a ~Rhizome~.
Many plants have such stems, particularly ferns, as you can see very
well if you dig up a bracken.
[Illustration: Fig. 46. Underground stem of the Solomon’s Seal, called
a Rhizome. It has many scaly leaves, ~s~ and a shoot A which will
come out into the air bearing green leaves. B is the scar left by the
similar shoot of last year. ~r~ are the adventitious roots which come
out all over the stem.]
[Illustration: Fig. 47. A Potato: ~s~, the stem attaching it to the
main stem; ~e~, scale-leaf; ~b~, bud in its axil; ~t~, tip of the
Potato with several buds, some of which are sprouting.]
~Some of the underground stems which store food are still more
modified~, so that it is very hard indeed to tell what they really
are. This is the case in the potato, which you would naturally think
at first is a swollen root, like those we saw in the dahlia (fig. 39).
That it is really a stem you can see by examining the “eyes” carefully.
The eyes (~see~ fig. 47) are buds with scale leaves round them, and
at the tip of the potato we can see several such buds together (fig.
47 ~t~). The whole potato is a very much swollen stem which is packed
with food and has all its other parts so reduced that it is difficult
to recognise them. Such special stems are called ~Tubers~.
Certain stems take on the work of leaves, and sometimes they are so
much modified for this that the plant has no true leaves at all. This
is what has happened in the case of a cactus. If you can get a cactus,
examine it carefully and you will see that the whole plant consists
of a thick mass of green tissue, which apparently is not divided into
stem and leaves. But the truth is that the whole of the thick mass of
tissue is the stem, and the little tufts of spines and hairs are really
reduced leaves. So that in the cactus the ~green stem does all the food
building work instead of the leaves~.
[Illustration: Fig. 48. Cactus plant, showing its fleshy stem, which is
green, and does the food building for the plant. The tufts of spines
and hairs represent the leaves.]
In some plants this is not so much marked, even though the stem does
some of the work for the leaves. In such cases the stem is generally
green and broad or winged and the leaves small, as in our common broom
and the whortleberry, where the leaves very soon drop off. Quite a
number of plants have stems which do this, and it is sometimes a great
advantage to the plant, for the big leaves are often very wasteful of
water, as you will see in Chapter XVIII.
In other cases we find that the side branches of stems may be modified
to protect the plant, and so take on the form of strong spines or
thorns, as in our blackthorn, where the sharp pointed ~spines~ are
~modified side shoots~.
There are many other pieces of work which stems may do; we must just
mention the climbing and twining stems, where the stem is delicate and
requires to be supported, which we are going to examine more carefully
in Chapter XIX.
Sometimes, instead of continuing to grow into the air, the stem may
bend over into the earth again, as often happens in big bushes of
bramble (~see~ fig. 49), and then from the tip of the stem a number
of adventitious roots (~see~ p. 56) grow out and hold it firmly in
the ground. If, then, this branch gets separated from the rest of the
plant, it can build a complete new individual.
[Illustration: Fig. 49. Leafy branch of Bramble which has bent into the
earth and given rise to a cluster of adventitious roots at the tip;
~l~, level of soil; P, point where the branch was cut from the parent
plant.]
In the case of the bramble notice how the leaves get smaller and
smaller towards the tip of this branch as it bends down to the earth,
and of course, they do not develop at all as true leaves under the soil
(~see~ fig. 49).
From these examples, and the many others you should be able to find
for yourselves, you see that stems may take on other duties beyond
their two chief ones, but that, however much they change their form
and appearance, we can always find out that they are really stems by
studying them with a little care.
CHAPTER XIII.
LEAVES
The late spring and summer are the best times to study leaves, for,
as you must have noticed, the woods begin to lose their green in the
autumn, and the leaves have fallen in the winter. This tells us that
the fresh greenness of the leaves (which you know is so important for
the plant) does not last very long, and when they are no longer green
the leaves are useless and drop away. As you know, =the chief work of
leaves is to build starchy food=, for which they require their green
colour.
[Illustration: Fig. 50. Simple leaf of the Cherry.]
When you go into the woods or gardens to study the leaves, first look
at single ones, collecting as many kinds as you can. Though their shape
varies very much, you will find that in almost all cases they are
green, expanded, and flat. Let us first examine a single simple leaf,
like that of a cherry. You will see that the expanded part (called
the ~leaf blade~ or ~lamina~) narrows down to a small stalk, which
connects the blade with the stem from which the leaf is growing; this
stalk is called the ~leaf stalk~ or ~petiole~. Then at the base of it,
just where it joins on to the stem, there are two little leaf-like
structures which are not true leaves, but which belong to the leaf and
are called ~stipules~; they are attached to the base of the petiole,
which spreads out to clasp the stem, and is called the ~leaf base~
(~see~ fig. 50). Such a leaf shows us all the parts of a ~simple~ leaf;
but some leaves have no stalks, others no stipules, and so on.
[Illustration: Fig. 51. Compound leaf of the Rose.]
Let us compare a rose leaf with the simple leaf of a cherry, oak, or
beech. In the rose you will find five or seven small leaflets arranged
on a single main stalk, and each of these leaflets separately is very
much like a single leaf of the beech. Such a leaf as this we call
~compound~, for it is divided up into several parts, each of which
looks like a whole leaf (~see~ fig. 51).
Leaves are of very many different kinds and shapes, and special names
have been given to each kind, which you can look up in a book if you
want to classify them.
[Illustration: Fig. 52. Peltate leaves of Nasturtium, showing the stalk
attached in the middle of the lamina.]
Let us just notice a few of the types. The cherry, beech, and others
which are simple with slightly pointed ends, we may call by the proper
term ~ovate~. Then there are leaves like those of the nasturtium, where
the leaf blade is circular and the leaf stalk does not come in at the
base of the leaf, but is attached to the middle of it; such leaves as
that are called ~peltate~.
The broad or rounded leaves, which spread out like the palm of a hand,
such as the ivy (~see~ fig. 26), are called cordate or lobed, and when
compound, as are those of the horse chestnut, ~palmate~.
[Illustration: Fig. 53. Needle leaves of Pine growing in pairs.]
All the grasses and the many plants belonging to their family have very
long, narrow leaves, which we call ~linear~, while those of the pine
trees are sharp and pointed, and are called ~needle~ leaves.
[Illustration: Fig. 54. Seedling of Rose; (~c~) cotyledons; (~a~) next
leaf, simple, but toothed; (~b~) next older leaf divided into three
leaflets.]
As we noticed in comparing the leaves of the rose and cherry, some
plants have very much more complicated leaves than others. Now such
complicated structures do not develop on a plant all at once, as you
can see if you examine a very young rose seedling. The first pair of
leaves or ~cotyledons~ do not remain inside the seed as they do in the
bean, but grow outside into the air and become green; they are quite
simple leaves with smooth edges. The next leaf which unfolds is also
simple, but it has a deeply toothed edge (~see~ fig. 54), while the
leaf following that is a compound leaf, divided into three leaflets.
The other leaves gradually get five and then seven leaflets as the
seedling grows up.
This is just one example of what usually happens in the history of
plants with compound leaves, or leaves with any special shape; the
young seedling’s earlier leaves are much more simple than the later
ones. You should collect as many seedlings as possible and make
drawings of them if you can, to show the various stages leaves pass
through before reaching the full-grown complex form.
[Illustration: Fig. 55. Skeleton of a leaf, showing the fine network of
the small veins.]
Now let us look again at the actual structure of leaves. Hold up those
of the rose, or lilac, or lime tree to the light, and look at the
“veins” running in them. There is a chief central vein or mid-rib, and
from it a number of side branches come off and divide and branch again
and again till they form a fine net-work throughout the whole of the
leaf blade (~see~ fig. 55). If you now look at a grass or lily leaf,
you will find that there are very many veins about equally important,
running from end to end of the leaf and remaining nearly parallel to
each other. This difference between ~parallel~ veins and ~net-work~ (or
~reticulate~) veins is quite important, and is one of the characters
which help to separate two very big families of flowering plants (~see~
Chapter XXIII).
[Illustration: Fig. 56. Alternating pairs of leaves of the Dead Nettle.]
[Illustration: Fig. 57. Pair of Honeysuckle leaves with no leaf stalks.]
Now let us see in what way the leaves are arranged on the stem. If you
pick a branch of dead nettle you will see that the leaves are attached
by their stalks to the stem in pairs, two leaves coming off from the
same level at opposite sides of the stem (fig. 56); while fig. 57 shows
that the leaves of honey-suckle really do the same thing, only they
grow out directly from the stem as they have no leaf stalk. Now look
once more at the leaves of the dead nettle, choose one particular pair
to start with, and then look how the pair above it are placed. You
will see that they do not lie directly above the pair you chose, but
are arranged on the opposite sides of the stem, so that the two pairs
alternate. If then you look at the pair next above them, you will see
that they are arranged in just the same way as the first pair, and so
alternate with the second. In this way ~every pair of leaves~ on the
stem ~alternates~ with the pair above and below it. Now examine a pear
or cherry twig, and you will see that the leaves are arranged singly
on the stem. Fasten a piece of thread to the stalk of one leaf and
twist it round the base of the next, then on to the next above and so
on. You will find that the thread makes a spiral round the stem, and
finally comes to a leaf higher up it, which lies exactly above the
one you started from. Very many plants have their leaves arranged like
this in a ~spiral~ on the stem with the youngest at the top. There
are different kinds of spirals for the arrangement of leaves in the
different plants. You can see this by making the spiral of thread and
counting how many leaves you pass on your way up the stem till you
reach the leaf which lies just immediately above the one you start from.
[Illustration: Fig. 58. Branch of Cherry (leaves cut off to make
it clearer), with a string twisted from leaf stalk to leaf stalk,
showing the spiral arrangement. Note that leaf 5 is the first to come
immediately above the one you started from.]
[Illustration: Fig. 59. Leaves arranged in a whorl in the Horsetail.]
Sometimes the leaves are arranged in a circle all round the stem at the
same level; this is the case in the horsetail (~see~ fig. 59), and such
an arrangement is called a ~whorl~, but it is not very common in plants.
In the goose grass the leaves look very much as though they were really
in a whorl (~see~ fig. 60), but there are only two true leaves; the
others are the stipules, which are so much like the leaves that it is
very difficult to tell them apart.
[Illustration: Fig. 60. Leaves of Goose grass looking like a whorl.]
As we found out already, leaves require light and air, and usually
arrange themselves so as to get them; hence, in a general way, we may
observe that the leaves all grow to face the light. If you go under a
beech tree, for example, and look up, you will find that you can see
nearly all the big branches on the inside, while the leaves form a
covering or dome on the outside. Special cases of leaves so arranged as
to get a good light we noticed before (~see~ pp. 36 and 37).
=As well as their own particular work, leaves may take on extra
and different work=, so becoming modified to suit their different
occupations, and unlike true leaves. We already noticed in the cactus
(~see~ fig. 48) that the ~leaves become like sharp spines~ which
protect the fleshy stem, and can do none of the usual work of leaves,
because they have lost their green colour.
