Fig. 35. Leaf of Sensitive Plant quite closed, and the leaf-stalk fallen, after being touched.
Some foreign plants swing their leaflets round slowly like the arms of a windmill, but we have not yet found out why they do this. Also in many flowers we find movement, and in flowers it is generally in relation to the insects which visit them. For example, some orchids shut up their big front petal with a sudden snap when an insect alights on it and shoot the astonished fly towards the middle of the flower.
Parts such as these, which have more power of movement than the rest of the plant, are called sensitive parts, but though in them we see it more clearly than in most plants, they only illustrate what is common to all, and that is some power of movement.
The movements which you have seen so far in plants are different from those of most animals in one way, and that is in the fact that the whole plant remains rooted in one place, and only parts of it can move as the circumstances require, while, though an animal moves its parts separately, the result of some of those movements is to carry its whole body about. This may appear to you a great difference between plants and animals, but it is not quite so great as it seems; nor must we forget that there are some simple slimy-looking plants which slowly crawl along the ground, as well as many minute, green plants, which you could only see with a microscope, which move their whole bodies and swim about just in the way that tiny animals swim.
SUMMARY OF PART I.
We have now done a number of experiments with plants, and found out many facts about their way of life, and I think you will agree that we have collected enough evidence to prove the statement made at the beginning of Chapter II.—that on the whole plants show the same “signs of life” as do animals.
We have seen that like animals they breathe in a part of the air, and that they breathe out with the air the added carbonic acid gas, which is the characteristic “waste product” of the out-breathing of animals.
They practically “eat” when they take in substances as food into their bodies, even though they have no gaping mouths which can open and close. We noticed, too, the interesting parallel between young plants and young animals, where both (the plants in the food in the seed, the animals in their mothers’ milk) are supplied with ready-made food at first, and as they get older have to find what food they require for themselves. As regards their feeding, the plants do more work than the animals, for they manufacture the starchy food for themselves out of simpler elements, while the animals require their starch to be ready made.
Then the fact that plants grow, increasing in size and forming new structures, has been known to you ever since you were a baby yourself. Although we noticed here an important difference between the kind of growth in plants and animals, yet the growth itself is alike in the two cases, for both plants and animals build up their living bodies out of simpler substances which they take in as food and change till the not-living food becomes part of themselves and is living.