Sometimes the leaves are arranged in a circle all round the stem at the same level; this is the case in the horsetail (~see~ fig. 59), and such an arrangement is called a ~whorl~, but it is not very common in plants. In the goose grass the leaves look very much as though they were really in a whorl (~see~ fig. 60), but there are only two true leaves; the others are the stipules, which are so much like the leaves that it is very difficult to tell them apart. [Illustration: 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. [Illustration: 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. When we come to look at ~Flowers~, with all their special shapes and brightly coloured parts, we are really looking at ~modified leaves~. But they are so very much modified that we have come to consider flowers as things by themselves, and so we will study them later on. Some plants which do not have true flowers, yet have leaves of two kinds. For example, the “flowering fern” has the usual green leaves and others which form rather brownish golden spikes, and which are covered with spore[6] cases. Then again, some leaves are very specially modified and are changed from the usual structure in order to act as traps for insects (~see~ Chap. XXI.). [6] ~Spores~ are simple little structures which do much of the work of seeds. ~See~ the Chapter on Ferns.