[Illustration: Decoration]
HOW TO LOOK AT A LEAF
To-day we begin to learn what we can about the leaves of plants. I hope
that enough fresh green leaves have been brought to school to allow
every child here to have one on the desk before him, so that he may
see, as far as is possible, just what is being talked about.
This picture (Fig. 133) shows you the leaf of the quince. Now, suppose
you held in your hand a leaf fresh from the quince tree, and were asked
to describe it to a blind person, how would you tell about it?
[Illustration: FIG. 133]
You would begin, I fancy, by saying, “This leaf is green,” for the
color of an object is one of the things we notice first.
Next perhaps you would describe its shape. “This quince leaf is
rounded, yet it is too long to be called a round leaf.” So you would
use the word “oblong.”
Like most leaves, then, it is green; and like some other leaves, it is
oblong.
Now look at this picture (Fig. 134) of the maple leaf. This, you see,
is not oblong, but three-pointed.
What other differences do you notice between these two leaves?
You do not seem quite sure as to whether there are any other
differences. But do you not notice that the edge of the maple leaf is
cut into little teeth, like the edge of a saw, while the edge of the
quince leaf is smooth?
And let me tell you here, that when I speak of a leaf, I speak not only
of the enlarged flat surface we call the “leaf blade,” but also of the
“leafstalk.” A leaf usually consists of a leafstalk and a leaf blade,
though some leaves are set so close to the stem that they have no room
for any stalks of their own.
[Illustration: FIG. 134]
“Oh! then,” you answer, “the leafstalk of the maple is much longer than
that of the quince, and there are little bits of leaves at the foot of
the quince leafstalk which the maple is without.”
You have done very well, and have noticed just those things which you
should.
The shape of the leaf blade, whether the edge of this is toothed, the
length of the leafstalk, and whether this has any little leaves at its
foot where it joins the stem, are things always worth noticing.
Now take your leaves and hold them up against the light, or else look
at the picture of the quince leaf, and study carefully the make-up of
the blade.
You see that this is divided lengthwise by a heavy rib which juts out
on the lower side. From this “midrib,” as it is called, run a great
many smaller lines. These are called “veins.” And from these branch off
still smaller veins that bear the name of “veinlets.” And somewhat as
the paper is stretched over the sticks of a kite, so from the leaf’s
midrib to its edge, and from vein to vein, is drawn the delicate green
material which makes up the greater part of the leaf.
What I wish you to learn this morning is, _how to look at a leaf_.
Before using our brains rightly, we must know how to use our eyes. If
we see a thing as it really is, the chances are that our thoughts about
it will be fairly correct.
But it is surprising how often our eyes _see wrong_.
If you doubt this, ask four or five of your playmates to describe the
same thing,—some street accident, or a quarrel in the playground, which
all have seen, or something of the sort,—and then I think you will
understand what I mean by saying that few people see correctly.
THE MOST WONDERFUL THING IN THE WORLD
It would be quite a simple matter to interest you children in plants
and their lives, if always it were possible to talk only about the
things which you can see with your own unaided eyes.
I think a bright child sees better than many a grown person, and I
think that it is easier to interest him in what he sees.
And then plants in themselves are so interesting and surprising, that
one must be stupid indeed if he or she finds it impossible to take
pleasure in watching their ways.
But about these plants there are many things which you cannot see
without the help of a microscope, and these things it is difficult to
describe in simple words. Yet it is necessary to learn about them if
you wish really to feel at home in this beautiful world of plants.
After all, whatever is worth having is worth taking some trouble for;
and nothing worth having can be had without trouble. So I hope when you
children come to parts of this book that seem at first a little dull,
you will say to yourselves, “Well, if we wish really to know plants, to
be able to tell their names, to understand their habits, we must try to
be a little patient when we come to the things that are difficult.”
For even in your games you boys have to use some patience; and you are
quite willing to run the risk of being hurt for the sake of a little
fun.
And you girls will take no end of trouble if you happen to be sewing
for your dolls, or playing at cooking over the kitchen stove, or doing
something to which you give the name “play” instead of “work.”
I only ask for just as much patience in your study of plants; and
I think I can safely promise you that plants will prove delightful
playthings long after you have put aside the games which please you now.
So we must begin to talk about some of the things which you are not
likely to see now with your own eyes, but which, when possible, I will
show you by means of pictures, and which, when you are older, some of
you may see with the help of a microscope.
Every living thing is made up of one or more little objects called
“cells.”
Usually a cell may be likened to a tiny bag which holds a bit of that
material which is the most wonderful thing in the whole world, for this
is the material which has _life_.
Occasionally a cell is nothing but a naked bit of this wonderful
substance, for it is not always held in a tiny bag.
[Illustration: FIG. 135]
This picture (Fig. 135) shows you a naked plant cell, much magnified,
that swims about in the water by means of the two long hairs which grow
from one end of the speck of life-giving material.
The next picture (Fig. 136) shows you a seed cut across, and so
magnified that you can see plainly its many cells.
In the middle portion of the seed the cells are six-sided, and laid
against one another in an orderly and beautiful fashion, while the
outer ones are mostly round.
All animals, we ourselves, all plants, began life as a single cell.
Sometimes a cell will spend its life alone. When the time comes for it
to add to the life of the world, it divides into two or more “daughter
cells,” as they are called. These break away from one another, and in
like manner divide again.
But usually the single cell which marks the beginning of a new life
adds to itself other cells; that is, the different cells do not break
away from one another, but all cling together, and so build up the
perfect plant or animal.
By just such additions the greatest tree in the forest grew from a
single tiny cell.
[Illustration: FIG. 136]
By just such additions you children have grown to be what you are, and
in the same way you will continue to grow.
Every living thing must eat and breathe, and so all living cells must
have food and air. These they take in through their delicate cell
walls. The power to do this comes from the bit of living substance
which lies within these walls.
This strange, wonderful material within the little cell is what is
alive in every man and woman, in every boy and girl, in every living
thing, whether plant or animal.
We know this much about it, and not the wisest man that ever lived
knows much more.
For though the wise men know just what things go to make up this
material, and though they themselves can put together these same
things, they can no more make life, or understand the making of it,
than can you or I.
But when we get a good hold of the idea that this material is contained
in all living things, then we begin to feel this; we begin to feel
that men and women, boys and girls, big animals and little insects,
trees and flowers, wayside weeds and grasses, the ferns and rushes of
the forest, the gray lichens of the cliffs and fences, the seaweeds
that sway in the green rock pools, and living things so tiny that our
eyes must fail to see them,—that all these are bound into one by the
tie of that strange and wonderful thing called life; that they are all
different expressions of one mysterious, magnificent idea.
