Think over the results of these three experiments, and you will see that it is only when the tip of the root is not cut off that the plant seems to “realize” that it is not in the right position. When the tip is removed it does not bend down even when the whole plant is lying horizontally, and in the other case (fig. 30, C1, D) it will keep on bending even after it has been put in its right position.
We noticed that it is not the very tip itself which bends, so that we see that the very tip is the part which “feels” what is happening, while the part just behind it grows and bends according to the need of the plant.
This is a somewhat similar case to what happens when you realize with your brain that you are in danger on the road, and your feet hurry you across.
When we come to consider why the root should grow downwards in this persistent way, we find that there is an outside influence at work on the plant. You know when a stone is left without any support that it always falls to the ground, and we say that it is attracted toward the centre of the earth by the force of gravitation. It has been proved that the strong tendency of roots to grow down into the soil is largely the result of the same attraction, while the stem is not attracted by it but driven away, and therefore grows away from the centre of the earth. To prove this, however, requires more complicated apparatus than you are likely to be able to use at present.
From the experiments which we have done already we see that plants, as well as animals, are affected by their circumstances, and can in some measure realize them, and move to alter themselves in accordance with them. Later on we shall find that plants have a similar power in relation to light, supply of water, and other things. Have we not already observed in plants nearly all the signs of life we set out to look for? (see p. [4]).
There is one very important point about the growth of plants which is strikingly different from the growth of animals. A young kitten has four legs, a head, and a tail, and as it grows to be a cat these only alter a little in shape and get larger and stronger; the number of its legs remains the same. A baby plant, on the other hand, has its little root and shoot with a few tiny leaves, but as it gets older these increase very much in number, till it may have many branches and thousands of leaves. In fact, the number of its parts is much more indefinite than those of an animal; its body is built on quite a different plan. Yet both plants and animals show the same important thing in their growth, that is the increase of their living body, which they build up out of their non-living food.
CHAPTER X.
MOVEMENT
While we have been examining plants to find out some of the facts about their other life properties, we have at the same time seen many cases of movement in their different parts.