"It's looking for a soft spot!" says the high school boy, "just as the river does."
NO HIT-OR-MISS METHODS FOR MR. ROOT
Exactly. But not in the sense that this phrase is used in slang. The root has certain work to do, and it does it in the quickest and best way. It can get food more quickly out of mellow soil than out of hard, and so it constantly hunts it up. I mean just that—hunts it up. For it isn't by aimless rocking back and forth that roots just happen upon the mellow places. It's the other way around; it's from a careful feeling along for the mellow places that the rocking motion results.
"But how on earth do the roots do this? What makes them do it?"
That's what any live boy would ask, wouldn't he? So you may be sure that's what the science people asked, and this is the answer:
The roots, like all parts of the plant—like all parts of boys and girls and grown people, for the matter of that—are made up of little cells. Well, these cells, first on one side of the root and then the other, enlarge, and so pump in an extra flow of sap. Now, as we know, the sap contains food for the plant, just as blood contains food for our bodies; and more food means more growth. So the side of the root where the cells first swell out grows fastest and thus pushes the root over on the opposite side. Then the cells on this opposite side swell, and the root is turned in the other direction again. So it goes—right and left, up and down. And when these two motions—the up and down and right and left—are put together, don't you see what you get? The round-and-round motion!
Precisely the same thing happened right now when you turned your finger round and round to imitate the motion of the root. (I saw you!) The muscles that did the work swelled up first on one side and then on the other, just as they do when you bend your elbow, when you walk, when you breathe, when you laugh.
And more than that: You know how tired you get if you keep using one set of muscles all the time—in sawing fire-wood, for example. Yet you can play ball by the hour and never think of being tired until it's all over; because, for one thing, you are constantly bringing new muscles into action as you go to bat, as you strike, as you run bases. It's the same way with the roots, it seems. For the theory is that after the cells on one side have swelled, they rest; then the cells on the other side get to work.
"But what starts the movement?" you may say. "The idea of moving my arms and legs starts in my brain."