Part 7

If you want to know just where the stomata are situated in a leaf, plunge it in water, then shake the drops off and notice what part of the leaf has not been wet. Wherever the leaf is dry, there are the stomata. In many plants, as, for instance, the jewelweed, it is quite impossible to wet the leaf. Soak it in water for an hour, and when you take it out it is dry! The parts that cannot be wet usually have a silvery, glistening appearance. Put the leaf in water and notice where it glistens; there are the stomata,—sometimes all over the under side of the leaf, sometimes in lines or patches, sometimes on both sides of the leaf.

Wax, gum, and resin are not the only things plant glands secrete. There are the glands in the flower cups that secrete nectar. In some plants this breaks through the delicate plant skin and runs into and fills up the little hollows or horns we call nectaries. In others the nectar is provided with stomata by means of which it can escape from the interior of the plant.

You may be surprised to learn that the flower is not the only part of the plant that can secrete nectar!

In some plants the stipules do it, and in some even the stems.

This is not to call visitors to the flowers, but perhaps to keep them away. Where ants trouble the flowers, certain kinds have invented this very clever way of stopping the unwelcome visitors. They do not want the ants to take the honey from the flowers, so they secrete honey on the leaves or stems, and the ants take that instead of traveling on to the flowers.

Of course each living skin cell contains protoplasm. The protoplasm lies in a thin layer against the walls and builds, builds, builds, until the skin is thick enough.

When a good thick wall has been built, the protoplasm passes out through tiny openings in the inner wall into the inside cells, where it goes to work doing something else. The skin cells are then empty of protoplasm; they are only filled with air, and we say they are _dead_ cells. Their hard walls are a good protection to the plant. In stems there is often a layer of thick cells behind the skin cells which also protects. These are called cork cells.

All very young plants have their stems covered with living skin.

Older plants, particularly woody ones, have their stems covered with the tough, dead skin. And trees have finally a thick layer of dead cork cells. In tree trunks the skin cells have disappeared entirely. The skin protected the young shoot; then its empty cells finally peeled off, as the cork cells formed underneath and made a thick bark. The bark then does the work of the skin. It protects the stem. It becomes very thick sometimes, as layers are constantly added beneath. The outside of the bark keeps peeling and scaling off.

Of course there are no stomata in bark. We find them only in the living skin. Bark does not need stomata, as it does not regulate the water supply. The young green parts of the plant do that by means of their covering of living skin. Living skin is usually transparent like glass.

It is tough and yet transparent. You see, the light must get through it to the cells which lie behind it.

There is usually no green color in skin. Sometimes there are other coloring materials, though not as a rule.

The living skin covers the leaf or stem or other part of the plant like a window of tough glass. Even where the skin is several cells thick, the light can pass through, just as it can through thick glass.

TUBE CELLS.

The top of a tree is a long way from the roots. Yet the leaves must have food from the roots, and the roots must have food from the leaves.

It is not an easy matter to move all this food material up and down, you may be sure.

I wonder how _you_ would manage it?

Why, you say, if I had to raise sap from under the ground to the top of the tree, I should certainly build some pipes and have a pump at the top.

That is the way the plant has decided. So pipes there are, plenty of them,—pipes or tubes of many sizes and shapes.

You know how cells grow, lying next each other. Well, tube cells are long and contain protoplasm in the beginning. They lie end to end. But, you see, it would not be very easy for the sap to pass through _millions_ of cell walls on its way up.

So when the protoplasm has built a row of cells with good thick walls, it passes out through thin places or openings it has left in the walls. The end partitions between the tube cells are thin and break away, and lo and behold! we have a long, strong tube with nothing in it but air. Up this tube the sap creeps or down it the sap runs. A great many of these tubes, which are as fine as hairs or much finer in some cases, are needed in a plant. They run all through the stems and out into the leaves. They are collected into bundles, and form part of the veins and the framework of leaves. I do not know what the plant would do without them.

But what makes the sap run _up_ the tubes?

Now you are asking questions! It took a long time for people to find that out, for there is more than one reason why the sap runs up.

For one thing, the root cells keep drawing in water and other things, and the fluid already in is pushed up by that behind; so there is a sort of pump at the bottom of the plant, you see,—a force pump. The sun shining on the leaves and stems evaporates the water above, and the water below then easily takes its place; so there is a sort of suction pump at the top.

Then the tubes are so _very_ fine that the fluid in them tends to move up, just as water will soak up into a towel if the fringe happens to get into the water; for you know that if you hang a towel so that the fringe dips into a basin of water, after awhile the whole towel will be wet, as a result of what we call capillary attraction. For all these reasons the sap creeps up the stems through the tubes the cells have made.

