Fig. 6.—Diagram of Straight and Curved Stems.
a, Stretched cells; b, crowded cells.
Mr. Darwin was not a man to be satisfied with finding that a thing is so. He never rested until he found just how it came about. I do not mean to say that he was the only man who studied these things, for there were many others who did; but he wrote about what he had studied in such a clear and simple and interesting way that anybody could understand him, and so people who don't pretend to be very wise in such matters read Mr. Darwin's account and nobody's else, and are apt to forget, though he is always careful to mention their names and what they have done, that any one else deserves any of the credit.
By closely studying the little cells of which the leaf or stem is made up, he found that when, for any reason, a plant needed to turn in a certain direction, the water in the stem rushed from the inner to the outer part of the curve, making the cells on the inner side of the stem a little smaller and those on the outer a little larger than usual. After a while the stretching of the outer cells makes them grow and stay larger (see in the figure how it must be, Fig. 6), and so the curve remains. You can not straighten a stem curved in this way without breaking it.
Every movement of stems and leaves comes from the movement of the water that fills their cells. But besides the water, there is something else just as important, and that is the sun. The water is only a servant, which obeys the light as its master. Many flowers turn their bright faces always to the light. They follow the sun as he moves through the heavens all the day long from his rising to his setting. This comes from the effect the sun has on the water in the stem, and not because the flower is beginning to "take notice," as the baby's bright eyes do of a lamp when it is moved about a room, though it does remind one of it.
The movement of climbing plants is only one of many curious movements that are made by stems and roots and leaves and flowers, though the cause is the same in all cases.