While writing that last sentence, I almost forgot that I was writing
for boys and girls, or indeed for any one but myself; and I am afraid
that perhaps you have very little idea of what I am talking about.
But I will not cross it out. Why not, do you suppose?
Because I feel almost sure that here and there among you is a girl or
boy who will get just a little glimmering idea of what I mean; and
perhaps as the years go by, that glimmer will change into a light so
bright and clear as to become a help in dark places.
But the thought that I hope each one of you will carry home is
this,—that because this strange something found in your body is also
found in every other living thing, you may learn to feel that you are
in a way a sister or brother, not only to all other boys and girls, but
to all the animals and to every plant about you.
[Illustration: Decoration]
HOW A PLANT IS BUILT
Now we know that the plant, like yourself, began life as a single cell;
and we know that the perfect plant was built up by the power which this
cell had of giving birth to other cells with like power.
Suppose that a brick were laid upon the earth as the foundation of a
wall; and suppose that this brick were able to change into two bricks.
Suppose that the new brick were able to form another brick in the
same manner, and that this power should pass from brick to brick; and
suppose that all these bricks were able to arrange themselves one upon
another in an orderly fashion, so that they could not easily be moved
from their places.
Now, if you can see this brick wall growing up, you can see something
of how the cells of a plant grow up and arrange themselves.
But though it is fairly easy to see how the plant cells form one from
another, that does not explain how they come to make a plant, with its
many different parts, with its root and stem, its branches, leaves, and
flowers.
One thing can divide and make two things of the same sort; but it is
not easy to see how it can make things that are quite different from
itself.
Now, if this difficulty as to the building-up of plants and animals has
come into your minds, you are only puzzled by what has puzzled hundreds
of people before you; and all these hundreds of people have found the
puzzle quite as impossible to solve as the king’s horses and the king’s
men found it impossible to put Humpty Dumpty together again.
A good many questions that we cannot answer come into our minds; but if
we look honestly for the answers and do not find them, then we can be
pretty sure that for the present it is safe to leave them unanswered.
As cell is added to cell in the building-up of plant life, some
wonderful power forces each new cell to do the special work which is
most needed by the growing plant.
Sometimes this new cell is needed to help do the work of a root, and so
it begins to do this work, and becomes part of the root; or else it is
needed to do stem work, and goes to make up the stem, or leaf work, and
is turned over into the leaf.
A healthy cell is born with the power to do whatever is most needed.
[Illustration: Decoration]
HOW A PLANT’S FOOD IS COOKED
Some time ago we learned that the little root hairs, by means of their
acid, are able to make a sort of broth from the earthy materials which
they could not swallow in a solid state.
But before this broth is really quite fit for plant food, it needs even
more preparation.
Why do we eat and drink, do you suppose?
“Because we are hungry.” That is the direct reason, of course. But we
are made hungry so that we shall be forced to eat; for when we eat, we
take into our bodies the material that is needed to build them up,—to
feed the cells which make the flesh and bone and muscle.
And this is just why the plant eats and drinks. It needs constantly
fresh nourishment for its little cells, so that these can live, and
grow strong enough to make the new cells which go to form, not bone and
flesh and muscle, as with you children, but fresh roots and stem and
leaves and flowers and fruits.
If these little cells were not fed, they would die, and the plant would
cease to live also.
And now what do you think happens to the broth that has been taken in
from the earth by the root hairs?
As we have said, this broth needs a little more preparation before it
is quite fit for plant food. What it really wants is some _cooking_.
Perhaps you can guess that the great fire before which all plant food
is cooked is the sun.
But how are the hot rays of the sun to pierce the earth, and reach the
broth which is buried in the plant’s root?
Of course, if it remains in the root, the earth broth will not get the
needed cooking. It must be carried to some more get-at-able position.
Now, what part of a plant is usually best fitted to receive the sun’s
rays?
Its leaves, to be sure. The thin, flat leaf blades are spread out on
all sides, so that they fairly bathe themselves in sunshine.
So if the broth is to be cooked in the sun, up to the leaves it must be
carried.
And how is this managed? Water does not run uphill, as you know. Yet
this watery broth must mount the stem before it can enter the leaves.
Water does not run uphill ordinarily, it is true; yet, if you dip a
towel in a basin of water, the water rises along the threads, and the
towel is wet far above the level of the basin.
And if you dip the lower end of a lump of sugar in a cup of coffee, the
coffee rises in the lump, and stains it brown.
And the oil in the lamp mounts high into the wick.
Perhaps when you are older you will be able somewhat to understand
the reason of this rise of liquid in the towel, in the lump of sugar,
in the lamp wick. The same reason accounts partly for the rise of the
broth in the stem. But it is thought that the force which sends the
oil up the wick would not send the water far up the stem. And you know
that some stems are very tall indeed. The distance, for example, to
be traveled by water or broth which is sucked in by the roots of an
oak tree, and which must reach the top-most leaves of the oak, is very
great.
Yet the earth broth seems to have no difficulty in making this long,
steep climb.
Now, even wise men have to do some guessing about this matter, and I
fear you will find it a little hard to understand.
But it is believed that the roots drink in the earth broth so eagerly
and so quickly, that before they know it they are full to overflowing.
It is easier, however, to enter a root than it is to leave it by the
same door; and the result is, that the broth is forced upward into the
stem by the pressure of more water or broth behind.
Of course, if the stem and branches and leaves above are already full
of liquid, unless they have some way of disposing of the supply on
hand, they cannot take in any more; and the roots below would then be
forced to stop drinking, for when a thing is already quite full to
overflowing, it cannot be made to hold more.
But leaves have a habit of getting rid of what they do not need. When
the watery broth is cooked in the sun, the heat of the sun’s rays
causes the water to pass off through the little leaf mouths. Thus the
broth is made fit for plant food, and at the same time room is provided
for fresh supplies from the root.
If you should examine the lower side of a leaf through a microscope,
you would find hundreds and thousands of tiny mouths, looking like the
little mouths in this picture (Fig. 137).
[Illustration: FIG. 137]
Some of the water from the earth broth is constantly passing through
these mouths out of the plant, into the air.
A STEEP CLIMB
It is all very well, you may think, to say that the pressure from the
root sends the water up through the stem; but when we cut across such a
stem as a tree trunk, one finds it full of wood, with a little tightly
packed soft stuff in the center, and not hollow like a water pipe, as
one would suppose from all that has been said about the way the water
rises in the stem.
No, a stem is not a hollow pipe, or even a bunch of hollow pipes, it is
true; and it does seem something of a question, how the water can force
its way through all this wood; and even if one hears how it is done,
it is not an easy thing to make clear either to grown people or to
children. But I will see what I can do; and I know that you really love
these plants and trees, and will try to be a little patient with them
and with me.
