The plant, in short, is a very sluggish animal, shut up tight in a wooden box, so that only the ends of its roots and shoots stick out where we can see them move. We know, however, that all the living jelly of the plant does move, tho we cannot see it inside the wood. To be sure, the plant moves only very slowly. A leaf will turn when a lighted match is held near it; but if somebody held the same lighted match equally near your nose, your jump would be something like four thousand times quicker. Nevertheless, some of the slowest animals are not a bit more rapid of movement than the quickest plants. The fig tree and the fresh water clam, for example, are equally slow to move when they are touched, but move they both do.
But we must not forget the turning of the plants’ leaves toward the light, for that is, after all, the one movement of plants which we have all seen for ourselves.
The curious thing about it is that the leaf turns, not because the light falls on the leaf itself, but because the light falls on the stem. If we cover the blade of the leaf, but let the light fall on the stem, then the leaf will turn; but if we shade the stem and leave the blade uncovered, then the leaf will not turn. Or in case there is a joint in the stem where the turning takes place, as in the clover leaf, then there is where the plant does its seeing. Allow the light to strike the leaf, cover that spot only, and the leaf is blind, but cover everything else, and the leaf turns as before.
Nor is it only sunlight toward which the plant turns its leaves. The great Darwin, who was one of the first to study this matter carefully, had a plant that after being kept a long while in the dark, screwed round its leaves to face a small lamp twelve feet away. Some of the so-called “sensitive plants,” will start turning toward a candle ten seconds after they first catch sight of it.
Oddly enough, however, the leaf will move in exactly the same way, if instead of letting light strike the stem, one rubs salt on it, or brings a hot wire near by. In fact, leaves, tendrils, and other soft green parts will turn toward a red hot wire till they touch it and are burned to death. So the plant is after all much like the infusorian. It can do one thing, which is generally right; but it does that one thing just the same, even when that is the worst thing it possibly could do.
XXIV
Some Plant-Like Doings Of Animals
The plants, then, know enough to do two things—to grow up or down with stem or root, and to turn toward the light or away from it. This really, if you can call this knowing, is about all they do know. Now I am going to tell you about some common animals which are not much better off than the plants; which know up and down, and know the direction of the sunlight, and know mighty little else.
Happy is the community which does not know the brown-tail moth. Wherever it appears, it spreads like a pestilence, eating every green leaf off a tree, and leaving it in mid-summer as forlorn and bare as at Christmas time. A great tree that has taken a hundred years to grow, the progeny of one moth will kill in three.
The brown-tail moths, their cousins the golden-tail moths, and several other sorts of moths, lay their eggs in the late summer and early fall. The little caterpillars hatch out that same season, grow to be something like a quarter inch long, and spend the winter in a cocoon-like nest which they spin for themselves much as does a silk-worm or a spider. In the spring, having eaten nothing all winter, they leave the nest, crawl to the ends of the branches, and proceed to devour the new leaves.