Every plant has these tubes, from the tiniest weed in the garden to the tallest forest tree. Although so small, they are often very prettily marked by lines and dots.

STRENGTHENING CELLS.

Plants need something more than cells of working protoplasm and something more than tubes, just as we need more than flesh and blood vessels.

We would be in a sad plight if we had no bones to keep us in place, and plants would be in a sad plight if they had no—well, not exactly _bones_, but something to serve the same purpose.

Think of the weight a tree has to bear. You could not _begin_ to lift the crown of a large tree, yet the tree trunk has to hold it up in the air. Not only that,—it has to hold on to it when the wind blows, which is a much harder task. Even small bushes and tender garden plants have quite a weight to bear and quite a task to keep their leaves and stems from being blown away. They could _never_ hold on to them if it were not for the wood and other tough cells they have,—never in the world.

These wood cells and other tough cells are made by protoplasm, of course.

The protoplasm builds them very much as it does the tube cells, long and slender, as you see in the picture at the beginning of the chapter, and then when the hard, tough walls are all done, the protoplasm slips out and leaves the strong framework of tough fibres to do its duty. This framework is not only strong, it is elastic, so it can bend easily. If it were not, the first strong wind or the first thing that happened to bend the plant would snap it off short.

You cannot break wood easily, and, if you do succeed, it always bends more or less first. Some wood bends more easily than others, as you know. A willow twig can be tied into a knot, it bends so easily.

Nearly all land plants have these stiffening cells. They run out of the stems down into the leaves and help make their framework of “veins.” The tubes and the strengthening fibres run along in bundles side by side. You see this saves space. If the tubes and strengthening fibres each took a different road, that would not leave much space for the chlorophyll and other working cells. But all the tubes and fibres are closely packed together and run lengthwise, through the stem. All around these long fibres are placed the other cells which are not long and do not form tubes or fibres. Most of those other cells in the leaf contain chlorophyll. They contain protoplasm, and do the work of transforming food materials into plant material.

WE AND THE PLANT PEOPLE.

We live and the plants live. Probably neither we nor the plants spend much time thinking about what we owe to each other.

The plants are excusable for this, for they are not great thinkers, at least so far as we know.

But we owe so much to them, we ought to stop and think about it once in a while. We are indebted to them not only for the food we eat, but for the air we breathe.

We know about chlorophyll and the starch it makes, and how this starch is stored up in potatoes and wheat and corn and rice and all sorts of food grains and vegetables.

We know, too, how the roots suck up substances from the earth which we need in our bodies, and how they are stored away with the starch or sometimes by themselves. We know, in short, how all the food we eat is made first or last by the plants. Not only do we owe our food to the plants, but all animals do.

You see, animal cells are not able to take carbon dioxide and water and ammonia and other gases and minerals and work them up into living cells.

The plants have to do this for them; and then the animals eat the plants, for animal cells are able to work starch and sugar and plant protoplasm over into animal protoplasm, which can build all sorts of animal cells. So all the animals in the world get their food from the plant world. If the plants were to stop living, all the animals in the world would soon starve to death. The word “animals,” you know, means every living thing that is not a plant; in this sense flies and bees and oysters and caterpillars are animals as well as dogs and cats and such large creatures. Last of all, we ourselves are animals.

So the animal world would be in a sad predicament if anything should happen to the plants.

But there is more to thank the plants for than food. That is a pretty large item certainly; but what do you think of having to thank them for the air we breathe as well? Yet this we shall have to do if we begin thanking them at all.

You know about oxygen, of course. It is one of the gases that make up the air; and I may as well remind you that air is composed principally of oxygen and nitrogen gases,—about four times as much nitrogen as oxygen, but the oxygen is the most important to us. We do not use the nitrogen in the air at all probably. It serves the purpose of diluting the oxygen, which would be too strong for us if it were not mixed with nitrogen. But what we do use is the oxygen.

That goes into our lungs, and some of it does not come out again. It passes into the lung cells and from them into the blood, and is carried by it all over our bodies to all the millions of cells.

We need a great deal of oxygen, and if the supply should be cut short we would die.

All animals need oxygen; even the worms in the ground and the fishes and oysters in the water must have it. So great quantities are being used up all the time.

Now, you know, when the plants pull carbon dioxide to pieces, they keep the carbon and return the oxygen to the air. In this way we get it to breathe.

But there is more than this to the matter in hand. We are all the time breathing out carbon dioxide as an impurity; so are all the millions upon millions of animals in the world.

The air might in time contain enough carbon dioxide to kill us if there were not some way of getting rid of it. You know what that way is.

The plants use it up. So by giving oxygen into the air and taking out carbon dioxide, the plants keep the air fit for us and all animals to breathe.