The water, or liquid, when it mounts a stem or tree trunk, takes a path
that leads through the new-made cells. Each young cell wall is made of
such delicate material that it allows the water, or broth, to filter
through it, just as it would pass through a piece of thin cloth. And so
it makes its way from cell to cell, along the stem, more slowly than
if it were passing through a hollow tube, but almost as surely. It is
true, the earth broth does not reach the leaves above without having
given up something to the little cells along the road. These seem to
lay hold of what they specially need for their support, while the rest
is allowed to pass on.
I want your teacher to prove to you by a little experiment that water
makes its way up a stem.
If she will place in colored water the stem of a large white tulip,
cutting off its lower end under the liquid, those parts whose little
cells are in closest connection with the stem will soon begin to change
color, taking the red or blue of the water; for a freshly cut stem has
the same power as the root to suck in water eagerly and quickly.
[Illustration: Decoration]
HOW A PLANT PERSPIRES
We cannot see the water as it passes from the tiny leaf mouths into the
air. Neither can we see the water that is being constantly carried from
the surface of our bodies into the air. But if we breathe against a
window pane, the scattered water in our breath is collected by the cold
of the glass in a little cloud; and if we place the warm palms of our
hands against this window pane, in the same way the cold collects the
water that is passing from the little mouths in our skin, and shows it
to us as a cloud on the glass.
Heat scatters water so that we cannot see it, any more than we can see
the lump of sugar when its little grains are scattered in hot water;
but cold gathers together the water drops so that we are able to see
them.
This is why you can “see your breath,” as you say, on a cold day. The
cold outside air gathers together the water which was scattered by the
heat of your body.
If you place against the window pane the under side of the leaves of a
growing plant, the water passing from the tiny leaf mouths collects on
the glass in just such a damp cloud as is made by the moist palms of
your warm hands.
When water passes from your hands, you say that you are perspiring;
and when water passes away from the plant, we can say that the plant
perspires. Some plants perspire more freely than others. A sunflower
plant has been known to give off more than three tumblers of water a
day by this act of perspiration.
There is a tree, called the _Eucalyptus_, whose leaves perspire so
freely that it is planted in swampy places in order to drain away the
water.
Of course, the more quickly the leaves throw off water, the faster the
fresh supply pushes up the stem.
If the leaves do their work more quickly than the roots make good the
loss, then the plant wilts.
When a leaf is broken from a plant, it soon fades. Its water supply
being cut off, it has no way of making good the loss through the leaf
mouths.
Just as the air in a balloon keeps its walls firm, so the water in the
leaf cells keeps the cell walls firm.
As a balloon collapses if you prick it with a pin, and let out the air,
so the cell walls collapse when the cells lose their water; and when
the cell walls of a leaf collapse, the leaf itself collapses.
[Illustration: Decoration]
HOW A PLANT STORES ITS FOOD
We see that the water is drawn away from the earth broth into the air
by the heat of the sun, just as water is drawn from the broth we place
on the stove by the heat of the fire; and that when this has happened,
the plant’s food is cooked, and is in condition to be eaten.
But this broth does not lose all its water. There is still enough left
to carry it back through the leaf into the branches and stem, and even
down into the root once more.
In fact, the prepared food is now sent to just those parts of the plant
which most need it.
Perhaps it is laid up beneath the bark, to help make new buds which
will burst into leaf and flower another year.
Or perhaps it goes down to help the roots put out new branches and
fresh root hairs.
Or possibly it is stowed away in such an underground stem as that of
the lily, or the crocus bulb, and is saved for next year’s food. Once
in a while some of this prepared food is stored in the leaf itself.
When a leaf is thick and juicy (“fleshy,” the books call it), we can
guess that it is full of plant food.
Do you recall the _Bryophyllum_,—the plant we talked about a few days
ago? Its wonderful leaves, you remember, gave birth to a whole colony
of new plants.
You may be sure that these leaves had refused to give up all the food
sent to them for cooking in the sun. You can guess this from their
thick, fleshy look, and you can be sure of this when you see the baby
plants spring from their edges; for without plenty of nourishment
stored away, these leaves could never manage to support such a quantity
of young ones.
LEAF GREEN AND SUNBEAM
But the earth broth which the roots supply is not the only article of
importance in the plant’s bill of fare.
The air about us holds one thing that every plant needs as food.
This air is a mixture of several things. Just as the tea we drink is a
mixture of tea and water, and milk and sugar, so the air is a mixture
of oxygen and nitrogen, and water and carbonic-acid gas.
Oxygen, nitrogen, and carbonic-acid gas,—each one of these three things
that help to make the air is what we call a gas, and one of these gases
is made of two things. Carbonic-acid gas is made of oxygen and carbon.
Now, carbon is the food which is needed by every plant. But the carbon
in the air is held tightly in the grasp of the oxygen, with which it
makes the gas called carbonic-acid gas.
To get possession of this carbon, the plant must contrive to break up
the gas, and then to seize and keep by force the carbon.
This seems like a rather difficult performance, does it not? For when a
gas is made of two different things, you can be pretty sure that these
keep a firm hold on each other, and that it is not altogether easy to
tear them apart.
Now, how does the plant meet this difficulty?
You cannot guess by yourselves how this is done, so I must tell you the
whole story.
Certain cells in the plant are trained from birth for this special
work,—the work of getting possession of the carbon needed for plant
food. These little cells take in the carbonic-acid gas from the air;
then they break it up, tearing the carbon from the close embrace of the
oxygen, pushing the oxygen back into the air it came from, and turning
the carbon over to the plant to be stored away till needed as food.
Only certain cells can do this special piece of work. Only the cells
which hold the green substance that colors the leaf can tear apart
carbonic-acid gas. Every little cell which holds a bit of this leaf
green devotes itself to separating the carbon from the oxygen.
Why this special power lies in a tiny speck of leaf green we do not
know. We only know that a cell without such an occupant is quite unable
to break up carbonic-acid gas.
But even the bit of leaf green in a tiny cell needs some help in its
task. What aid does it call in, do you suppose, when it works to wrench
apart the gas?
In this work the partner of the bit of leaf green is nothing more or
less than a sunbeam. Without the aid of a sunbeam, the imprisoned leaf
green is as helpless to steal the carbon as you or I would be.
It sounds a good deal like a fairy story, does it not,—this story of
Leaf Green and Sunbeam?
Charcoal is made of carbon. About one half of every plant is carbon.
