Chapter XI: Part V: Leaves (2)

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.