But there is more than this we have to thank them for.

They shade the earth and regulate the rainfall and the water supply.

Where forests grow there are always streams of water, and the large water courses are kept full the year round.

The Mississippi River depends upon the far-away forests for its broad stream.

The spreading crowns of the trees shade the earth and prevent the water which falls as rain or dew from evaporating rapidly. It collects into streams and flows through the land, keeping the earth fresh and beautiful.

More than this,—large forests cause the rain to fall and the dew to collect. Their leaves condense the moisture in the air and cause it to fall as rain or be deposited as dew.

When people recklessly cut down the forests in a country, the water courses dry up, and even the largest rivers are affected.

When the spring rains fall over a country whose trees have been cut away, the water rushes down the little streams all at once and causes a terrific flood in the large rivers. It soon drains away; then the rivers fall lower and lower until they nearly dry up. This state of affairs is a great calamity, because the people can no longer raise crops on the land near where the old forests stood, for it is parched and dry months at a time.

Moreover, boats laden with coal and grain and all sorts of things can no longer pass up and down the rivers, because the water is too low.

People ought to think of these things and not destroy too much forest land. After awhile we shall have to go to work and plant trees instead of cutting them down or burning them; but it takes a long time for trees to grow, and a wiser way would be for us to take care of those we have.

You have heard a great deal about plants eating and the good they do us by eating the carbon dioxide in the air. They take this in through their leaves, and you remember they take in all their other food materials—water, nitrogen compounds, sodium, potassium, magnesium, and many other substances—through their roots.

But they do more than eat; they also breathe.

They breathe everywhere over the surface of their bodies where there are stomata or where the skin is not too thick for the air to penetrate it.

And I must tell you they breathe just as we do,—that is, they take in air, use the oxygen, and give off the carbon dioxide.

It seems rather inconsistent of them to take in carbon dioxide as food and throw it off as a waste at the same time, but that does not trouble _them_; they do not care whether they are consistent or not. And it is true they take in carbon dioxide and give off oxygen, and take in oxygen (in the air) and give off carbon dioxide, in one breath as it were.

You see, it is different parts of protoplasm at work that does this; one part—that in the chlorophyll bodies—is attracting carbon dioxide, breaking it up, and casting out oxygen. Other protoplasm in the cells outside the chlorophyll bodies attracts and uses the oxygen, while the carbon dioxide comes to the stomata from different parts of the plant as a waste material, just as it comes to the cells of our lungs to be cast out.

So plants, by breathing, make the air a little impure, but they destroy or break up so much more carbon dioxide than they make that on the whole they act as powerful purifiers of the air.

When we think of the great forests of the tropics, all overgrown with luxuriant vegetation, we may remember that those tangles of vines and trees and strange growths are our friends no less than the grass and bushes in our dooryard.

For there is a carrier always at work bringing the pure air to us and carrying away the impure air which we create. This carrier is the air currents. The great winds sweep about the earth, bearing the oxygen from the forests to the crowded cities, and sweeping away the carbon dioxide from the cities to the fields and woods. The winds, too, stir up the water where the water plants and fishes live, and help keep it full of air for the things in it to breathe; the tides and currents help, so as far down in the water as there are living things, you may be sure there is air for them to breathe. There would not be air enough for you, because you need so much; but for them there is plenty.

Swirling around the earth go the winds, carrying the oxygen to the people and the carbon dioxide to the plants, for the plants are as glad to get the carbon dioxide we breathe out as we are to get the oxygen they give off.

And we are glad, when we come to think about it, that we are able to give them something in return for all they give to us.

You see, we need each other,—plants and people, and the winds are friends to us both.

WHAT ARE THE FLOWERS MADE OF?

I think flowers are “made of sugar and spice and everything nice.” At least, if it is not that, it is something very like it, as I have good reason to believe.

What flowers and all other parts of the plant are made of depends upon protoplasm; and if protoplasm can make sugar and spice and build up flowers that way, we should like to know it.

We _do_ know about sugar and how the little green chlorophyll people run their starch factories in all the green parts of the plant,—under the skin of stems sometimes as well as of leaves, for wherever a stem is green, we may be sure chlorophyll is at work making starch in it. And we know how the protoplasm in the different cells changes the starch into sugar.

We know, too, how wood and other tough substances are made of starch.

But there is something else in plants as important as starch and very different,—the protoplasm. Protoplasm itself is not made entirely of starch; it requires materials not found in starch.

These materials are nitrogen, sulphur, and phosphorus.

Nitrogen is the most important, and this the plant gets chiefly through the roots.