The coal we burn in our fireplaces is the carbon left upon the earth
by plants that lived and died thousands of years ago. It is the carbon
that Leaf Green and Sunbeam together stole from the air, and turned
over into the plant.
If one looks at a piece of coal with the eyes which one keeps for the
little picture gallery all children carry in their heads, one sees more
than just a shining, black lump. One sees a plant that grew upon the
earth thousands of years ago, with its bright green leaves dancing in
the sunlight; for without those green leaves and that sunlight, there
could be no coal for burning to-day. And when we light our coal fire,
what we really do is to set free the sunbeams that worked their way so
long ago into the plant cells.
It is more like a fairy story than ever. Sunbeam is the noble knight
who fought his way into the cell where Leaf Green lay imprisoned,
doomed to perform a task which was beyond her power. But with the aid
of the noble Sunbeam, she did this piece of work, and then both fell
asleep, and slept for a thousand years. Awakening at last, together
they made their joyful escape in the flame that leaps from out the
black coal.
In truth, a sunbeam and a flame are not so unlike as to make this story
as improbable as many others that we read.
And because I have told it to you in the shape of a fairy story, you
must not think it is not true. It is indeed true. Everywhere in the
sunshiny woods and fields of summer, the story of Leaf Green and
Sunbeam is being lived. But when the day is cloudy or the sun sets,
then there is no Sunbeam to help the Princess, and then no carbon is
stolen from the air.
PLANT OR ANIMAL?
Did you ever stop to ask yourself, “What is the difference between a
plant and an animal?” because this is the place where that question
should be answered.
“Why, an animal is _altogether_ different from a plant,” you answer,
perhaps a little scornfully. “I have no trouble in telling which is
which.”
It is very natural that you should feel this way. A cow or a horse, for
example, is not at all like a tree; and when you think of animals, you
think of the ones you know best, and likewise of plants.
But wise men have discovered plants that look and act so much like
animals, and animals that look and act so much like plants, that at one
time they say, “Now, these are animals, surely,” and a little later
exclaim, “No, after all, these are plants;” and they take a long time
to make up their minds as to whether certain objects are plants or
animals.
And already even you children have discovered that the plants you know
best belong to families, and have children, and care for them in a very
motherly fashion; that they drink earth food with their roots, and eat
carbon food with their leaves; and soon you will find that they do many
other things which once upon a time you would have thought it a great
joke to be told a plant could do.
You remember my telling you of one little plant cell that could swim;
and there are some animals, you know, that are rooted to one spot as
we usually think only a plant is rooted.
What, then, _is_ the difference between a plant and an animal?
Leaf Green and Sunbeam between them put life into what had no life
before; and the living plant matter, which they help to make, is that
which animals cannot make themselves, yet which they cannot live
without, for this living matter is absolutely necessary to them as food.
And the one real difference between a plant and an animal is this,—a
plant can make out of certain dead substances the living matter that
all animals must have for food; an animal cannot do this.
HOW WE ARE HELPED BY LEAF GREEN AND SUNBEAM
The cell in which Leaf Green lives has no little mouths such as we saw
in the picture some time ago.
Its walls are so delicate that the carbonic-acid gas passes through
them quite easily,—as easily as the gas escaping from an unlighted jet
in the schoolroom could pass to your nose even if you wore a veil, or
as easily as water would pass through a piece of muslin.
But between Leaf Green’s cell and the outer air are other cells,—those
which make up the outer covering or skin of the leaf. These are
arranged so as to form the openings or mouths about which we have read.
By means of these mouths the gas makes its way through the leaf’s thick
skin.
The plant needs as food the carbon in this gas, and so keeps fast hold
of it; but the oxygen is not needed for this purpose, and so it is
pushed back into the air.
Now, we learned in the last chapter of one very great service rendered
to animals by plants. We learned that plants took carbon from the air,
and turned this into food for animals.
But there is still another way in which plants serve animals. And
once more it is the work of Leaf Green and Sunbeam that is of such
importance to us; for when they take hold of the carbon, making it into
living food for man and beast, they take from the air the gas that
is poisonous, and send back into the air the gas which gives life and
health.
This poisonous gas which they lay hold of, you remember, is
carbonic-acid gas; and carbonic-acid gas is what we animals send out
of our bodies with every breath, for it is the part of the air which
poisons us. When the schoolroom is so close that our heads ache, it is
because so many children have been breathing out this gas, and we are
forced to take it back into our bodies again.
But when this gas is stolen by the plant, and robbed of its carbon, it
is no longer carbonic-acid gas. Nothing of it is left but the oxygen
which is pushed out through the cell walls; and this oxygen is as good
to breathe as the other gas (carbon and oxygen mixed) is bad.
So the plant finds good what we find poisonous. It takes in and keeps
that which hurts us (the carbon), and sends out that which helps us
(the oxygen).
So you see that our lives depend on the lives of plants in two ways:—
1. The plants give us the food we need for life.
2. The plants take from the air the gas that poisons us, and give to
the air the gas which we need for life and health.
And in both cases it is Leaf Green and Sunbeam who are making life
possible for us.
Remember the great services of these two fairies when next you pass a
green tree which is bathing itself in sunshine.
HOW A PLANT BREATHES
Perhaps you have heard people say that it is not good to sleep in a
room with plants.
They say this, because they have heard that at night the plant does not
give out oxygen, but that it does give out the poisonous carbonic-acid
gas.
Now, you children know that part of this statement is true.
You know that the plant cannot give out oxygen at night, because at
that time there is no Sunbeam about to help Leaf Green tear apart
carbonic-acid gas and send the oxygen back into the air.
But how about the other part of the statement?
Is it true that at night plants give out the poisonous carbonic-acid
gas?
Both day and night, plants give out carbonic-acid gas; for though
plants, save in the sunlight, cannot eat by means of their little green
cells, they can breathe through the tiny mouths (Fig. 137) on the under
side of the leaf by night as well as by day.
And when either a plant or an animal breathes, it takes the life-giving
oxygen from out the air mixture, and keeps it for its own use. But
poisonous carbonic-acid gas is sent back into the air. Now, the
question is, whether a plant does most good or most harm to the air by
taking in and sending out the different gases.
Of course, it does good when it lets the oxygen out through its cell
walls, and stores away the carbon within itself; and it may seem to do
harm when through its leaf mouths it breathes in oxygen and breathes
out carbonic-acid gas.
There is only one key to unlock the matter, and that is this,—to find
out whether the plant does most towards poisoning or towards purifying
the air.
And that has been found out already.
