Part 26

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.