Nitrogen is found in the earth combined with hydrogen and other substances. The protoplasm tears to pieces these nitrogenous substances which the roots suck up, and so enables the plant to take the nitrogen.

The other two substances which the protoplasm needs, sulphur and phosphorus, the plant gets partly from the air and partly from the earth.

Sulphuric acid exists in _very_ small quantities in the air and goes in through the stomata, attracted, no doubt, by the protoplasm inside. But other sulphurous and phosphorous compounds are taken up by the roots.

So we see protoplasm is complicated. It contains carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus united in a very complicated way.

Although protoplasm itself is made only of carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus, it can make use of a great many other things. When the protoplasm of certain cells wants to build hard, tough walls, it uses potash and soda or even silica, which you know glass is made of. Just draw a blade of sedge grass through your fingers if you want to feel the silica in it. You will probably cut your fingers, but that will help make you remember about silica. Then the protoplasm uses iron to color the petals and other parts of the plant. It uses magnesia, too, and salt and lime and a number of other materials for building walls or making dyes or something else.

Every material in our own bodies is found in plants, and sometimes the plants have materials that we do not have.

Of course materials are put together differently in plants from what they are in us. When Mother Nature combines her carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, magnesia, iron, and all the other things to make a plant, she does not go to work as she would if she were going to make an animal.

Just what the difference is it would be difficult to tell, but there _is_ a difference.

Plants contain a good deal of sugar as a rule, and if you remember cloves you will admit that at least _some_ flowers are made of spice, for cloves are the dried flower buds of the clove tree.

Cinnamon is the bark of a plant, and if you are acquainted with orange trees you will be willing to say they are “made of sugar and spice and everything nice,” for the whole tree, wood, bark, stems, leaves, flowers, and fruit, is fragrant and spicy.

Oil is another common substance in plants, and it is made from the materials of starch which, as we know, are carbon, hydrogen, and oxygen; cotton-seed oil, olive oil, and castor oil we are all familiar with.

All nuts contain a great deal of oil, and the skin of a fresh-picked orange is so full of it that it runs down our fingers when we cut the orange.

All the things in a plant—starch, sugar, oils, spices, wood, bark—everything is made by the wonderful protoplasm in the cells.

Starch and the food taken up by the roots pass through all parts of the plant by the sap tubes, and as the sap goes along, each living cell draws into itself the substances from the sap that it needs, and these it combines into the things it wants to make. Some of the cells in an orange skin, for instance, attract out of the sap the materials to make the fragrant, stinging oil that fills the fresh skin, while other cells attract the materials to build the white cottony covering inside the outer skin, and so the cells in each part of the plant take out what they need to build with.

WHAT BECOMES OF THE FLOWERS?

Early in the spring the snowdrops and crocuses peep out, and then they go away.

We do not think much about it, for other flowers have come in their places.

Spring beauties and bloodroots shine in the woods, and then they go away. But the mandrakes have come with their umbrella leaves, and then the columbines and roses ask for a welcome.

After awhile we can find no more mandrakes and columbines, only yellow apples and brown seed-pods.

Jack-in-the-Pulpit jumps up quite early in the summer, and then we cannot find him, only in the late summer we sometimes come across little clusters of bright red berries lying on the ground.

We would scarcely suspect them of having any relation to Jack, yet they are his berries. But what has become of Jack?

In the autumn the rose leaves fall off, and there is left only red stems and red berries.

The morning-glory vine wilts and turns black at the first frost; it sinks to the ground and we see it no more, or else its stems linger brown and hard for a time, but in the end it all disappears. What has become of it?

And the nasturtiums—what a wreck the frost makes of them! The leaves are wilted and black; the stems, too, are soft and lie flat on the ground.

Why, you say, the frost has killed them. But that does not at all tell what has become of them. Besides, the frost did not kill the snowdrops and crocuses and blood roots and spring beauties nor Jack-in-the-Pulpit nor the umbrella leaves of the mandrakes. Yet they are all gone. All we can find of Jack and the mandrakes are red berries and yellow apples. Not a sign of the snowdrops or spring beauties or crocuses is left.

If you will just step down with me under the earth a few inches I will show you something.

Make believe you are a gnome or a fairy and can see as well in the dark earth as anywhere else and come along. Now look about.

Did you ever dream of anything so cunning in all your life? Everywhere and everywhere old mother earth is packed full of little white and brown bulbs.

There they are as snug as peas in a pod, thousands of them, in every direction as far as you can see.

And besides these bulbs, there are thick, fleshy root stems, red and brown and yellow, everywhere and everywhere. Do you want to know who they are?

They are our little friends of the early summer,—snowdrops and crocuses and spring beauties and dogtooth violets; mandrakes, too, and Jack-in-the-Pulpit.