Wise men say that Leaf Green and Sunbeam do much more good to the air
than the little breathing mouths do harm. The two good fairies take
away a great deal of poison, and send back a great deal of the helpful
oxygen; while the tiny mouths neither rob the air of much oxygen nor
give it much poison. Indeed, the harm they do is so small compared with
the great good accomplished by Leaf Green and Sunbeam, that even at
night you need not worry at the thought that you have plants in your
room.
Perhaps you wonder that a plant does these two things that are so
exactly opposite to each other.
But a plant must breathe as well as eat; for when it breathes, it takes
in the precious oxygen which is just as necessary to its life as to
ours.
In summer, by the dusty roadside, you see plants almost white with
dust, looking quite ill and lifeless.
And they are both ill and lifeless; for their little leaf throats are
so choked that they cannot breathe in the oxygen they need, and in
consequence they are being slowly suffocated.
THE DILIGENT TREE
Now we have learned three things about plants, and especially about
leaves. We have learned—
1. That they perspire.
2. That they eat and drink.
3. That they breathe.
They perspire when the water passes through the leaf mouths into the
air.
They eat when Leaf Green and Sunbeam together manage to take the carbon
out of the carbonic-acid gas which has made its entrance through the
leaf mouth and the cell wall. They drink when the roots suck in water
and earth broth.
They breathe when the leaf mouths take from the air the oxygen, and
give back to it carbonic-acid gas.
The veins and veinlets, of which you see so many running through a
leaf, act in something the same way as the water pipes of a city; for
through these veins the watery food, the earth broth, is carried to the
different cells.
When one knows all that we know even now about a plant, one looks at a
tree covered with leaves with a good deal of admiration.
Just think of what is being done inside that quiet-looking tree! Think
of the millions of cells that go to make it up, each cell having its
own work to do! Think of the immense amount of business being carried
on within the trunk, inside the branches, and especially in each green
leaf! And when you have the chance, notice how hard each leaf tries to
get just as much sun and air as it possibly can.
In the first place, the thin, flat leaf blades are so spread out that
every part is exposed to the light and air.
Then notice how the leaves are placed in reference to one another.
Almost every single one is fastened to the tree so as to get its fair
share of sunshine.
When you think of the many thousands of leaves borne by one tree, it
astonishes you to see how seldom one leaf gets in another’s light.
And the shapes of leaves are always suited to their arrangement on the
tree.
If you should take the leaves of a chestnut tree and replace them by
the leaves of a maple, you would find the maple leaves all getting in
each other’s way, or else you would see that they were taking up a
great deal more room than necessary.
But when a leaf is studied on its own tree, one sees that its shape is
the very best that could be imagined for its position.
And in the smaller plants we notice this same thing.
And when you remember that Leaf Green cannot feed the plant unless
Sunbeam comes to her assistance, you realize how necessary it is that
each leaf be within the reach of Sunbeam’s visits.
LEAVES AND ROOTS
You will be surprised to learn that the way in which a plant’s leaves
grow tells us something of the way in which its roots grow.
Many of you have been overtaken far from home in a rainstorm, and have
sought shelter under a spreading tree. The ground directly beneath the
tree has kept almost dry even after some hours of rain, but the earth
just under the tips of the spreading branches got very wet: for the
great tree acted like a large umbrella; and when the raindrops fell
upon the smooth leaves, which sloped outward and downward, they rolled
from leaf to leaf till they reached the very lowest, outermost leaves
of all. From these they fell to the ground, just as the drops that
gather upon your umbrella run outward and downward to the umbrella’s
edge, and then off upon the ground.
So you can see that the circle of earth which marks the spread of the
branches above must be specially wet, as it received a great part of
the rain which fell upon the whole tree.
And whenever you see a tree which sheds the rain water in such a
circle, you can be pretty sure that, if you should dig into the earth a
ditch which followed this circle, you would soon reach the tips of the
new root branches of the tree.
You know that the root does the drinking for the plant; and only the
newest parts of the root, the fresh root tips, are really good for work
of this sort. You remember that the earth food is carried up the stem
to the leaves in a watery broth; and that if the water supply should
give out, the new plant cells would not get the broth which helps them
to grow, and to put out other cells, and so to build up the plant.
Now, as only the new root branches, near their tips, are able to
drink, if the water should leak through the earth in equal quantities
everywhere, much of it would be wasted; but when this water is
collected in certain spots within reach of the new root branches, there
is good reason to believe that these will be able to satisfy their
thirst.
By the shedding of the rain from the tips of the spreading branches
above, the water is collected in a ring, and so sinks into the earth;
and the root branches below spread out in just the same direction as
the tree branches above, till they find what they need, and drink their
fill.
[Illustration: FIG. 138]
So by the way in which a tree sheds the rain, you can tell just where
its root branches reach out underground.
In smaller plants you see much the same thing. Fig. 138 shows a plant
called the _Caladium_. You can see that the raindrops must roll outward
down these leaves, and fall upon the earth just above the tips of the
root branches.
Fig. 139 shows you the rhubarb plant. This has quite a different sort
of root. Now, if the rhubarb leaves were like those of the _Caladium_,
unless the rhubarb root-branches changed their direction, these
root-branches would grow very thirsty indeed.
[Illustration: FIG. 139]
But as it is, the water pours down these leaves toward the center of
the plant, and reaches the ground almost directly over the straight,
fleshy root, with its downward-growing branches; and we see that these
root-branches are watered by the leaves above just as carefully as are
those of the _Caladium_.
By knowing one thing about a plant, often you can guess that another
thing is so.
You understand now that when the leaves of a plant shed rain water
after the fashion of the _Caladium_, the chances are that its
root-branches spread out as far as the drip of the water; and that the
root of the rhubarb points almost straight downward, is told you by the
drip of water from the rhubarb leaves.
LEAF VEINS
Some time ago you learned that from the stem of a plant you could guess
the number of seed leaves which it brought into the world, and that in
the same way from the seed leaves you could guess what kind of a stem
it would build up.
From the way in which a leaf is veined you can guess both of these
things. You can guess what sort of a stem belongs to the plant, and
with how many seed leaves it began life.
When the little veins run in and out, forming a sort of network, we say
that the leaf is “net-veined.”
[Illustration: FIG. 140]
[Illustration: FIG. 141]
[Illustration: FIG. 142]
These leaves of the quince (Fig. 140), the maple (Fig. 141), and the
basswood (Fig. 142) are all net-veined.
Net-veined leaves are borne by plants which brought into the world more
than one seed leaf; and with the net-veined leaf we can expect to find
that stem which comes with more than one seed leaf,—a stem where the
skin or bark, the woody rings, and the soft central pith, are clearly
separated one from another.
