Fig. 60. Leaves of Goose grass looking like a whorl.
As we found out already, leaves require light and air, and usually arrange themselves so as to get them; hence, in a general way, we may observe that the leaves all grow to face the light. If you go under a beech tree, for example, and look up, you will find that you can see nearly all the big branches on the inside, while the leaves form a covering or dome on the outside. Special cases of leaves so arranged as to get a good light we noticed before (see pp. [36] and [37]).
As well as their own particular work, leaves may take on extra and different work, so becoming modified to suit their different occupations, and unlike true leaves. We already noticed in the cactus (see fig. 48) that the leaves become like sharp spines which protect the fleshy stem, and can do none of the usual work of leaves, because they have lost their green colour.
Fig. 61. Leaf of Pea, showing leaflets modified as tendrils (t); expanded leaflets (o).
In some plants leaves, or parts of leaves, may change into fine tendrils which become very sensitive to touch, and can twine round supports and cling to them, and so help the plant to climb. Such tendrils we saw (fig. 31) move very quickly; they are quite different in their structure from ordinary leaves. This happens in many plants, and you may see it very well in the sweet pea (see fig. 61), where only two leaflets of the compound leaf remain leaf-like, the others having been changed into tendrils.