[Illustration: Fig. 61. Leaf of Pea, showing leaflets modified as
tendrils (~t~); expanded leaflets (~o~).]
In some plants leaves, or parts of ~leaves, may change into fine
tendrils~ which become very sensitive to touch, and can twine round
supports and cling to them, and so help the plant to climb. Such
tendrils we saw (fig. 31) move very quickly; they are quite different
in their structure from ordinary leaves. This happens in many plants,
and you may see it very well in the sweet pea (~see~ fig. 61), where
only two leaflets of the compound leaf remain leaf-like, the others
having been changed into tendrils.
When we come to look at ~Flowers~, with all their special shapes and
brightly coloured parts, we are really looking at ~modified leaves~.
But they are so very much modified that we have come to consider
flowers as things by themselves, and so we will study them later on.
Some plants which do not have true flowers, yet have leaves of two
kinds. For example, the “flowering fern” has the usual green leaves
and others which form rather brownish golden spikes, and which are
covered with spore[6] cases. Then again, some leaves are very specially
modified and are changed from the usual structure in order to act as
traps for insects (~see~ Chap. XXI.).
[6] ~Spores~ are simple little structures which do much of the work of
seeds. ~See~ the Chapter on Ferns.
~Other leaves~, instead of being very much developed, or specially
developed along some line, ~are simply reduced~, that is, are very
little developed indeed. For example, as you saw in the under-ground
part of the potato and many rhizomes growing horizontally, the leaves
never become large and green, but remain as simple brown scales. Some
~scale leaves~ have quite a special work to do in the way of protecting
the very young green leaves while they are in the buds, and we will
look at these carefully in the next chapter.
We have now seen that leaves, like all the other parts of the plant,
can modify themselves in a very great number of ways, and may do
many extra pieces of work above and beyond their chief work of food
manufacture.
CHAPTER XIV.
BUDS
The proper time to study buds in nature is the spring, but then you
will have to wait long to see all the different stages of their slow
unfolding. But they can be made to open artificially, and it is really
wise to study buds in winter, when there are not so many other things
to do. You can arrange this very well if in the late autumn you cut off
fairly big branches with buds on them (horse chestnut is particularly
good for this) and keep them in a warm room. You must, of course, keep
the cut ends of their stalks in water, which you should change every
three or four days, sometimes cutting off a piece from the ends of the
branches so that they have a fresh surface exposed to the water. In
this way they should live for months, and may just begin to unfold and
show fresh young green leaves about Christmas time, when the buds on
the trees out in the cold are still tightly packed up.
[Illustration: Fig. 62. Buds of the Horse Chestnut beginning to unfold.]
Watch the buds as they unfold, and you will find that round each bud
are several dry brown scales; these drop off, and within them are more
green, leaf-like scales enclosing the true young leaves, which are
still curled up and very small when they first come out.
If you examine a big bud which has not yet begun to unfold, and
carefully pull off all the parts separately with the help of a needle
and knife, you will see how the outer scales fold over one another like
a coat of mail, and where they are exposed to the outside air they
are hard and shiny, and in many plants are covered over by a sticky
waterproof substance like tarpaulin. These outer scales keep off the
rain and snow, and keep the inner parts dry and unharmed. Within them
the scales are softer and often quite green, and they, too, wrap round
each other, so that there is no crack left which could allow the cold
rains to enter to the little leaves within. In many cases also the
young leaves are wrapped up in soft, long hairs which look almost like
cotton wool. These hairs grow on the leaves themselves, and you can
see them after they have opened out, but as the leaves are then much
bigger, the hairs are scattered further apart and do not show so much.
[Illustration: Fig. 63. Bud of the Horse Chestnut, showing the
overlapping of the scales.]
If you cut right through the length of a bud with a sharp knife, you
will see how all these scales and young leaves are packed together, as
in fig. 64.
[Illustration: Fig. 64. Bud cut through lengthways, showing the
bud-scales and young leaves packed within them.]
Take another bud and carefully pull off all the scales one by one and
lay them in a row, beginning with those right outside; you will see
that they get less scale-like and more like real leaves as you go in
towards the centre of the bud (~see~ fig. 65). The outside simple brown
scales scarcely look like leaves at all, but the inner ones are green
and soft, and in some plants, those right inside have quite a leaf-like
appearance.
[Illustration: Fig. 65. A series of bud scales from a Horse Chestnut;
(~a~) and (~b~) are entirely hard and brown; (~c~) and (~d~) are brown
at the tips and green at the base, where the others cover them; (~e~)
is quite green, soft, and leaf-like.]
This helps us to see that ~bud scales are really only modified leaves,
which are altered for their special work of protection~ of the young
leaves through the winter.
Of course, you know that the buds are already on the trees in the late
autumn after the leaves have fallen; but have you seen the buds already
there in the summer while the leaves are still fresh and green? If
you look for buds you will be sure to find them, and at the same time
you will learn where they grow on the stem. You must look right at
the base of the leaf stalk, in the angle made by the leaf stalk where
it joins the main stem; this is called the ~axil~ of the leaf, and it
is in the axil of the leaf that you will find the small green buds in
summer-time. These buds grow out in the following year, so that a new
leaf comes in very nearly the same place as the old one, or, what is
more usual, there grows out a new branch which may bear several new
leaves. Now examine a twig of horse chestnut or sycamore from which
the leaves have dropped; notice that, where the ~buds~ are to be seen
on the stem, they ~lie immediately above scars~ of a definite shape,
~which are the scars left by the fallen leaf stalks~, as you can see by
comparing them in the autumn with leaf stalks which are just falling
away (~see~ fig. 66, ~l~, ~b~, and ~s~).
On the stem there are other scars, which are different from the
ordinary leaf scars, and which are like bands of fine lines round the
stem. What are these? Now if the single big leaf stalk leaves its
scar so clearly on the stem, what kind of scar would a number of thin
scales lying close together be likely to leave? Will it not be a number
of narrow scars in a band, just such a scar as we have here (fig. 66,
~a~, ~a~^1, and ~a~^2). If you mark a bud on a tree or one of the
branches in your room and watch it unfold, and keep a note of it till
the autumn, you will find at its base where the bud scales were, that
there is then a scar just like this. Whenever you see such a scar you
will know that it has been left by a bud. Now you know that, as a rule,
trees have buds only once a year, so that each of the bud scars along
the stem must represent a past year’s bud, and if you count these scars
along the length of the stem it will tell you the number of years the
stem has been growing. For example, in fig. 66 the twig shows us five
years’ growth if you count the last bud which will grow out to form a
shoot.
[Illustration: Fig. 66. Branch of Sycamore, showing leaf stalk (~l~)
with bud (~b~) in its axil. Scars of leaf stalks (~s~) and large
terminal bud (~t~) with scars (~a~), (~a~^1), and (~a~^2) left by the
terminal buds of past years.]
The buds which come in the axils of the leaves along the stem may form
new leaves, or may develop into side shoots with new stems and leaves.
There is another bud, generally bigger than these, which grows at the
end of the shoot (~t~, fig. 66). This has just the same structure as
the others, but it will certainly grow out to form a stem and carry on
the line of growth of the main shoot, unless it is injured.
The amount that the shoot grows in one year depends on very many
things, on the light and warmth it gets, on its food and the growth
of its neighbours. Hence, in the growth of different shoots in the
same year, or the same shoot in different years, we find very great
differences. Sometimes a number of bud scars lie very close together,
showing that for several years it had only grown a small amount, while
in the years following it may have added very much to its length. In
some plants there are little side shoots which never grow much, and
always remain quite short; for example, in the larch each tuft of
leaves grows on a little stunted stem which represents several years’
growth, and which never reaches any length (~see~ fig. 67).
[Illustration: Fig. 67. Larch, with tufts of leaves growing from short
side shoots.]
Not only do we get leaves and stems packed away in buds, but the
flowers for next year are there also. For example, examine several of
the big horse chestnut buds from the outer branches of the tree, and
you will be sure to find tiny sprays of young flowers packed away in
the hearts of some of them.
There are ~some quite special buds~ which we must notice, and which
at first sight appear very different from real buds. They have been
given a different name, and are called ~Bulbs~. Cut right through a
tulip or hyacinth bulb lengthways, and compare it with a horse chestnut
bud to which you have done the same. The arrangement of the parts of
the two things seems to be very similar. If you examine the bulb in
detail, you will find that it is protected on the outside by brown,
hard scales, and that the softer leaves within are folded over each
other very much like those in the true buds. Now the bud of the horse
chestnut is attached to the parent stem--is there nothing corresponding
to the stem in the tulip bulb? Look carefully at the base, and you will
see a little mass of tissue which holds the scales together (~see~ S,
fig. 68); this is the stem, which is short and very much reduced, being
unlike a usual stem. There is also one great difference between the
scales in the bud and the bulb. In the bud they are rather thin and
dry, but in the bulb they are thick and white and very fleshy, and if
you test them with iodine, you will find that they contain much starchy
food. They form the storehouse of the tulip, and this food will be used
by the plant when it begins to grow. In the axils of these thick fleshy
leaves you may often find small buds, which will get large and fleshy
by next year and form the new bulbs (~see~ fig. 68 ~b~).
[Illustration: Fig. 68. Bulb cut through, showing the overlapping
scales (~s~) packed with food attached to the shortened stem S. B is
the bud, which will grow out into the air, and (~b~) the bud which will
form a new bulb next year; (~r~) adventitious roots growing from the
base of the stem.]
Sometimes little bulb-like structures grow in the axils of ordinary
leaves, for example, in the tiger lily; these drop off when they are
ripe, and can grow into whole new plants. They are really half-way
between bulbs and true buds.
CHAPTER XV.
FLOWERS
If you have ever noticed a pea-flower fading, you will have seen that
from its heart there grows a little green pea pod which ripens till
there are full-grown peas in it (~see~ fig. 80); and a yellow dandelion
flower turns in the end to a white puff ball which scatters a hundred
floating fruits. In fact, almost all flowers which have not been
spoiled by the gardeners and “over-cultivated” leave in their place
when they die fruits and seeds in some form or other. ~This gives us
the key to the secret of the structure of the flowers themselves.~
=They are the forerunners of the fruits=, ~containing living seed,
and their structure and all their parts are adapted in some way to
help in the formation of fruit~. Now let us examine the flowers, never
forgetting that fact.
[Illustration: Fig. 69. Flower of Harebell; (~s~) sepals; (~p~) petals.]
Let us choose, for example, a harebell. On the outside we find five
separate green parts, and if we examine a bud which has not yet opened
we shall find that these fold quite tightly over the inner portions
of the flower and protect them, as they do in the rose and in almost
all flowers (~see~ fig. 69). In this they correspond to some extent
to the bud scales, and their special work is that of ~protection~.
In all harebells and roses there are five of these parts, but in the
wallflower you will find only four, and in poppies two, and so on.
