But, you may say, leaves have a definite work to do; how can the plant live without them? In the cactus the thick stem is green and does the work of food building; naturally it cannot do so much for the plant as many big leaves could, but it does enough to allow it to live and grow slowly and surely for many years, though it cannot grow in each year nearly at the same rate as can the sunflower. If you cut through the stem of a cactus you will find that its skin is very thick and tough, and this thick coat protects the plant against the fierceness of the sun far more completely than the thin skin of a sunflower does. At the same time, the tissues of the two stems are different; the sunflower is hollow and delicate, but the cactus is very thick and juicy, and each cell contains much gummy stuff which has the power of holding water strongly. So that we see in many important points the structure of a cactus is different from that of a usual green plant, and is specially suited to the dry conditions of the desert.
Many desert plants are built on the plan of the cactus, but there are also others which are not at all like them, and yet they are able to live in deserts and very dry places. It you examine them, however, you will find that they all have some special way of protecting themselves from being dried up. Some of them have hard, dry, woody stems, well protected by corky layers, and they only put out green leaves in the rainy season, and lose them directly the hottest weather begins. Others, which grow from seed every year, learn to sprout, flower, and fruit very quickly while there is some moisture, and they form well-protected seeds, which wait till next rainy season. One very curious desert plant has only two leaves, which last it the whole of its life, and which are very hard and leathery. There are endless varieties of things which the plants may do to protect themselves from being dried up, and we can only look at a few special examples.
To find plants growing in desert places we do not need to go out of England, because from the point of view of the plant, one which is growing on a dry rock or on a patch of bare dry sand, is really growing in a little desert. For it the supply of water is the chief problem, even though we never get hot tropical sunshine in England. Look, for example, at the plants growing on the sand dunes which are very like deserts in appearance, and the plants on dry walls, or on the “screes” of broken rock at a hill foot; they are all growing in deserts.
In many cases plants growing in such positions have small thick leaves, nearly round, or shaped like sausages, so that they have much water-storing tissue in proportion to a small transpiring surface. This is the case in the stone-crop (see fig. 98) and the house-leek, where each separate leaf has followed the same principle as the cactus stem, and exposes relatively little surface to the air. Such plants frequently have very long roots, which penetrate deeply between the cracks of the rocks and find hidden sources of water.
Fig. 98. Thick fleshy leaves of Stone-crop.
Other plants, instead of having leaves of this type, have exceedingly small leaves which may soon drop off, while the stem is green and does some of the food building. Small leaves are assisted by the green stem in gorse (fig. 99), which often lives in very dry places, though it can grow equally well under usual conditions.
