Certain stems take on the work of leaves, and sometimes they are so much modified for this that the plant has no true leaves at all. This is what has happened in the case of a cactus. If you can get a cactus, examine it carefully and you will see that the whole plant consists of a thick mass of green tissue, which apparently is not divided into stem and leaves. But the truth is that the whole of the thick mass of tissue is the stem, and the little tufts of spines and hairs are really reduced leaves. So that in the cactus the green stem does all the food building work instead of the leaves.
Fig. 48. Cactus plant, showing its fleshy stem, which is green, and does the food building for the plant. The tufts of spines and hairs represent the leaves.
In some plants this is not so much marked, even though the stem does some of the work for the leaves. In such cases the stem is generally green and broad or winged and the leaves small, as in our common broom and the whortleberry, where the leaves very soon drop off. Quite a number of plants have stems which do this, and it is sometimes a great advantage to the plant, for the big leaves are often very wasteful of water, as you will see in Chapter XVIII.
In other cases we find that the side branches of stems may be modified to protect the plant, and so take on the form of strong spines or thorns, as in our blackthorn, where the sharp pointed spines are modified side shoots.
There are many other pieces of work which stems may do; we must just mention the climbing and twining stems, where the stem is delicate and requires to be supported, which we are going to examine more carefully in Chapter XIX.
Sometimes, instead of continuing to grow into the air, the stem may bend over into the earth again, as often happens in big bushes of bramble (see fig. 49), and then from the tip of the stem a number of adventitious roots (see p. [56]) grow out and hold it firmly in the ground. If, then, this branch gets separated from the rest of the plant, it can build a complete new individual.