There are different numbers of them in the different kinds of flowers.
They are also of different shapes and sizes; sometimes each of the
five parts is free, and sometimes they are all joined up together to
form a true cup, as in the primrose (~see~ fig. 72). These outer green
protective parts have a special name, and are called the ~calyx~ or
cup, while each of the separate parts which makes the cup is called a
~sepal~.
[Illustration: Fig. 70. Buds of the Rose; A with the calyx covering the
inner parts, B with the petals opening out.]
[Illustration: Fig. 71. Flower of the Rose, with separate petals.]
Directly within the calyx we come to the parts which are generally
bright and prettily coloured, and which give the chief beauty to the
flower. In the harebell which we are examining these parts are joined
up to form a bell, but in the rose they are each separate (~see~
fig. 71). In both harebell and rose we find five of these parts, and
the same number in the primrose, where you will find that they are
joined up at the base to form a long, narrow tube, and then spread out
separately like those of a rose. Both in the harebell and primrose,
where they are joined up, we can tell the number of parts which go to
make the whole bell or tube (and this is nearly always the case in bell
flowers), while, of course, where the parts are free it is quite easy
to count them, and we find that for each kind of flower their number
is always fixed. For example, we find five in the harebell, rose,
primrose, and many others, four in the poppy, wallflower, cress, and
so on. These parts are called the ~petals~, and in almost all flowers
you will find that they are bright and pretty, and stand out from the
surrounding green leaves, so that they are easily seen. When the cups
or bells hang down they may protect the parts within from the rain, but
that is not generally their chief work. The ~first duty of the petals
is to be attractive~. You will understand better why this is so after
we have gone further into the flower.
[Illustration: Fig. 72. Primrose flower cut open, showing the stamens
(~s~) attached to the tube formed by the petals.]
[Illustration: Fig. 73. Flower of Speedwell, with only two stamens
(~s~).]
Within the petals, and, in most cases, lying at the base of the bell,
you find several yellowish dusty sacs, on fine thread-like stalks. In
most flowers they are all free from each other and from the petals,
but in the primrose they are fastened to the tube of the petals (~see~
fig. 72). In some flowers you will find a great many of these, as you
do in the wild rose (~see~ fig. 71) and the poppy, where there are so
many that you can hardly count them. In other flowers there are very
few; for example, there are only two in the blue speedwell (~see~ fig.
73). In most flowers the single ~stamens~, as they are called, are very
much alike in their structure, and they all have the same work to do.
Look at these structures in a tulip or lily, where they are very big,
and carefully pull one off and examine it (fig. 74). You will find
that it consists of a stalk which we call the ~filament~, with two
long sacs at the tip which hold the yellow dust, and which we call the
~anthers~. If you examine a fully blown flower of the tulip or lily,
you will find that the sacs split open right down their length and let
out a fine yellow powder. ~This powder is the important thing about the
stamens, and is called the~ =pollen=. In all stamens you will find the
anthers or pollen sacs, while the stalk, which is less important, is
not always developed. Sometimes the sacs split right open like those in
the lilies, but there are other ways of opening; as for instance, in
the rhododendron you will see a little round hole at the tip of each
anther, which lets the pollen shake out like pepper out of a pepper-pot.
[Illustration: Fig. 74. Single stamen from Tulip flower; A, anthers, or
pollen sacs; F, filament, or stalk of stamen.]
[Illustration: Fig. 75. Flower of Tulip laid open, showing the
three-cornered central green box containing the young seeds.]
~Now we have come to the heart of the flower, and find there the most
important thing in it.~ Examine a sweet pea, for example, and you
will find in the middle of the flower a tiny green structure very
like a pea pod, with a little sticky knob at the tip. In the heart of
a tulip you will find a long green box with a sticky, three-cornered
knob at the top (fig. 75), while in a buttercup there are a number of
these structures instead of one (~see~ fig. 76 ~s~), each of which
has very much the appearance of a little pea-pod. Open the pea-pod,
or the box of the tulip, and you will find within it a number of very
small balls of a clear green colour. These are the young structures
which will become seeds when they are older, and they are the most
important things in the flower. The green box which protects them is
called the ~carpel~ in the case of the pea-pod, where there is one
space in it. In the tulip you will find that the box is divided into
three compartments, and each of these is called a carpel (~see~ fig.
77). You may think of the tulip carpels as being the same thing as
three pea-pods joined very tightly together. Some flowers have only one
carpel; others have three or five joined up like those of the tulip,
while others like those of the buttercup have a very large number of
single separate carpels.
[Illustration: Fig. 76. Buttercup flower laid open, showing that
there are many seed-boxes (~s~) in the centre of the flower. R, the
receptacle is the swollen end of the flower stalk.]
[Illustration: Fig. 77. Carpels of the Tulip cut open to show that
there are three spaces with seeds, each division representing a single
carpel.]
In the pea, tulip, buttercup, and many others, the carpels are in the
centre of the flower, above the petals, and attached to the swollen end
of the flower stalk, which is called the ~receptacle~, as in fig. 76 R.
Other flowers have the receptacle hollowed out like a cup or goblet,
and the carpels sunk right in it. When this is the case, we generally
find that the sepals and petals lie above the carpels and not below
them, as in fig. 78. This is also the case in the rose, where in fig.
70 flower B shows clearly the swollen part below the bud, which is the
hollowed receptacle containing the carpels, and the same is true of
the harebell (~see~ fig. 69) and many flowers.
[Illustration: Fig. 78. Flower of Cherry cut open to show the hollowed
receptacle, R, below the level of the petals, and containing the
carpel, C.]
What can be the use of the sticky tip that we found on the carpels?
Examine the tip of the carpel of a lily which is well open, and you
will very likely find some of the yellow pollen sticking to its
surface. It is a curious fact that the little structures within the
carpels which will become seeds cannot ripen into true seeds unless
they are waked up to growth by the pollen grains. ~The sticky tip of
the carpels~ (or ~stigma~, as it is called) ~catches the pollen grains
and holds them; then they grow down into the carpels and carry with
them the nuclei~ (see p. 92) ~that enter the undeveloped seeds. These
stir the cells to divide, and after many divisions the embryos are
formed and the seeds ripen.~ Sometimes the stigma has a long stalk
which places it in a good position to catch the pollen grains. This
stalk is called the ~style~, and is to be found in many flowers (~see~
fig. 72).
The pollen dust is fine and light, and may be carried by the wind on to
the stigma, as it sometimes is in poppies, and always is in pine-trees;
but this is rather a wasteful way, because the wind blows so
irregularly that very much pollen is lost and never reaches the stigma.
In order to save some of this loss, and ~to make the pollinating more
certain~, flowers have arranged their parts so as to make use of the
help of insects. You know that very many flowers have sweet honey in
them which the bees like, and come to collect, going from flower to
flower to do so. When the bee settles on the flower it gets covered
with the pollen dust, and then when it goes to the next flower and
walks over it, it is almost sure to leave behind it some of the pollen
sticking on the stigma. Of course, in this way also some pollen is
lost, but insects are far more reliable than the wind. We now see the
use of the bright coloured petals; they help to attract the bees to the
flower. The flowers have made the bees and other insects their special
carriers of pollen, and they pay the insect with honey, and some of
the surplus pollen. Bees generally go from flower to flower of the same
kind on any one day’s journey, so that the flowers get pollen from
others of their own kind. This is important, for “foreign pollen” (as
the pollen from quite different kinds of flowers is called) does not
help the young seeds at all.
We have now found a use for all the parts of the flower.
[Illustration: Fig. 79. A, Violet, a two-sides flower. B, Primrose, a
circular flower.]
There are many special things about flowers which we must leave till
later on, but we may just notice now how some are regular, like the
primrose, rose, poppy, and so on, which are after the pattern of a
circle, and appear the same from whichever side you look. Others, like
the violet, larkspur, or sweet pea, are not regular, but have only
two sides alike. This difference is very often due to some special
structure of the flower in relation to the insects which visit it, and
if you examine and compare the two-sided with the circular flowers
you will generally find that the two-sided flowers are the more
complicated. Some of them become very complicated indeed, like the
orchids, which have such strange flowers, and in which the relation
between the insects and the flower has become very special.
We must leave these more complicated cases till Chapter XXII., and come
back to ~the simple important facts about the work which all flowers
have to do~. =They must make sure that in some way or other, seeds are
formed for the plant.= If the flower does not do this, then it is not
doing the work for which it was made.
You will find a number of flowers in gardens which do not do their work
properly, and very often have no seeds at all, but they are specially
cultivated by gardeners to do other things. For the study of the true
structures of flowers, it is generally better to examine wild flowers
instead of garden ones, which are often much altered by the rather
unnatural conditions in which they are made to live.
CHAPTER XVI.
FRUITS AND SEEDS
[Illustration: Fig. 80. A, Pea-flower. B, the same beginning to fade,
with the ripening carpel breaking through the keel. C, the same carpel
much enlarged, the petals and stamens quite faded.]
Within the flowers we saw, protected and shut in, the carpels or
seed-box, within which are the very young structures which will become
seeds. Now let us watch them develop. In such flowers as the sweet
pea, for example, in summer-time, this will not take very long. Mark a
special flower, and watch it each day; you will find the little green
pod will gradually grow bigger, till it splits away the petals which
are beginning to wither, and pushes out between them. As the pod gets
larger you can see the seeds within growing too, if you look at the
pod carefully against the light. The stigma does not grow any further,
as its work was finished when it had caught the pollen grains. After
a time the petals and stamens drop right away, and only the calyx
remains; it does not grow very much, but it keeps fresh and green for
some time, as it has still to act as a cup to hold the pod. It only
takes a few days for these things to happen, then till the pea pod is
quite ripe may take a week or two more. The pod continues to grow and
turns yellowish brown and dry, then one day when the sun is warm you
may see and hear it split open suddenly down its central ridge, and
shoot out the brown, dry seeds. Then the work of the flower is quite
over, and the seeds have started to make their own way in the world.
[Illustration: Fig. 81. A, ripe carpel of pea or “pea pod.” B, pod
suddenly split and twisted up, scattering the seeds.]
Let us pick a nearly ripe pea-pod and examine it; it is the ripe
carpel, with several ripe seeds in it, and together they form what is
called a ~fruit~. In the case of the pod the “fruit” itself is a dry
pea-pod husk, but in other plants the fruits may be very different.
Examine a marrow, for example. Watch it in the course of its growth,
if possible, and you will find that the marrow flower is one of those
with its seed-box below and outside the calyx and petals. As the marrow
ripens this swells with the food stored in it, and the many growing
seeds, till the flowers are only small shrivelled structures at one
end. If you then cut the ripe, or nearly ripe, marrow fruit across,
you will find that its wall is very thick and fleshy, and that the
many seeds are buried in a soft pulp. The melon shows us just the same
thing. Such fruits seldom split suddenly to shoot out their seeds
(though some foreign ones do); they depend more on animals which may
eat them and so scatter the seeds about.