[Illustration: FIG. 143]
[Illustration: FIG. 144]
But a leaf such as that in Fig. 143 or that in Fig. 144, where the
veins do not branch off in a network, but run in unbroken lines side by
side,—such leaves as these tell you that they are borne by plants which
started life with only one seed leaf, and which have such a stem as the
cornstalk, where you see no woody rings or central pith.
These leaves are called “parallel-veined.”
I fear that you find all this a little difficult to understand and to
remember; but if you read it patiently, when you study the botany for
older children, I think it will come back to you and make your lessons
easier.
[Illustration: Decoration]
LEAF SHAPES
As I told you before, we should notice always the shape of a leaf.
[Illustration: Leaves]
It is much easier to describe some new plant we have met on our walks
if we remember the shape of its leaves.
Next summer I hope you will make a collection of leaves, pressing and
keeping them. I think you will be amazed at their great variety in
shape.
[Illustration: FIG. 145]
[Illustration: FIG. 146]
Some you find long and narrow, others almost round. Some are
arrow-shaped, others star-shaped, others needle-shaped (Fig. 145). Some
are three-pointed like the maple leaf (Fig. 146); others deeply-parted,
like the oak leaf (Fig. 147).
Sometimes a large leaf is cut up into several little leaves. These
little leaves are called “leaflets.”
[Illustration: FIG. 147]
[Illustration: FIG. 148]
The clover leaf (Fig. 148) has three leaflets.
[Illustration: FIG. 149]
The locust leaf (Fig. 149) is cut into a great many leaflets.
The edge of one leaf (Fig. 150) is smooth, while that of another is cut
into little teeth (Fig. 151) like the teeth of a saw.
[Illustration: FIG. 150]
[Illustration: FIG. 151]
I should like to know how many of you children, without looking even at
a picture save such as you carry in that little gallery in your head,
could describe correctly the shapes of some of our common leaves. I
should like to ask you to draw on the blackboard the rough outlines of
any leaves that you remember. If you think you could not do this, will
you not try, when next you see a leaf, to carry off in your mind such
a picture of it as to enable you to outline it on the blackboard when
you go back to the school room?
Really it does not take any more time to see a thing correctly than to
see it incorrectly. It takes a little more sense, that is all.
It takes some sense to give even one minute of honest thought to the
thing you are looking at.
You know some children who never seem to have all their thoughts in one
place at a time, and who in consequence never see anything really well.
It is better to stop doing a thing altogether than to do it in a
foolish sort of way; and it is foolish to start to do even the smallest
thing, and yet not do it.
The child who looks at even a leaf in a way to make it possible for
him to draw the outline of that leaf five minutes later, is likely to
be the child who goes in for both work and play with all his heart,
and who comes out as far ahead on the playground as he does in the
schoolroom.
Now, after that lecture, which some of you need badly enough (and which
I will tell you, as a great secret, I need not a little myself), I want
to point out a few more of the things that are worth noticing in a leaf.
But perhaps it is better to save them for another chapter.
HAIRY LEAVES
Notice always whether a leaf is smooth or hairy. Do you remember the
mullein that sends up its tall spires over the hill pasture? The
grayish leaves of this mullein are so hairy that they feel almost like
wool. What is the use of all this hair? It is not likely that a plant
would wrap itself in this hairy coat except for some good reason.
It is believed that this coating of the mullein prevents animals from
eating the leaves, and so destroying the plant. In the mouth, these
hairs slip from the leaf blade, and cause a most unpleasant sensation.
But usually the hairs on a leaf are helpful because they prevent too
much perspiration or giving-off of water. The more freely the hot sun
beats upon a leaf, the more quickly the water is drawn away from it.
You can see just how this is by hanging a wet towel in front of the
fire. In a very short time the heat from the burning coals draws the
water from the towel. But put a screen between the fire and the towel,
and the water passes off more slowly.
Now, the hairs on that side of the leaf which faces the sun act as a
screen from its fierce heat. We have learned how important it is that
the leaf should not part with its water more quickly than the roots can
make up the loss. We know that when a leaf does this, it wilts just as
a leaf wilts when it is picked and cut off from its water supply, on
account of the collapse of the walls of the many little cells which are
emptied of water.
So you can understand that plants which grow in dry, sunny places,
where there is little drinking water for the roots, and where the sun
beats constantly on the leaves, must take every care that there is no
waste of water.
And if you keep your eyes open, you will discover that many of the
plants which grow in such places screen themselves from the full heat
of the sun by a coat of hairs.
The plant called “life everlasting” is one which grows in dry, open,
sunny places. It clothes its leaves with silky hairs, and so prevents
them from throwing off too quickly the small amount of water its roots
are able to provide. Without this silky coat, the sun would suck its
leaves quite dry of water.
Sometimes a leaf has only a few of the little leaf mouths through which
most of the water passes. As these mouths are wide open only in the
sunlight, and as often the rest of the leaf is covered with a thick
skin which prevents the water from slipping away (as a little of it
nearly always does) through the cell walls, such a leaf will hold its
water supply and keep fresh for a long time. Such leaves as these we
find on what we call “evergreen” plants. The pines and hemlocks which
light up the woods all winter have these thick-skinned, few-mouthed
leaves, which throw off so little water that even when the ground is
frozen hard, and gives no drinking water to the roots, they are able to
keep fresh by the careful way in which each one hoards its own little
supply.
WOOLLY AND “DUSTY” LEAVES
Curiously enough, some plants put on a hairy coat for just the opposite
reason from the one which makes life everlasting clothe itself in that
fashion. Life everlasting fears lest its leaves throw off their water,
or perspire too quickly.
Down by the stream that runs through the meadow grow great clusters of
the pink-flowered steeple bush. If you look at the lower sides of the
leaves of the steeple bush, you see that they are very woolly. As this
wool is not between the sun and the leaf blade, it cannot be meant to
protect the leaves from the heat of the sun; and indeed in this wet
meadow, close to the river, never mind how quickly the leaves throw
off their water, the roots can have no difficulty in finding close by
more than enough to make good the loss. No, the fact is that these
leaves need to throw off water very freely indeed to make room for
the ever-fresh supply that is pushing up the stem, and their woolly
covering is intended to help them do this very thing. Its object is
to aid perspiration. In swampy places the moisture rises every night
from the wet ground, and settles on the plants about. The little mouths
on the under surfaces of the leaves of the steeple bush would soon be
clogged with the moisture rising from below, if they were not protected
in some way; and if they became so clogged, they could not throw off
the water with which the whole plant is charged. Thus, by having this
thick coat of hair, the water that otherwise would cling to the outer
surface of the leaf blade is kept at a distance from the little mouths,
and these are not interrupted in the performance of a duty so necessary
to the health of the plant.
