For example, we found (Chapter IX.) how the tips of roots move round to get back their vertical position if they are placed horizontally, and how the shoots of young plants bend over towards the light when they are grown in a dark box where it can enter only from one side (Chapter VIII.). Then, too, as the root tip grows into the soil or between the crevices of rocks it bends round the stones or other things in its way, and it is also attracted towards water, thus showing a continual, slow movement in its growth. The shoot shows a parallel kind of movement in following the light and placing itself as advantageously as possible with regard to it.
Fig. 31. Tendrils of the Pea; A young tendrils which have not yet been touched; B beginning to curl fifteen minutes after being rubbed with a twig.
You may see a still faster movement if you carefully examine a twining tendril. Notice how the young tendrils of a sweet-pea are at first almost straight, growing out into the air (see fig. 31). Now choose such a one for the experiment, and another like it which you do not touch, but keep to compare with the one on which you have experimented.
Gently rub one side of the tendril with a small rough twig, and then leave it alone. You will see that in about five or ten minutes it has begun to curve, and in a quarter of an hour may have bent round completely. Such movement is more rapid than that in the ordinary growth, and this power of bending so quickly is one of the special characters of tendrils, and one that is very important in helping them to do their work for the plant and to seize on any support within reach as quickly as possible.
Fig. 32. Leaves of Wood-sorrel; A in the day position, B “asleep” at night.
Then there are other movements, one of which you must have often observed in the “sleep” of plants. Many flowers and leaves close up and bend down at night, taking up their usual position again next day. This is not the same thing as the opening of buds, for it may occur again and again in the fully grown parts of plants. For example, you may mark certain leaves of wood-sorrel or common clover, and watch them close up at night and re-open in the morning many times. These movements are not very fast, and you cannot see the plant moving as you can see a kitten waving its tail, but the difference is only one of degree.