In all cases it is better for the plant to have the seeds scattered so
that they do not sprout too near together, but have room enough to grow
without crowding each other out.
In the pea and marrow there are many seeds, but there are large numbers
of fruits in which we find only one. For example, in plums, cherries,
and peaches we have a fleshy outer fruit-case with a stony lining
covering over one large seed. Such fruits do not open, for there is
only one seed within, and so the fruit is scattered whole. These fruits
nearly always get scattered by animals, for the flesh is very sweet
and attractive to eat, and then, as a rule, they get rid of the stone
(which contains the seed) at some distance from the parent.
[Illustration: Fig. 82. Cherry fruit cut open, showing the flesh (~f~),
stone (~s~), and seed S.]
Sometimes we find a number of fruits just like the cherry clustered
together, only instead of each of them being large, they are all very
small, so that the whole cluster of fruits may be the size of a single
cherry. This is the case in the common blackberry and the raspberry,
where each of the little fruitlets really corresponds to a cherry.
Then there are many fruits which belong to quite a different class, and
arrange to scatter themselves by the help of the ~wind~, such as the
fruits of the dandelion, thistle, and many others, which have light
“parachutes,” and therefore blow away with the least puff of wind when
they are ripe and dry (~see~ fig. 83).
[Illustration: Fig. 83. A, head of Dandelion fruits, with most of them
scattered. B, single Dandelion fruit.]
Other fruits like the sycamore have big side-wings which catch the wind
as they fall, and get twirled for some distance. ~In these cases each
of the separate parts which flies is really a~ =fruit=, only in the
case of the dandelion, thistle, and many others, each of these fruits
contains only one seed, and the fruit itself is so small and dry that
we get into the way of speaking of the whole fruit as a “seed.” This
is not correct, however, because even though there is only one seed
present, yet it is surrounded by the dry remains of the ripe carpel,
and is therefore a fruit.
~Simple seeds which have wings~ are rather rare, but we find them on
pine seeds (~see~ fig. 125), and the seeds of the willow herb are
covered with a number of silky hairs, which make them so light that
they fly in the wind. It you watch a spray of willow herb ripening, you
will find that the old carpels, or fruits, split up into four parts and
let out a number of fluffy white seeds. These are true ~flying seeds~
(~see~ fig. 84).
[Illustration: Fig. 84. Fruit of the Willow herb, opening and allowing
the flying seeds to escape.]
Other seeds get scattered by the wind although they do not fly. For
example, in the poppy the fruit is the hardened ripe carpels which have
become quite dry, and together look like a little round box, within
which there are many tiny dry seeds. When the box or ~capsule~ is quite
ripe openings come in it, just below the projecting top, and then, when
the weather is dry and they are open, a strong wind may bend the stalk
of the fruit and shake the capsule strongly. The seeds come scattering
out like pepper from a pepper-pot, and may get carried some distance
from their parent plant (~see~ Plate II. and fig. 85).
[Illustration: Fig. 85. Ripe Poppy capsule, showing the little pores at
the top which let out the ripe seeds when the capsule is shaken.]
Some fruits are covered with spines and hooks, which catch on to the
wool of animals, and so get carried quite a distance before they are
dropped. This gives the seedlings a good chance of reaching a new spot
where they can grow away from the parent, and so not be too crowded.
Well-known fruits of this kind are the burs, which stick tightly to
one another with their dozens of little hooks, the “bur” being really
a cluster of many fruits together. Simple fruits of the same kind are
the bidens, each with its two long spines, and the small fruits of the
goose grass, which are covered with the finest hooks.
[Illustration: Fig. 86. A Bur, which is a cluster of hooked fruits.]
[Illustration: Fig. 87. Simple fruits: (~a~) of the Goose grass with
its hooks; (~b~) the Bidens with its harpoon-like spines.]
[Illustration: Fig. 88. Strawberry. Each of the little “seeds” is a
whole fruit, and the “flesh” the swollen receptacle.]
Quite a special kind of fruit is the ~strawberry~, which, as you know,
has a thick fleshy pulp covered with a number of small, yellow “seeds.”
In reality, each of these “seeds” is a whole fruit, and the thick flesh
which we eat is the swollen end of the flower stalk which we call the
“receptacle.” Therefore ~a strawberry really consists of a large number
of fruits~ and a piece of stalk which is altered to form the fleshy,
attractive mass which induces birds and people to eat the whole, and so
scatter the little dry fruits.
There are very many other kinds of fruits which all have special
devices to make sure that their seeds are scattered, and all proper
fruits have seeds in them. But, just as we found that some garden
flowers are grown only for their beauty, and do not set any seed, so
we find that some fruits are grown specially without any seeds, such
as bananas and some oranges. Such fruits are the result of our liking
to eat the soft, sweet pulp without the trouble of the seeds, but such
fruits are of no use to the plant.
Now let us look at the structure of the ripe seeds themselves, and see
how they are fitted to go out alone into the world prepared to make a
new plant. Seeds are all very much alike in the important points of
their structure, although they vary much in the shape, size, and colour
of their parts. We already know what beans are like from our careful
study of them at the beginning of our work (~see~ Chapter III.), and
beans show us particularly well all the important parts of a true
seed, so that we may take them as being typical of one large family of
flowering plants. The maize embryo (~see~ p. 10) is typical of the rest
of the flowering plants. ~In the ripe seeds~ of both of these groups
(you should examine them again if you have forgotten any of the facts)
we find that ~the important thing is the baby plant~, which is supplied
with food and ~protected by two seed coats~, till it is time for it to
grow out and form a new plant like its parent.
[Illustration: Fig. 89. A, outside of Bean; (~h~) black scar showing
where the bean was attached to the pod; (~r~) ridge made by young root;
B, bean split open; (~n~) nurse leaves; (~p~) embryo; (~a~) scar where
the embryo was separated from the nurse leaf on that side.]
[Illustration: Fig. 90. A, outside of Maize, showing the embryo (~e~)
on one side; B, sprouting, showing the root (~r~) and shoot (~s~); C,
the same further grown.]
CHAPTER XVII.
THE TISSUES BUILDING UP THE PLANT BODY
[Illustration: Fig. 91. Two cells from plant tissue. (~c~) Living
contents; (~n~) cell nucleus; (~v~) spaces filled with sap; (~w~) wall
of cell (much magnified).]
In our study of plants up to the present, we have only looked at their
structures from the outside. We have examined the form, uses, and life
of the parts of their bodies without looking for the details which
might answer the question--“How are they built up?” Just as a house as
a whole has a definite form, with rooms, and doors, and windows, each
with their definite form and use, and at the same time every one of
these things is built up of small separate bricks, tiles, pipes, and
pieces of wood: so we find that the whole plant is composed of a number
of definite parts, which are themselves built up of tiny individual
parts, which we may take to correspond with the bricks of a house.
Of course, they do not do this completely, for a plant is a living
thing, and is far more complicated than a house, and each of the tiny
individual parts is also a living, growing thing. These little building
structures are called ~cells~ both in plants and animals, and they are
so very small that you cannot study them fully without a microscope,
and that is a very complicated and expensive thing, so we will leave it
alone and only study what we can see of the structure of plants without
it. Try, however, just to see a few cells under the microscope, so
as to know what they are like. A ~typical cell~ has a ~wall~, within
which is the actual ~living substance~, a clear, jelly-like mass, which
contains many granules of food and stored material. Within this living
substance is a more solid mass of still more actively living substance,
which is called the ~nucleus~. Cells like these, or cells which were
like these when they were young, and which have become modified for
special work, build up the whole plant body (~see~ fig. 91).
[Illustration: Fig. 92. Piece of thin section across Water-lily stem,
showing mesh-work tissue seen with a magnifying glass.]
You can get a good hand magnifying-glass for several shillings, and
with this and a very sharp knife, you can find out something of the
structure of the insides of plants, even though most of the cells
are so small as to be out of sight except when looked at through a
microscope.
Let us first cut as thin a slice as possible across a water-lily stem,
and put it on a small piece of glass and hold it up to the light.
Examine it with the magnifying glass, and you will see that it is not
a solid mass of tissue, but that it is built up of a fine network like
lace, with quite large spaces between the threads (~see~ fig. 92).
These spaces are air spaces, and the fine lace-work threads are meshes
built up of single rows of cells, which you may be able to see if your
glass is a good one. Cells may be packed loosely like this, or they
may be in more compact form something like a honeycomb, as you may see
in the pith of an elder twig and many other stems. You can crush these
soft cells between your fingers, and we cannot imagine that they could
build up the hard, firm branches of trees.
Now examine the stem of a seedling sunflower, by cutting a very thin
slice across it; you will see in it a ring of strands where the cells
are smaller than those of the soft tissue and also much more closely
packed (~see~ fig. 93). Then cut a thin slice longways down the stem,
and you will see that these more solid strands are the cut ends of
long strings of such tissue which run through the stem. The cells
which build up these strings are not quite ordinary cells, but are
exceptionally long, like water-pipes, and they have thickened walls.
These cells do the carrying of water and liquid food up and down the
plant (~see~ fig. 94).
[Illustration: Fig. 93. Piece of the stem of a seedling Sunflower cut
across, showing strands of “water-pipe” cells.]
[Illustration: Fig. 94. Piece of stem cut across and then split
lengthways, showing the strands of thicker “water-pipe” walls.]
You can see that the water travels up these cells if you cut across
a stem near the root and place it in a little red ink. After a few
hours if you cut a section several inches from the bottom of the stem
you will find that these strands are coloured red by the ink which
has passed through them, while the rest of the stem is very little
coloured, or quite colourless. This shows us that these strands are the
special water-pipes of the plant.
[Illustration: Fig. 95. Cross section through a Lime twig three years
old seen with a magnifying glass.]
Large numbers of such cells closely packed together, and with some
other hard cells between them, make up the wood in woody stems. Cut
across a small twig of lime or oak and examine it with your lens.
Outside is the brown bark, then within that some green cells and a
little soft tissue, while most of the stem is made up of a mass of hard
wood cells, among which you can see some of the larger water vessels
distinct from the rest. All this hard tissue really corresponds to the
joined up separate strands which we saw in the sunflower stem (~see~
fig. 95). Trees like the lime and oak, which live for a long time, grow
for a certain amount every year, and each year they add a ring of wood
to their stems. In old stems you can see clearly the rings of wood
which have been formed by each year’s growth. This is another way of
telling the age of the stem, and you should compare your results from
this method with those you got from counting up the bud scars (~see~ p.
75).
Leaves, as you know, require much water, which comes to them up
the stem through the “water-pipes.” You saw already the course of
the water-pipes in leaves, for they are the “veins” which we found
sometimes make a complete network, and sometimes run parallel in the
tissue of the leaf. If you put a leaf stalk in red ink, you will see
that the veins are connected with the water-pipe strands in the stalk,
for they will both get coloured by the ink as it passes along them.
