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." WHERE MR. ROOT KEEPS HIS BRAINS Just so again. The root has a brain, too, or what answers for a brain. And the root's brain, is in its head; at least in the vicinity of its nose--that is to say, its tip. It's the tip that first finds out which side of the road is best, and passes the word back to the part of the root just behind it to bend this way or that. It's also the tip that feels the pull of gravity and knows that it's the business of roots to keep under cover. And Mr. Root just _will_ have it that way! You can't change his mind. Mr. Darwin tried it and he couldn't; although he finally changed human people's minds a lot. [Illustration: WHERE MR. ROOT WEARS HIS CAP A root wears its cap right where you do--over its brain department; that is to say, the tip. It is called the "root cap" and protects the tip from injury.] This is how he tried it on a root. He took a bean with a little root that had just started out into the world. He cut off the tip and then set the bean so that the root stuck straight up. It continued to grow that way for some little time. Finally, however, a new tip had formed. Then there was a general waking up, as if the tip said to the rest of the root: "Here, here, this will never do! Where are you going? You must bend _down_!"