Part 23

Just suppose that plants had no strong roots twisting around stones and bits of earth underground and holding them fast! What a time there would be whenever the wind blew.

Even a light breeze would be worse than a cyclone at present, for it would send the wheat in the wheatfields flying before it.

All the plants would go hurry-skurry wherever the wind blew—excepting the morning-glories and others that were twined about trellises or fences or rocks; and even they would be blown all out of shape.

And when a strong wind came, if the trees had no roots to anchor them they would go hurry-skurry in the direction in which the wind blew, even if they were balanced so that they could not fall over; and we should see the forests sliding about the country and probably right on our houses, knocking them down, so we would not be able to have any houses, but would have to live in caves. It is a very good thing for us that the plants are held fast by their roots.

Well, the root hairs do the most important work of the plant after all. It is they who go poking their noses through the soil, and with their cells draw up water and potash and nitrogen and sulphur and iron and many other things which have become dissolved in the water. They are even able to dissolve rocks and such delicacies for themselves.

Now a growing root tip is a very delicate thing. You could not expect it to go pushing its tender tip through the hard earth without some kind of protection. And it does not: it wears a cap. This cap fits over the tip of the root and is hard. The cap is not alive, that is, the outside of it is not. The growing part of the root tip is just behind the cap.

The root tip grows by adding on new cells and so pushes the root cap ahead of it. The hard root cap finds its way between the particles of earth and so opens a channel for the growing root tip behind it.

The cap wears off on the outside as the bark does on a tree, and, like that, is continually renewed from the inside where the cells are alive.

SKIN CELLS.

Skin covers over and protects what is underneath. It is thin compared with what it covers, but it is important, as we discover when we lose a piece of our own skin. A fluid substance or even blood oozes out, and the spot where the skin is off is very painful.

Plants have a skin too, and it does for them what our skin does for us. It is tough and protects the soft inner parts and keeps the sap from oozing out.

Skin, of course, is built up of cells. These cells generally lie close together, touching each other, except at certain spots, where there is an opening.

Skin cells are usually long and wide, and their outer walls, as you would expect, are thicker than the inside walls. The protoplasm builds up hard material on the outside to protect the rest of the leaf or stem. Leaves and young stems and roots and flower parts all have skin.

The skin is alike in all in a general way, just as all houses are alike in a general way. They all have a roof, walls, partitions, doors, and windows, though these are of different sizes and arranged differently in different houses to suit the needs of the people who live in them. So with plants. The skin cells are different in size and shape and thickness in different plants to suit the needs of the plants, though in all there is a general resemblance.

Here is a row of skin cells (a) with other cells (b) back of them. See how thick the skin cells are on the outside (c). They are very tough there too. d is an opening between two cells, and all is magnified several hundred times.

Sometimes there are several layers of skin cells where the plant needs a particularly thick skin; a in the illustration is an example of such a skin.

But it would not do to have an air-tight skin, even for a plant.

Our own skins are full of holes, or pores, as you know, to let out the extra water and other waste materials in what we call perspiration. The plants need such an arrangement as much as we do. So in their skin we find pores. You see the plant needs a great deal of water. The water is used in making the substance of the plant. It is also used in the sap to carry food about from place to place. Sap contains a great deal of water in order that it may flow easily. This water cannot all be used by the plant, and when it comes up from the roots in the sap a large part of it has to be got rid of by the leaves.

If the skin were solid, the water could not escape. But you know what protoplasm can do.

If the skin needs pores, it will make them. And this is how it does it.

If you peel off a bit of skin from the under side of a leaf and put it under the microscope, you will see something like this.

The round forms are the pores. The crooked lines between are the edges of the cell walls, and you are looking at them right through the outer wall of the skin, which is transparent like glass, otherwise you could not see the edges of the partitions.

Let us look at these pores, or stomata as we must call them, if we want to talk like botanists.

One of the stomata is called a “stoma”; stoma comes from the Greek and means a “mouth,” or “opening.” These little mouths, or stomata, are made of two cells lying close together. These cells reach through the skin into an open space back of it.

There are open spaces between many of the inner plant cells, and there is always one behind a stoma. There are very few spaces between skin cells, excepting, of course, the openings between the two cells of a stoma. The two cells which make a stoma are called “guard cells,” because they guard the opening into the plant.

They are shaped, you see, something like half-moons. When the plant is full of water these half-moons swell up and their edges are drawn apart—so. [Illustration: x]

This, you see, makes an opening (x) into the plant. This little mouth through the skin opens into the space back of the skin, and this space connects with other spaces all through the plant. Through these stomata all parts of the plant can communicate with the outer air. The extra water and other waste materials pass out through the open stomata and air and other gases pass in and out.

Now, if the air outside is very dry and the earth is dry so that the roots are not able to send up much water, these wise little guard cells do not swell up and separate.