Just as in animals the whole body is covered over with a ~skin~, so
in plants we find a special outside sheet of cells, which protect the
inner tissues and form a thin skin. You can get this off very well if
you break across an iris leaf, and pull along the thin, colourless
layer on the outside. If you examine it with your lens, you may perhaps
see something of the mosaic-like pattern of the cells which build it
up. You should certainly see that it is colourless, although the tissue
of the leaf beneath it is quite green.
On the large branches of trees and the bigger plants, we do not find
this delicate protecting layer, but instead there is a thick brown
~cork~. When the cork layer gets very thick it splits irregularly as
the tree grows too big for it, and so forms a rugged bark. The cork
layers have much the same duty as the fine skin, only they are thicker
and stronger, and more suited to hold out through the winter. You know
already from daily life the practical use of cork, for you put it into
bottles to keep the liquid in the bottle and the damp and dust in the
air from entering. Just what the cork does for the bottle, the sheets
of cork wrapping round the branches do for the plant. They prevent it
from being dried up by cold winds, and they keep out the heavy rains
of winter which would injure it. Roots have a cork coating also when
they get old. As you may remember, it is only the tip of the root which
can absorb water for the plant, so that in the young part of the root
a cork layer would be very much out of place, and you will never find
it there. You will find instead the little delicate root-hairs, which
absorb water and pass it on to the older parts; these old parts do no
more absorbing, they are only the water carriers and food storers, and
so have no hairs and are protected by a layer of cork.
As we found before, plants ~breathe in~ air like animals, and you
may ask how they can do this when they are covered with their thick
air-tight layers of cork. Examine a fairly old elder twig, and you will
see all over its brown skin numbers of darker brown spots. If you look
at these with your magnifying glass, you will see that they are quite
spongy and soft. They are the special entrances for air, and are the
breathing spots or ~lenticels~ (~see~ fig. 96). They are to be found in
all corky stems, although they are not always so easy to see as in the
elder.
[Illustration: Fig. 96. Piece of Elder twig, showing the breathing
pores in the bark.]
On the leaves and stems of many plants you will find a large number
of ~hairs~. In some cases there are so many as to make the whole
plant quite woolly, like the mouse-ear leaves. These hairs are
protective, and keep the leaf warm and dry, and in some cases may
shelter it from the sun. Hairs may consist of one cell, or several in
a row, or of cells which are branched in a complicated way. Certain
hair-cells protect the plant by stinging, as you can see if you watch a
nettle-leaf with your magnifying-glass, and then rub your finger along
it, only touching the hairs. You will find that it is they which sting
you, and not the leaf itself.
Now we have found several kinds of tissues in plants, the ~skin~ and
~cork~ covering all over and protecting the rest; the ~central vessels~
or ~water-pipes~, corresponding to the veins and arteries of animals,
the soft ~white ground~ tissue, which in some stems may be very loosely
packed, and the soft ~green tissue~ in the leaves and young stems,
which we found was the food-manufacturing part of the plant. There are
also strands of simple ~strengthening tissue~, both by the water-pipes
and in separate bundles in the soft tissue; these we may take as
representing the bones of animals.
We have noticed (Chapter VIII.) that plants are sensitive to light
and bend towards it, that they feel heat and cold, and that the stem
and root seem to know when they are growing in the right or wrong
positions, and bend accordingly. We know that we ourselves and the
animals recognize such things by the help of nerves which carry
messages to the brain. But where is the brain in plants, and the
nerves? No true nerves have been found in plants, and it seems as
though different parts of the plant were specially sensitive without
there being any “brains.” So that we cannot speak of a central nervous
system in even the highest plants as we can in the animals. In this
respect they are built on quite a different plan from animals.
PART III.
SPECIALISATION IN PLANTS
CHAPTER XVIII.
FOR PROTECTION AGAINST LOSS OF WATER
If you go along the lanes and in the gardens in the height of summer
when it is hot and dry and the sun beats on the plants all day, you may
see them beginning to wither for want of water. The roots are not able
to find enough moisture in the soil to supply the leaves, which, being
in the hot air, continue to transpire away the water resources of the
plant, so that in the end each of its cells must suffer and the whole
become limp and droop. This happens because the ordinary green plants
of our country make no special preparation for such dry weather. Our
hot season is short, and even in the summer we have frequent showers
which keep the soil moist enough to provide the plants with water from
day to day, so that they have not become accustomed to long periods
when there is no prospect of rain.
[Illustration: Fig. 97. A Cactus with needle-like spines for leaves,
and a thick green stem.]
Compare one of our usual green plants, a sunflower, for example, with
such a thing as a cactus, which you may get growing in a pot of dry
sand. The cactus is able to withstand the hottest sun for days, though
it gets very little water, and sometimes apparently none at all; yet it
does not wither, but grows, and may bear the most lovely flowers. From
travellers we learn how the huge cactus plants grow in dry and stony
deserts, standing every day in the blazing sun. Such is, of course,
their home, and they are used to it; but how is it that they are able
to flourish under conditions which would kill one of our own green
plants?
Let us look at their structure and see in what they differ from a usual
plant. First, they have no green leaves, for these have developed into
spines (~see~ p. 62), while the sunflower has many large ordinary
leaves.
You will remember that the surface of leaves is continually giving off
water from its many pores. When a plant has a number of big leaves this
transpiring area is large, while when it has no leaves at all, but a
thick, green stem instead, then the amount of surface from which water
vapour is being given off is very much reduced, even though there may
be about an equal quantity of actual tissue in the two plants. You can
see that this is the case if you take a ball or thick block of dough
and roughly measure its surface, then roll it out till it is fairly
thin and measure it again; you will see that the thinner you roll it
the more surface there is; all the time, of course, the amount of
actual dough remains the same. So that of two plants of the same bulk,
the one with broad, thin leaves will expose the most surface to the
air, and so lose more water than one with very thick leaves or none
at all. The latter would therefore be better fitted to live under dry
conditions.
But, you may say, leaves have a definite work to do; how can the plant
live without them? In the cactus the thick stem is green and does the
work of food building; naturally it cannot do so much for the plant as
many big leaves could, but it does enough to allow it to live and grow
slowly and surely for many years, though it cannot grow in each year
nearly at the same rate as can the sunflower. If you cut through the
stem of a cactus you will find that its skin is very thick and tough,
and this thick coat protects the plant against the fierceness of the
sun far more completely than the thin skin of a sunflower does. At the
same time, the tissues of the two stems are different; the sunflower is
hollow and delicate, but the cactus is very thick and juicy, and each
cell contains much gummy stuff which has the power of holding water
strongly. So that we see in many important points the structure of a
cactus is different from that of a usual green plant, and is specially
suited to the dry conditions of the desert.
Many desert plants are built on the plan of the cactus, but there are
also others which are not at all like them, and yet they are able to
live in deserts and very dry places. It you examine them, however, you
will find that they all have some special way of protecting themselves
from being dried up. Some of them have hard, dry, woody stems, well
protected by corky layers, and they only put out green leaves in the
rainy season, and lose them directly the hottest weather begins.
Others, which grow from seed every year, learn to sprout, flower,
and fruit very quickly while there is some moisture, and they form
well-protected seeds, which wait till next rainy season. One very
curious desert plant has only two leaves, which last it the whole of
its life, and which are very hard and leathery. There are endless
varieties of things which the plants may do to protect themselves from
being dried up, and we can only look at a few special examples.
To find plants growing in desert places we do not need to go out
of England, because from the point of view of the plant, one which is
growing on a dry rock or on a patch of bare dry sand, is really growing
in a little desert. For it the supply of water is the chief problem,
even though we never get hot tropical sunshine in England. Look, for
example, at the plants growing on the sand dunes which are very like
deserts in appearance, and the plants on dry walls, or on the “screes”
of broken rock at a hill foot; they are all growing in deserts.
In many cases plants growing in such positions have small thick
leaves, nearly round, or shaped like sausages, so that they have much
water-storing tissue in proportion to a small transpiring surface.
This is the case in the stone-crop (~see~ fig. 98) and the house-leek,
where each separate leaf has followed the same principle as the cactus
stem, and exposes relatively little surface to the air. Such plants
frequently have very long roots, which penetrate deeply between the
cracks of the rocks and find hidden sources of water.
[Illustration: Fig. 98. Thick fleshy leaves of Stone-crop.]
Other plants, instead of having leaves of this type, have exceedingly
small leaves which may soon drop off, while the stem is green and does
some of the food building. Small leaves are assisted by the green stem
in gorse (fig. 99), which often lives in very dry places, though it can
grow equally well under usual conditions.
[Illustration: Fig. 99. Gorse, with green stem which does the work of
leaves.]
Many plants roll up their leaves when it is dry, so that the surface
with the transpiring pores is on the inside, and protected by the outer
side with its hard skin (~see~ fig. 100). In damp weather these leaves
unroll, and do all the work they can. Leaves like this are to be seen
in many of the grasses, particularly those growing on sand dunes and
moorland; while a number of the heaths and heather do the same thing to
protect their transpiring surfaces.
[Illustration: Fig. 100. Leaf of the Sand-grass. A, rolled up; B, open.
(~a~) and (~b~), sections across the same.]
~You will find that in nature, water is one of the most important
things in the surroundings of plants~, and in their struggles to get
it and keep it they have changed their forms in many ways, and in some
cases have become extraordinary-looking creatures as a result.
CHAPTER XIX.
SPECIALISATION FOR CLIMBING
If you go into a wood, or even a thicket, in summer, you can see how
the leaves of the big trees make, what is for us, a delightful shade.
But look at the ground under these tall trees, at a place where they
are growing thickly together, and you will find that there are very few
plants below them, and that the earth is almost bare except for dead
leaves, twigs, and a few mosses. In deep pine-woods there are great
patches without even the mosses, where are only dead pine-needles and
some toadstools. You can well understand, when you remember how very
important light is for the plants, that it is too dark for them to grow
under the heavy shadow of thick trees. Even in gardens you may see how
the tall, quickly growing plants kill off the smaller ones beneath them.
When many plants are growing together, it is easy to see that the
taller ones get most light, but if a plant grows very tall it requires
a strong stem to hold it up right, and that means the building of a
large amount of wood which takes a quantity of material, so that the
growth must be slow and costly.
Some plants, however, have learned to grow up into the light without
building a firm stem for themselves, because they use instead the
support of other plants, and especially of trees. You must often have
noticed in a wood great sprays of honeysuckle sprawling high up over
the trees; sometimes one of the festoons of honeysuckle may lie over
the branches of several trees, and so get into the best positions for
the light. The Travellers’ Joy, or white clematis, grows all over the
tall hedges, and may sometimes completely smother a young tree, so that
one can see nothing but the leaves and light green and white flowers of
the clematis. Then, too, there is the ivy, which you know may sometimes
grow up trees to a very great height, covering over the leaves so that
the whole looks like a giant ivy bush. These plants all get their
support from trees, which have built themselves strong stems. Pull down
a big branch of honeysuckle or Travellers’ Joy from the supporting
tree-trunks, and you will see that it cannot remain upright but falls
limply to the ground. It is true that these plants have some wood in
their stems--sometimes clematis and ivy may have woody stems several
inches thick, but they are never strong enough to support the weight
of the crown of leaves and branches. By clinging to others in this way
these plants can economise much building material and reach the light
far quicker than they could do otherwise.