This same habit of coating its lower leaf surfaces with hair, you
notice in the speckled or swamp alder, a shrub which grows also in wet
places, and therefore runs the same risk of having its leaf mouths
clogged with water.
So when you see only the upper surface of a leaf covered with hair,
you can guess that the object of the plant is to prevent too much
perspiration; but when you see only its lower side clothed in this same
way, you can guess that the plant fears too little perspiration.
Sometimes you find a plant with leaves which have a coating of what
looks almost like dust on one or both of their surfaces. This dust we
call “bloom.” We see it in apples and grapes, as well as on leaves. It
is made up of a waxy material which is put forth by the plant just as
it puts forth hair. This bloom the plant uses also as a help to free
perspiration. By thus clothing its leaves it shields the little mouths
from water clogging; and so you can be sure that the little mouths have
not been filled with water, and thus prevented from doing their work.
The cabbage leaf has mouths on both of its surfaces, and so both sides
are covered with this protecting bloom. If you dip a cabbage leaf in
water and then shake it, the drops roll off and leave it quite dry.
PRICKLES AND POISON
Leaves need to protect themselves from other enemies than too great
heat and too much water.
We found that the prickly armor of the thistle enabled it to live in
pastures where the cattle had killed most of the other plants.
Many animals like to eat green leaves, so we are not surprised to find
that plants invent different ways of protecting themselves.
One look at the leaf of the thistle is enough to persuade us that it
would not be very good eating.
[Illustration: Prickers]
The red-berried holly, with which we decorate our churches and houses
at Christmas time, is another plant with prickly leaves.
Some plants cover their leaves with bristles, which the cattle dread
almost as much as the stout prickles.
As we read before, the mullein defends its leaves by a fuzzy coat of
hair. Such an armor as this is less warlike than that of the thistle,
but quite as effective.
Other plants fill their leaves with juices which are either poisonous
or unpleasant.
It seems as if animals guessed the presence of these unfriendly juices
by the plant’s smell, for they will munch the different growing things
all about such a plant as this, and leave the harmful leaves severely
alone.
The nettles cover their leaves with stinging hairs. These stiff hairs
break off when handled, burying themselves in the flesh, and sending
out a burning acid that punishes severely the meddler, man or beast, as
it may happen to be.
By this time I think you realize that leaves are well worth noticing.
And when you have looked at a leaf so fully as to be able to carry in
your mind its outline, I hope you will then discover whether it wears
a coat of hair, or a dusty bloom, or a prickly armor, or a thick,
evergreen skin, and that you will decide what enemies it is trying to
escape.
[Illustration: Decoration]
SOME CRUEL TRAPS
Have you ever seen a leaf like the one in this picture (Fig. 152)?
It is shaped something like a pitcher; and the plant on which it grows
has been named the “pitcher plant.”
The pitcher plant lives in low, wet place, such as the shaded swamp, or
the marsh down by the lake.
On account of its curious leaves it is brought to the cities, and is
sold on the street corners or at the florists’.
In June comes the great flower of the pitcher plant. Sometimes this is
a dull red; again it is a delicate pink or perhaps a light green; and
it has faint, pleasant fragrance.
[Illustration: FIG. 152]
Next June I hope that some of you children will find these beautiful
flowers and these curious leaves.
Why should a leaf be shaped like a pitcher, do you suppose?
These leaves are not only pitcher-like in shape, but also in their
way of holding water; for if you succeed in discovering a settlement
of pitcher plants, you will find that nearly every pitcher is partly
filled with rain water. Usually this water is far from clear. It
appears to hold the remains of drowned insects; and sometimes the
odor arising from a collection of these pitcher plants is not exactly
pleasant.
Perhaps you wonder how it happens that dead insects are found in every
one of these pitchers; and possibly you will be surprised to learn that
apparently these curious leaves are built for the express purpose of
capturing insects.
It is easy to understand that these odd leaves are not so well fitted
as more simple ones to cook the plant’s food in the sun, or to take
carbon from the air; but if they are unfitted to provide and prepare
ordinary food, possibly they are designed to secure food that is
extraordinary.
It seems likely that the pitcher plant is not content to live, like
other plants, upon the simple food that is taken in from the earth
and from the air. We are led to believe that it wishes something more
substantial; that it needs a _meat_ diet; and that to secure this, it
teaches its leaves to capture flies and insects in order that it may
suck in their juices.
These leaves are veined in a curious and striking fashion. The
bright-colored veins may convince the insects of the presence of the
sweet nectar in which they delight. At all events, in some way they
are tempted to enter the hollow leaf; and, once they have crawled or
tumbled down its slippery inner surface, they find it impossible to
crawl back again, owing to the stiff hairs, pointing downward, which
line the upper part of the pitcher.
Even if they have wings, it is difficult for them to fly upward in so
straight a line as would be necessary to effect their escape.
When tired out in their efforts to get out of this cruel trap, they
fall into the water at the bottom of the pitcher, and are drowned.
Their bodies decay and dissolve; and it is thought that this solution
is taken in by the leaf, and turned over to the plant as food.
It is just the old, sad story of the spider and the fly, you see, only
now it is the pitcher and the fly.
But be sure to examine one of these pitchers if you possibly can, and
then you will understand better how the whole thing is managed.
The leaf in this picture (Fig. 153), for it is a leaf, you cannot find
in our North American swamps. It grows on a plant called _Nepenthes_, a
plant which lives in hot countries far from the United States.
[Illustration: FIG. 153]
The leaf in the picture is full grown, and all ready for its work of
trapping animals. Before it was old enough to do this, the lid which
is now lifted was laid nicely across the opening to the pocket, and so
prevented any unseasonable visits.
Sometimes these pockets are so large as to be able to hold and to hide
from sight a pigeon. They are gayly colored, and the rim around their
border is covered with a sugary, tempting juice. So you can guess
that the animals in search of nectar are not slow in accepting the
invitation offered by color and sweets, and that some of these are
imprudent enough to venture across the sticky edge. In this event they
are pretty sure to lose their footing on the slippery inner surface
of the pocket, and to fall into the watery liquid with which it is
filled. Even if they do not slip immediately, their efforts to crawl
back over the rim are defeated by a row of teeth such as you see in the
picture.
The liquid at the bottom of the leaf is not rain water, as in the
pitcher plant. It is given out by the leaf itself; and it contains an
acid which dissolves the animals’ bodies, so that their more nourishing
parts can easily be taken in by certain little cells which line the
lower part of the pocket, and which have been brought up to this work.
