Fig. 24. Nasturtium covered over, so that the light only enters from below. The leaf surfaces bend over to face it.
Fig. 25. Spray of Maple showing stalks of leaves of the same pair of very different lengths, so as to place the leaves well as regards light.
In a small plant, or one with only a few big leaves, this desire for the light is easily arranged for, as there is room for each of them. But if all the leaves of a great tree were turned in the same direction, you will see that many of the under ones must be shaded by the others. This is not so bad as one might expect, however, owing to the wonderful way in which the leaves arrange themselves so as to use every bit of space they can, and yet to overlap and screen each other as little as possible. Particularly in plants which grow flat on the ground or against walls, and which therefore get all their light from one side, this is very well shown. In plants with the leaves in opposite pairs you will often find one leaf of the pair big, and the other one small, or that the leaf-stalks are of different lengths, and if you examine this pair in relation to the rest of the branch, you will see how it is developed in this way so as to use every bit of space it can and get as much light as possible without overlapping its neighbours (see fig. 25). Although it is true in one way that each leaf works as a separate individual, yet each separate leaf is only a small part of the plant, and they all work together for the good of the whole. Branches which have their leaves arranged in this way so that they seem to fit into a pattern, form what is called “Leaf Mosaic.” You may see this kind of arrangement among the leaves of very many plants (see figs. 25 and 26).
Fig. 26. Leaves of Ivy growing out from the stem so as not to overlap each other.
If, as we have already seen, light is so very important for the plant, what is the result of growing it in the dark? As you know, it will not be able to build itself food, and so would finally starve and die. If, however, we choose a plant which has already much food stored up and can therefore grow for a time without making a new supply, then we can study the effect of darkness on its growth.