If you examine their wood, you will see that it is not quite like that
of usual plants. Cut through the stem of a clematis which is about an
inch thick, and even before you look at it with a magnifying-glass you
will see how very loosely built the wood is, with wide rays of soft
tissue and very large water vessels. It is not built for strength and
support, but merely to carry supplies of water up to the leaves, for
although these plants use trees as supports, they do not get anything
more from them, and must supply themselves with all else they need.
You may often see that the central part of the wood is not in the true
centre of the stem, but is pushed to one side, and the rings of the
year’s growth are very irregular, being much more to one side than to
the other. This is because they lean against the supporting branches,
and so must grow chiefly on the side away from them. Sometimes as the
ivy grows right round the support, it will grow more, first on one and
then on the opposite side of its stem, and so the centre does not
remain in one place, but shifts round.
The other parts of these woody climbing plants are but little out
of the common. They have merely learnt to economise their own
stem-material, and at the same time to reach a good position in
the light, so that it is in their stems that we find their chief
differences from usual plants. The honeysuckle and clematis have no
special climbing organs, but the Ivy has clusters of adventitious roots
which come out from the back of its stem, and hold it on to the support
(~see~ p. 56 and fig. 101).
[Illustration: Fig. 101. Adventitious roots growing out from the stem
of Ivy between the leaf stalks.]
In climbing plants in which the above-ground parts live only for one
year and then die down, we do not get a woody stem. Such soft green
plants as the hop and convolvulus, for example, are entirely dependent
on others for their support. They have specially sensitive tips to
their stems, which feel the support and definitely twine round it in
a close spiral, which clings ever closer to the support as they grow
(~see~ fig. 102).
[Illustration: Fig. 102. Soft twining stem of Convolvulus.]
Climbers of this kind have only modified their stems; the rest of their
parts are not in any way specially altered by this habit.
Some plants which sprawl about on others hold themselves up by the
power of clinging and twining in their leaf-stalks, for example, in
the nasturtium we find that the plant is held up entirely by the leaf
stalks, which catch on to anything in their way (~see~ fig. 103).
[Illustration: Fig. 103. Nasturtium stem held up by the support given
by the leaf-stalks, which cling around any suitable prop.]
Very many plants which depend on others for support modify their
leaves, or parts of leaves, to form sensitive tendrils which twine
quickly round any prop they can find, and thus hold up the stem (~see~
fig. 104). The young tip of the stem continues to grow upwards, the
next young leaves unfold their soft green tendrils which twist round
a support directly they feel it, and so the plant goes on growing
higher and higher. You can see the fate of a pea-plant which does not
find supports, by growing one in a big pot all by itself. It will grow
upright at first, but it will soon have to creep along the earth and
fall over the edge of the pot, for its stem is not strong enough to
support its own weight.
[Illustration: Fig. 104. Sensitive tendrils of the Pea. (~t~) tendril
at the end of foliage leaf, (~o~) ordinary leaflets.]
In vines and marrows we also get tendrils, but they are not modified
leaves, but special branches which have become sensitive.
In some plants the sensitive tendrils do not twine, but instead form
little sticky suction pads at their tips whenever they come in contact
with the support, and these hold the tendril very firmly on, as you can
see in the ampelopsis, which grows right up the walls of houses. If
you look under the thick covering of leaves, you will find these tiny
padded tendrils clinging tightly to the wall (~see~ fig. 105). This is
the reason that the ampelopsis grows so well up the walls without being
held up artificially.
There are many other things you may find out about climbing plants, but
you will have seen enough to be able to look for more for yourself, and
to understand how it is that the climbing plants can reach such a great
height so quickly. ~They have learnt to avoid the trouble and expense
of building strong supporting stems for themselves, and by getting
their support from others, they are able to grow quickly out into
the good positions for the light which they could not otherwise have
reached.~
[Illustration: Fig. 105. Ampelopsis, which supports itself by the
little suction pads developed at the ends of the tendrils.]
CHAPTER XX.
PARASITES
We call a plant or animal a ~Parasite~ when it does no food-building
for itself, but adapts its whole structure to obtain and use the food
made by the work of other plants or animals. Plant parasites generally
attach themselves to a “Host” plant so closely that they suck their
food from it, and sometimes remain with it till they have finally
killed it, and so have destroyed their only source of food and means of
life.
Among plants, most of these degenerate creatures belong to the group of
~Fungi~. The rust and smut on wheat, the mildew on fruit, and nearly
all the thousand spots, blemishes, and diseases of cultivated and other
plants, are the result of the parasitism of some members of the family
of fungi. Plants which prey like this on others are without very many
of the characteristics of true plants; they become colourless, losing
their green substance, and with it all power of building food for
themselves, so that they are quite dependent on the host plant, without
which they must ultimately die.
Fungus parasites, of which there are many thousands, have become so
specialized that they are quite a study in themselves, and we will
leave them for the present and follow the history of a few of the
higher plants which have taken to this mode of life.
[Illustration: Fig. 106. Dodder plants growing over Clover. (~a~)
clusters of Dodder flowers.]
One of the most completely parasitic of the flowering plants is the
dodder, which you may often find growing on clover. In fields of clover
sometimes there are colonies of dodder, which live together and kill
the clover in great patches so that it almost looks as though it had
been burnt. Dodder grows on other plants, such as gorse, as well as
clover, and even on nettles. If you find a plant of dodder you will see
that it seems to consist of nothing but fine, white or pinkish threads,
twisted round and round the clover stems and hanging in festoons over
them. Pull off these fine threads carefully, and you will find that at
intervals along them there are little sucker-like pads which hold the
dodder quite firmly on to the plant on which it is growing. If you cut
through the middle of one of these pads and the clover-stem while they
are still attached, and look at the cut with your magnifying-glass, you
will see how the tissue of the dodder pad enters right into the tissue
of the clover stem (~see~ fig. 107). These pads act as suckers for the
dodder and draw from the clover all the ready-formed nourishment that
the dodder requires, so that it has no work to do in food building. It
has no roots because it needs none; the suckers act as roots in getting
all the water and also the manufactured food the plant uses; for the
same reason it requires neither leaves nor green chlorophyll, and its
body is only a colourless or pinkish mass of thread-like stems and
sucker pads.
[Illustration: Fig. 107. Section, A, across the Clover stem, with the
Dodder D attached. S, suckers of the Dodder, entering the Clover.]
There is one thing, however, that the clover plant cannot do for the
dodder, and that is, make its seeds. When the clover builds seeds,
then they are clover seeds and will grow up as new clover plants. The
dodder must build its own seeds if dodder plants are to grow from
them. That is why we find growing out from the simple reduced thread
of a stem, relatively large tufts of flowers (~see~ fig. 106), which
are very little different from usual flowers and which form seeds. The
dodder belongs to the same family as the convolvulus, and though its
flowers are small, if you examine them with a magnifying-glass you
will see that they are very much the same in structure as those of the
convolvulus.
When the young dodder plant grows out from the seed, it is a simple
little thread with no leaves, and it keeps on growing at the tip,
which it moves round till it feels some suitable host, then it quickly
fastens on to it and lives on its food.
This is the general history of all kinds of parasites, for when any
living thing ceases to use its structures and becomes a complete
parasite it loses nearly all its parts, as there is no longer any
need for them. So that parasites tend to sink to a lower level of
development simply as a result of their way of living.
A plant which is largely a parasite, but yet does a little work for
itself, is the mistletoe (~see~ fig. 108). Its leaves are greenish, but
not the true healthy green of a hard-working plant. If you can find a
bough of mistletoe growing on an oak or apple tree, you will see that
it has no root in the earth, but grows out of the bough of the host
tree. It has sucker-like roots at the base of its stem, which go right
into the stem-tissues of the host and get much nourishment from them.
[Illustration: Fig. 108. Young Mistletoe attached by its sucker-like
roots S to a twig of apple A, split open.]
In the winter, when the flow of food is very slow in the host, it is
likely that the mistletoe does some of its own food building in its
yellow-green leaves, which would be exposed to the full light, as the
host’s leaves would have fallen away. The mistletoe has soft, white
fruits which are scattered by birds, and as they are very sticky, they
hold for some time on to the branch where they are dropped, and there
the seedling sprouts and fastens itself on to the tissues of the host,
growing every year with its growth.
[Illustration: Fig. 109. P, parasite attached to the root R of a host
plant H (which is the Ivy). A is the host root on the other side of the
parasite.]
Quite a number of plants which grow in the ground attach themselves
with suckers to the roots of other plants, from which they get all
their ready-made food. Plants which do this are generally colourless
or brownish yellow, like the broomrape, which has only whitish leaves
which cannot do the proper work of leaves (~see~ fig. 109).
Then there are several plants which are partly parasitic, but which
you would never guess were anything but ordinary plants. For example,
the little eyebright with its green leaves, which do most of the
food-building, is yet partly parasitic. If you ~very~ carefully get out
a whole plant with its complete roots (this is rather difficult to do,
and you must not pull it hastily, or you will break the connections),
you will find that there are tiny suckers on them which connect them
with the roots of the plants which are growing near. So that the
eyebright gets some of its food ready-made from the neighbouring
plants. The meadow cow-wheat does the same thing, and so do the
lousewort and several others; but they are not complete parasites, for
they are green and do a lot of work for themselves, even though they
are not quite self-supporting, and tap the supplies of other plants to
some extent.
Among flowering plants, parasites are not common. We see in plants
like the eyebright and cow-wheat, which do a little thieving, that the
results are not very serious, and they are little altered by their
habit. In those which are entirely parasitic, however, like the dodder,
the result is the loss of nearly all the organs of the plant except the
flowers, which have to be kept in order to build seeds.
CHAPTER XXI.
PLANTS WHICH EAT INSECTS
As a rule, plants are the sufferers and are eaten by animals, but
there are cases known in which this state of things is reversed; the
plants catch and devour the tinier animals and small insects such as
flies. But, you may ask, how can they do that, for the insects move
so quickly, and the plants are fastened by their roots to one spot.
Just as a spider builds a web and then waits quietly beside it till
the flies are caught, so the plants build traps which catch the unwary
insects. There are not very many plants growing wild in England which
do this, but there are one or two that you might be able to find.
[Illustration: Fig. 110. Plant of Sundew, showing the round leaves
covered with tentacles.]
