But, of course, plants do not move to the same extent that animals do. Plants only move when there is something to be gained by doing so. As a rule they are fast-rooted in the soil, and they find all the food they need in the sunlit air around them and the soft rain that falls upon them, so there is no occasion for them to move. But when on account of the poorness of the soil they cannot get enough nourishment, certain plants have learned to move, just as many lower animals do, in order to obtain for themselves a sufficient supply of nitrogen. But it is not only to obtain food that plants move.
Many plants alter their position when they go to sleep at night.
Notice how the pansy in the garden bed bends down its head when evening comes, how the daisy draws its ray flowers over the delicate disk flowers in the center of its head. If you look about in the garden and in the meadows when the sun is going down in the west, you will see that not only the flowers, but the leaves of plants, often change their position when they go to sleep for the night. The clover folds the three leaflets in each of its leaves one over the other, each pair of leaves of the creeping stem of the chickweed lean towards each other, and the leaves of many other plants draw closer together or droop downward on their stems. Many plants, too, move their flowers and their leaves so that they face the sun the greater part of the day. There is, of course, a reason why plants bend towards the light in this way, for their very life depends upon the light of the sun.