[Illustration: FIG. 154]
The next picture (Fig. 154) shows you a water plant. It is called the
“bladderwort,” because of the little bags or bladders which you see
growing from the branches under water. The little bladders are traps
set for water animals, which swim into them in their wish, perhaps, to
escape some enemy. But they are quite unable to swim out again; for the
door into the bladder is transparent, and looks like an open entrance
with a nice hiding place beyond. It opens easily from the outside, but
is so arranged that it will not open from within. So when the poor
little animal hurriedly swims into what seems to it a cozy resting
spot, and draws a long breath of relief at getting safe inside, it is
hopelessly caught, and must slowly starve to death, for there is no
chance of escape. It may live for nearly a week in this prison; but at
last it dies. Its body decays, and is taken in as food by the cells set
apart for that purpose.
Strangely enough, though we ourselves do not hesitate to kill animals
for food, and sometimes, I am sorry to say, for nothing but amusement,
we give a little shiver of disgust when we find these plants doing the
same thing. Some lines that came out in one of the magazines a few
years ago express this feeling:—
“What’s this I hear
About the new Carnivora?
Can little plants
Eat bugs and ants
And gnats and flies?
A sort of retrograding!
Surely the fare
Of flowers is air,
Or sunshine sweet.
They shouldn’t eat
Or do aught so degrading.”
MORE CRUEL TRAPS
The plants about which we read in the last chapter do not take any
active part in capturing insects. They set their traps, and then keep
quiet. But there are plants which lay hold of their poor victims, and
crush the life out of them in a way that seems almost uncanny.
This leaf (Fig. 155) belongs to a plant which lives in North Carolina.
It is called Venus’s flytrap.
[Illustration: FIG. 155]
You see that the upper, rounded part of the leaf is divided by a
rib into two halves. From the edges of these rounded halves run out
a number of long, sharp teeth; and three stout bristles stand out
from the central part of each half. When an insect alights upon this
horrible leaf, the two halves come suddenly together, and the teeth
which fringe their edges are locked into one another like the fingers
of clasped hands.
The poor body that is caught in this cruel trap is crushed to pieces.
Certain cells in the leaf then send out an acid in which it is
dissolved, and other cells swallow the solution.
After this performance the leaf remains closed for from one to three
weeks. When finally it reopens, the insect’s body has disappeared, and
the trap is set and ready for another victim.
The next picture (Fig. 156) shows you a little plant which is very
common in our swamps,—so common that some of you ought to find it
without difficulty next summer, and try upon it some experiments of
your own.
[Illustration: FIG. 156]
It is called the “sundew.” This name has been given to it because in
the sunshine its leaves look as though wet with dew. But the pretty
drops which sparkle like dew do not seem so innocent when you know
their object. You feel that they are no more pleasing than is the bit
of cheese in the mouse trap.
When you see this plant growing in the swamp among the cranberry vines
and the pink orchids, you admire its little white flowers, and its
round red-haired leaves, and think it a pretty, harmless thing. But
bend down and pluck it up, root and all, out of the wet, black earth.
Carry it home with you, and, if you have a magnifying glass, examine
one of its leaves.
The picture (Fig. 157) shows you a leaf much larger than it is in life.
The red hairs look like pins stuck in a cushion, and the head of each
pin glistens with the drop that looks like dew.
[Illustration: FIG. 157]
But the ants and flies do not take these drops for dew. They believe
them to be the sweet nectar for which they long, and they climb or
light upon the leaves in this belief.
And then what happens?
The next two pictures will show you (Figs. 158, 159).
[Illustration: FIG. 158]
[Illustration: FIG. 159]
The red hairs close slowly but surely over the insect whose legs are
already caught and held fast by the sticky drops it mistook for nectar,
and they hold it imprisoned till it dies and its juices are sucked in
by the leaf.
I should like you to satisfy yourselves that these leaves act in the
way I have described. But a bit of fresh meat will excite the red
hairs to do their work quite as well as an insect, and I hope in your
experiments you will be merciful as well as inquiring.
So you see that the little sundew is quite as cruel in its way as the
other insect-eating plants. But its gentle looks seem to have deceived
the poet Swinburne, who wonders how and what these little plants feel,
whether like ourselves they love life and air and sunshine.
“A little marsh-plant, yellow-green
And tipped at lip with tender red,
Tread close, and either way you tread,
Some faint, black water jets between
Lest you should bruise its curious head.
“You call it sundew; how it grows,
If with its color it have breath,
If life taste sweet to it, if death
Pain its soft petal, no man knows,
Man has no sight or sense that saith.”
THE FALL OF THE LEAF
You know that in autumn nearly all the leaves fall from the trees. To
be sure, a few trees (such as the pines and hemlocks) and some plants
(such as the laurel and wintergreen and partridge vine) do hold fast
their leaves all winter; but these are so few as compared with the many
plants which lose their leaves, that they hardly count.
Perhaps you never stopped to wonder why most plants get rid of their
leaves before winter comes on; but you feel pretty sure now that there
is some good reason for a habit that is adopted by nearly all the
plants that live in this part of the country.
When we were talking about the way in which leaves defend themselves
from different dangers, we found that evergreen leaves, the leaves
which hold fast to the tree and keep fresh all winter, manage to keep
their water safe inside their cells by wearing a very thick skin, and
by not having too many little leaf mouths. For when a leaf has a thin
skin and a great many mouths, its water leaks away very quickly. And
if many such leaves should remain upon a plant into the winter, might
it not happen that they would let off all its water at a time when its
roots could not find any more in the frozen ground? And thus might not
the leaves kill the plant by draining it quite dry?
So you can see why it is well for most plants to shed their leaves
before winter comes on and the root’s drinking water is turned into
ice.
But when a plant is about to shed its leaves, it takes care not to
waste the precious food which they hold. This food it draws back into
its stem and roots, laying it away in safe places beneath the buds
which are to burst another year.
It is this action on the part of the plant which changes the color of
the leaves every fall. That material which makes them green is broken
up, and part of it is taken away. That which is left is usually yellow
or brown or reddish, and gives the leaves the beautiful colors we see
in our October woods.
So whenever you see the woods changing color, losing their fresh green
and turning red and yellow, you can be sure that the trees have begun
to prepare for winter. You know that they are stowing away their food
in warmer, safer places than can be supplied by the delicate leaves.
And when all the food has been drawn out of the leaves, and packed away
in the right spots, then the plant finishes a piece of work it began
some time before. This piece of work is the building-up of a row of
little cells just where the leafstalk joins the stem or branch. When
this row is complete, it acts almost like a knife, loosening the stalk
from the stem.
Then the leaf’s life work is over; and with the first breeze, the empty
shell, which is all that is left, breaks away from the parent plant,
and drifts earthward.