There is the sundew, which grows among bog-moss in wet, swampy places
at the edges of lakes, or on the wet patches on hillsides. It is fairly
common in such places, a little distance from big towns, but it does
not like smoke, so that it will not live within a few miles of London,
Manchester, or any big smoky town. It is a small plant with round,
reddish-coloured leaves, covered over with little fingers or tentacles
each with a sparkling drop of sticky moisture at the end, so that even
in the heat of the day when all the dew is dried up, the whole plant
looks as though it were spangled with tiny dew-drops. Perhaps it is
this cool, sparkling appearance which attracts the insects to it, but
when once a fly alights on one of the leaves, its fate is sealed. The
tentacles with their sticky tips bend over one by one till the fly is
quite covered in by them and cannot get away. It dies, and is digested
by the juices given out by the leaf, which are very much like the
digestive juices of animals.
[Illustration: Fig. 111. Single leaf of Sundew, with the tentacles
closing over a fly.]
You can watch the movement of the tentacles very well if you drop a
minute piece of meat or white of egg on to the leaf. They will close
over it one by one till it is quite shut in, and when the egg is
all digested, they slowly open out again. The time that this takes
depends a little on the health of the plant and the time of the year,
but generally all the tentacles are bent over in a few minutes. The
digestion takes longer, of course, at least several hours and often
more, partly depending on the size and nature of the piece of food.
The sundew leaves contain chlorophyll and do some of the usual work of
leaves, but the plant gets much of its nourishment from the insects it
catches.
[Illustration: Fig. 112. Butterwort, showing the rolled leaves which
catch flies and other small insects.]
In the butterwort there is a different arrangement for catching its
prey. You will find its little clusters of broad, spoon-shaped,
yellowish-green leaves growing in marshy places and beside streams in
hilly districts. In the spring one or two lilac flowers on long stalks
come up from the centre of the group of leaves. The leaves of this
plant also act as insect traps; they are covered with little sticky
glands, and when an insect settles on them, the edge rolls over and
shuts it in, keeping it there till the juices given out by the glands
have digested all that is worth digesting, when the leaf unrolls again,
and the remains of the feast are washed away by the rain.
[Illustration: Fig. 113. A piece of leaf of Bladderwort showing the
bladders on the branches.]
There is one more animal eater which you must try to see, which
grows in the water of slow-running streams and in ponds. It is the
bladderwort, on which we find very many tiny bladder-like structures
on the finely divided leaves under the water. The bladders are built
on something of the same plan as a lobster pot, with bristly hairs
pointing into the entrance, across which there is a little flap,
which makes it quite easy for the very minute animals living in such
abundance in the water, to swim ~into~ the bladder opening, but
extremely difficult or almost impossible for them to swim out again
(~see~ fig. 114). So there they must finally die, and their nourishing
juices are absorbed by little compound hairs, many of which are
developed on the inside of the bladder.
[Illustration: Fig. 114. A single bladder of the Bladderwort, much
enlarged, showing the pointed hairs and the flap at the opening.]
In the tropical countries there are many kinds of “pitcher plants” with
wonderful soup-kettle-like pitchers which catch insects. You may be
able to see these plants in a big greenhouse, and should certainly find
them in every botanical garden. Notice how large the pitchers are,
and that they are really modified leaves which have become different
from the other leaves of the plant because of their special work. They
generally contain a considerable quantity of water as well as the flies
they have caught, and are really “stock-pots” which keep the plant
supplied with nourishing, ready-made food in addition to the food which
it builds for itself in the green leaves.
[Illustration: Fig. 115. Pitcher leaf of Nepenthes, which acts as a
“soup-kettle.”]
Though these plants have specialised themselves to catch and use animal
food, still there are not very many plants that do so, and the old
fairy tales about trees with branches which caught men and devoured
them, as a sea-anemone catches and devours its food, are only fairy
tales, because no such plants exist.
CHAPTER XXII.
FLOWER STRUCTURES IN RELATION TO INSECTS
The relation between flowers and insects is one of mutual help and
advantage, and therefore is quite different from that in the cases
where the animals eat the plants or vice versa.
When we examined flowers in general, we found that the insects do a
very important work in carrying the pollen from flower to flower, and
that their structures are arranged to attract insects and to make it
easy for them to get covered with the pollen of one flower and leave
it on the next. If we look at the details in some of the flowers, we
shall see how elaborate their structures may be, and how carefully they
are planned to make sure that the bee gets the pollen on its body and
carries it with it to the neighbouring flowers.
[Illustration: Fig. 116. Circular flower of Rose, with many stamens in
the centre.]
In the simple circular flowers, such as roses, poppies, and lilies, the
bee can enter freely from any side that it chooses, and it generally
goes straight to the centre. Many of these simple flowers, therefore,
have large numbers of stamens which stand up in a crown in the middle,
so that the bee must touch and stir some of them as he dives in the
centre for the honey.
[Illustration: Fig. 117. Slightly two-sided flower of the Foxglove,
with the petal tube cut open to show the four stamens bending to the
front.]
In others which are nearly circular, there is a little difference
between the back and front of the flower, and the stamens are so placed
that the visiting insect must touch them. For example, look into the
bell of a foxglove, where you will find only four stamens, but they are
bent so that the anthers together form a kind of platform in the front
of the flower, over which the bees must pass as they enter (~see~ fig.
117). Frequently the stamens bend in this way towards the front of the
flower, and in many cases the whole flower becomes quite definitely
two-sided, with a front and back, and a special place for the entrance
of the bee. This is the case with the violet, pea, monkshood, and many
others (~see~ fig. 118).
[Illustration: Fig. 118. Two-sided flowers: A, Monkshood; B, Violet.]
When flowers have this form, you frequently find that the number of
stamens is quite small, seldom more than ten, and often less.
A plant of this kind very interesting to watch is the yellow gorse. If
you can get up and sit by a flowering bush from about half-past five to
seven one sunny morning, you will be able to learn a great deal about
the doings of the bees and flowers.
[Illustration: Fig. 119. (~a~) Flower of the Gorse after the insect’s
visit, showing the inner parts exposed; (~b~) young flower nearly ready
to be visited.]
First examine a flower so that you know how it is arranged. At the back
lies the big petal, or “standard,” as in the pea; there are two side
wings, and in the front the two petals close together forming the
“keel.” The two-sidedness of this flower is very well marked. Inside
the keel you will find ten stamens, all joined to form a tube except
the back one, which is free, and inside them lies the carpel with its
curved style. When the stamens are ripe they are so fitted that they
lie inside the keel of the petals in a bent form, and when they are
pressed from above they fly out with a little explosion and scatter
the pollen dust about. Now watch a bee alighting on the flowers; he
presses the two front petals with his legs to open them to get at the
honey, and the stamen explosion covers him all over with pollen. Then
he goes to the other flowers, but perhaps the next one he visits has
already exploded and the ripe stigma is exposed in the front of the
flower, and as he settles he touches it with his furry body all covered
with pollen, and leaves some on it. If you watch the bees doing this
yourself, you will find out a number of things which I have not told
you, while you may notice how some of the bees are lazy and enter the
wrong side of the flower, others are stupid and go to flowers which
have already been visited several times, and therefore are of no use,
while other bees which come late may open up buds which were not ready
for them and steal the honey before the stamens are ripe enough to
smother them with pollen. I have watched them opening buds which were
still so tightly closed that it took them all their strength to get in.
But we must not stop too long with one flower, for almost every flower
has some special arrangement of its own, and all are worth study.
[Illustration: Fig. 120. The two kinds of Primrose flowers, A, with
long style and stamens low in the petal tube; B, short style, with
stamens at the mouth of the petal tube.]
The primroses and cowslips are interesting, as they have two kinds
of flowers. It you gather a bunch of primroses and look into them you
will find that in some you can see the little central green ball of
the stigma, and in others at the top of the tube are the five small
anthers. These two kinds of flowers make an arrangement which ensures
that the pollen from the one kind of flower reaches the stigma of the
other. A big fly like the wasp-fly, and several others, visit these
flowers most frequently, and carry the pollen from flower A (~see~ fig.
120) to the stigma of B, and the pollen of B to the stigma of A.
[Illustration: Fig. 121. A, Flowerhead of the Daisy; (~b~) a single
little flower from the side with big petals fused together; (~c~) a
single little flower from the middle with very small petals.]
As we noticed before, the chief duty of the petals is to act as flags
to attract the visiting insects by their bright colours. Now we find
that some flowers club together, and grow clustering closely on one
head, so that it is sufficient for a few of them to have the flag
petals which attract the insect to the group, as it goes from one to
the other when once it is there. When a few of the flowers do this, the
rest can economise in petals and have quite small ones, and yet all
the same they have a good chance of insect visits. Such an arrangement
as this is found in the daisy (~see~ fig. 121). A single daisy is not
one flower, but a whole bunch of flowers, in which some of the outer
flowers of the bunch (~see~ fig. 121 (~b~)) form big petals, while all
the inner ones (fig. 121 (~c~)) are quite small and inconspicuous, and
by themselves would hardly attract any visitors. Just the same thing
happens in the cornflower, sunflower, and very many members of the
daisy family. The big outer petals attract the insect, and once on the
head of flowers it walks about over them, and they all get the benefit.
In such cases we get ~a division of labour among the flowers of a
head~, and this represents what is perhaps the highest state of
development that flowers have reached.
[Illustration: Fig. 122. Flowerhead of the Cornflower; (~a~) a single
flower from the side with big petals.]
PART IV.
THE FIVE GREAT CLASSES OF PLANTS
INTRODUCTORY
If you go out into the garden, or fields and woods in summer, and look
around you at the plants, you will find that nearly all of them are
flowering, or have flower-buds, or have the proof of having had flowers
in the shape of fruits and seeds. Even among the few which do not show
any of these things, many will probably be plants which you know to be
the same as others of their kind which you have seen flowering.
Generally flowers (such as roses and daisies) are easy to see, but in
some plants they are less showy, as in the oak, for example, where the
little green tails or catkins which come out early in the spring are
the flowers. On the whole, however, if you look carefully, you will
have no difficulty in seeing proof that ~nearly all of the conspicuous
plants of our gardens and woods bear flowers~.
All the same, there are very many other plants, some of them quite easy
to see, and others very small and inclined to hide, which do not have
flowers at all, and which are so different from the flowering plants
that even before you have studied them, you instinctively separate
them. The seaweeds or mosses, for example, are at once recognized by
any one as being of a different family from roses and lilies.
When you have studied all the plants carefully, you will see how true
is this instinctive separation of the chief families, and how nature
seems to have made five principal big families, so that both scientists
and quite unlearned people see more or less clearly the limits she has
set to each.
The family which is most highly advanced is that of the flowering
plants, but the others, too, are well worth study, and we will
now notice some of the points about their structure which are
characteristic of each of the families.
CHAPTER XXIII.
FLOWERING PLANTS
=All the plants which have flowers are put into one big family=, about
which you already know a good deal, because nearly all the plants we
have studied up to the present have been plants which have flowers. Let
us now go systematically over the chief points about their structure,
so that we may have a clear idea of their characters, and be able to
compare other families with them.
1. We find that ~the plant body is clearly marked out into root, stem,
leaves, and flowers~. The stem may be green and delicate, or it may be
thick and strong like an oak tree, and on the stem or its branches we
find the leaves.
2. The stem and root have definite strands of “water-pipe” cells, and
very often the stems have many rings of wood, one of which is added
every year.
3. The leaves are very various in the different plants, but they are
generally thin and big, though they are seldom much more compound than
those of the sensitive plant.
4. The flowers are easily recognized, as a rule, and consist of a
number of parts, some of which are often brilliantly coloured. The
stamens and carpels are generally in the same flower.
5. ~The seeds are always enclosed within the carpels~, and have
generally two seed-coats.
6. Within the seed ~are always either two cotyledons~, as in the bean,
~or one cotyledon~, as in the grasses. Thus when the seedling grows out
of the seed it may have two first leaves or one only.
These are the chief characters of the whole big family of the flowering
plants, but this big family is separated into two smaller groups
~according to the number of cotyledons in the seed~. Those that have
two form the group of ~Di~cotyledons, those with one the group of
~Mono~cotyledons. This may not seem a very important point to form
the ground for separating plants with flowers so alike as tulips and
roses, but we find that, as well as the number of cotyledons, many
other differences distinguish the two groups when we separate them
in this way. For example, the ~Dicotyledons have the veins of their
leaves so arranged as to form a network~, as in the lime, while the
~Monocotyledons have them parallel~, as we noticed in the grasses and
lilies.
We also find that it is ~only in the Dicotyledons~ that the plants have
~rings of wood in their stems~, as is the case in the lime, oak, and
many others.
In the numbers of the parts of the flower, we also find differences
between the two groups; for example, the ~Dicotyledons~ have generally
two, four or five, or a multiple of these numbers such as ten, as
we see in the poppy, primrose, rose, and many others; while the
~Monocotyledons~ have the parts of their flowers in three or multiples
of three, as in the lily, tulip, and daffodil.
These differences between the Monocotyledons and Dicotyledons, however,
are not nearly so important as their likenesses, for they agree in
the main points (1) to (6), and therefore belong equally to the great
family of the flowering plants, which is the most important family now
living.
CHAPTER XXIV.
THE PINE-TREE FAMILY
Since trees such as the oak, beech, and lime all belong to the family
of flowering plants, you may be surprised to find that the pine-trees
are separated from them. Yet all the trees like =pines, Christmas
trees, larches, and many others, form a family of their own=. You will
see why this is, if you look at a pine-tree carefully, and compare its
characters with those we saw in the flowering family. In the first
points the two families are alike.
1. We find that ~the pine-tree body is clearly marked out into root,
stem, leaves, and cones~.
2. Also that the stem and root have definite strands of “water-pipe”
cells, and that the stem has rings of wood, one of which is added every
year.
3. The leaves vary a little in the different members of the family,
but the commonest kind of leaf is the fine sharp “needle” leaf of the
ordinary pine (~see~ fig. 53). In almost all cases the leaves remain
on the tree for more than a year; they are evergreens (it is only the
larch among the English-growing members of the pine-tree family which
has new leaves every year), and the leaves are simple and strong, and
well protected.
4. ~There are no “flowers,” but the two kinds of cones which take their
place are easily recognised.~ The two kinds of cone generally grow on
different branches of the tree, the small ones only live a short time
and scatter the pollen, and the larger ones often remain two or three
years on the tree, and form the seeds. The wind scatters the pollen;
you will remember in the spring-time before the leaves are out, how the
“sulphur rain” showers down from the pine-trees; this is the yellow
pollen, which is blown in clouds on to the seed-bearing cones. There
are millions of pollen grains scattered in this way, and but few of
them ever reach a cone. You will remember that many of the flowering
plants could afford to make small quantities of pollen, as they had
special carriers in the insects to take it from flower to flower.
[Illustration: Fig. 123. A branch of Pine with a small young
seed-bearing cone, and a large ripening one.]
Besides the pollen cones, you should find two sizes of seed-cones on
the tree: some quite small, and green or pink, and some large ones
which are brown and ripe. It will be easier to see their structure at
first in the big ones; they consist of a number of brown scales packed
neatly one over another. If you pull these apart you will see that each
of them bears two seeds on its upper side.
[Illustration: Fig. 124. Larch. A and B, young scales, showing (~i~)
inner seed-bearing scale, and (~o~) outer protective one. A, side view;
B, front view; C and D, old scales, C from the side, D from the front,
showing how the inner scale increases more rapidly than the outer.]
5. ~The seeds are always seen to be lying quite openly on the upper
side of the scales, and are not covered in by closed carpels~ as they
are in the flowering plants. Each of the scales (which bears its two
seeds) corresponds in a way to the carpel in a flower, but there is
an important difference in the fact that it leaves the seeds open. In
old pine cones there seems to be only one scale to each pair of seeds,
but there is really a second smaller one outside it--which is sometimes
quite difficult to see. It shows better in the larch, where the outside
one is much the bigger of the two in the young cones, and gradually
gets left behind, as the inner scale grows very fast (~see~ fig. 124).
Notice, too, how the ripe seeds have one-sided wings, which split off
from the inner scale, as you can see if the cone is not too ripe. This
~wing is on the seed itself~, not on a fruit, as is often the case
among the flowering plants. The wing helps the seed to fly, and in the
late autumn (in many cases two years after it began to grow, for some
pines grow very slowly) it is scattered with its brothers. If you are
ever near pine trees when there has been snow, you may see it sprinkled
with these winged brown seeds.
[Illustration: Fig. 125. Winged seed of the Pine.]
6. You may never have seen a baby pine tree. If not, you must get some
seeds and grow them. They grow very slowly at first, and may take six
weeks to show above ground even in summer; but they are well worth
waiting for. Notice how they come up (~see~ fig. 126), and that at
the beginning of their growth, as they come out from the seed, they
have seven or even as many as twelve first leaves, and these leaves
are really the cotyledons, as you may see by cutting a seed across. So
that instead of the one or two cotyledons of the flowering family, we
find in the pine family that ~there are many cotyledons, and that their
number may vary from five to ten or more~.
[Illustration: Fig. 126. Stages in the growth of Pine seedlings; (~c~)
cotyledons.]
If you go back over these points, you will see that we have found a
large number of differences between the flowering plants and the pines.
Of these, the most important are the points (5) and (6), which alone
would be enough to make us place the pines and flowering plants in
separate families, though point (4) is also very important. We find,
however, that the pines are more like the flowering plants than are
any of the other families, so that they are the nearest relatives the
flowering plants have, even though they are rather far-away ones.
[Illustration: PLATE III.
TREE FERNS, SHOWING THEIR TALL THICK TRUNKS AND LARGE LEAVES, WITH
SMALLER FERNS GROWING BENEATH THEM]
CHAPTER XXV.
FERNS AND THEIR RELATIVES
=Perhaps there is no family of plants so easy to recognise as the
ferns.= It is nearly always a simple matter to know whether or not a
plant is a fern, for although there are hundreds of different kinds,
they all have the family characters plainly marked.
We have not very many ferns growing commonly in England, for they
generally require a moister air than is usual in this country. By far
the commonest is the bracken, which grows in all parts of the country,
and sometimes in very large masses (~see~ Plate I.). Some people
separate the bracken fern from the others, and speak of “bracken” and
“true ferns,” but this is not at all correct, for the bracken is just
as much a true fern as the others, only as it is so much commoner,
people are apt to value it less.
In some countries, particularly in the tropics, there are (as well as
ferns like ours) very large ferns with tall, strong, upright stems, and
crowns of large spreading leaves. Such ferns you can see in Plate III.,
and they are called tree ferns. Notice how thick the stem is, and how
large the leaves are compared with it, while the trunk seems to be all
rough and hairy, which is due to the jagged bases of the old leaves
which have fallen away. Yet even the tree ferns are easily recognised
as belonging to the fern family.
Let us examine ferns in order to find out what are the points about
them which are specially characteristic for their family, and which
help us to separate them from the other plants.
1. We find that ~the fern body is clearly marked out into roots, stem,
and leaves, but there appear to be neither flowers nor cones~.
2. The stem and roots have definite “water-pipe” cells, as you can see
if you examine a thin slice with your magnifying-glass, but there are
never rings of wood formed year by year, as in the higher families. The
stems are frequently short and stumpy, and often run underground. They
are usually covered by the rough leaf-bases of old leaves and by dry
scales.
3. The leaves are generally few in number, often only three or four,
but they are highly compound, and are split up into very many side
leaflets. They are generally thin and delicate. When they are young
they are rolled up in the bud in a close coil (~see~ fig. 127), and as
they unfold they bend back. This way of coiling up is quite a special
character of ferns. The buds are generally covered with flaky, shining
scales, which stick all over the young leaf-stalk.
[Illustration: Fig. 127. Young leaf of a Fern rolled in a close coil.]
4. ~You have never seen a fern with flowers or seeds~, yet there are
always plenty of new ferns every year. How are the young ones formed?
For a long time botanists did not know, so that people thought there
was some magic about it, but now we know the whole story, and it is
a very interesting one.
5. ~There are no seeds~, and
6. Therefore there are ~no seedlings to have cotyledons~.
You must have noticed little dark brown spots on the backs of some
fern-leaves. It is in them that you must look for the beginning of
the new fern plants. The little patches are at first hidden by green
coverings, but when they are ripe these bend back, and expose the
little brown clusters within. If you look at one of these ripe patches
with a magnifying-glass, you may be able to see a number of little
roundish boxes on stalks. Each of these contains a number of tiny
“spores” (which are ~single cells~ with the power to grow), and when
the spore-cases are ripe they open and shoot out the spores, as you may
perhaps see if you look closely at a ripe patch when it is taken into
warm, dry air.
[Illustration: Fig. 128. A small piece of Fern leaf showing the patches
of spore-cases on the under side.]
These brown patches are not at all like flowers, but in some way they
do the work of flowers, for they give rise to cells which can carry on
the life of the fern to a second generation. The way in which they do
it, however, is totally different from that of the seed, and is quite
the most special character of the ferns and their relatives.
The spores grow slowly when they come on to moist earth, but as their
development takes a long time, you had better get some from a gardener
which have already grown. As the spore grows, the one cell composing
it divides and divides again, until there is formed a little filmy
heart-shaped green structure called a ~prothallium~ (~see~ fig. 129),
which is not in the least like a fern plant, for it is not more than a
quarter of an inch across. It has no stem or leaves, and is only a thin
layer of green cells, with a few root-hairs on the under side. Two of
the cells formed on this little structure then unite and begin to grow
while still attached to it, and finally they grow into the form of a
very small, simple fern plant (~see~ fig. 129). So that between the
old fern plant and the little fern “sporeling” (for we cannot call it
a seedling) we find a whole new structure, the prothallium, which is
quite different from the usual fern plant. This curious alternation of
fern,--prothallium,--fern, and then again prothallium, is what we call
“alternation of generations,” and is very characteristic indeed of the
fern tribe.