CHAPTER XVIII.

FOR PROTECTION AGAINST LOSS OF WATER

If you go along the lanes and in the gardens in the height of summer when it is hot and dry and the sun beats on the plants all day, you may see them beginning to wither for want of water. The roots are not able to find enough moisture in the soil to supply the leaves, which, being in the hot air, continue to transpire away the water resources of the plant, so that in the end each of its cells must suffer and the whole become limp and droop. This happens because the ordinary green plants of our country make no special preparation for such dry weather. Our hot season is short, and even in the summer we have frequent showers which keep the soil moist enough to provide the plants with water from day to day, so that they have not become accustomed to long periods when there is no prospect of rain.

Compare one of our usual green plants, a sunflower, for example, with such a thing as a cactus, which you may get growing in a pot of dry sand. The cactus is able to withstand the hottest sun for days, though it gets very little water, and sometimes apparently none at all; yet it does not wither, but grows, and may bear the most lovely flowers. From travellers we learn how the huge cactus plants grow in dry and stony deserts, standing every day in the blazing sun. Such is, of course, their home, and they are used to it; but how is it that they are able to flourish under conditions which would kill one of our own green plants?

Let us look at their structure and see in what they differ from a usual plant. First, they have no green leaves, for these have developed into spines (~see~ p. 62), while the sunflower has many large ordinary leaves.

You will remember that the surface of leaves is continually giving off water from its many pores. When a plant has a number of big leaves this transpiring area is large, while when it has no leaves at all, but a thick, green stem instead, then the amount of surface from which water vapour is being given off is very much reduced, even though there may be about an equal quantity of actual tissue in the two plants. You can see that this is the case if you take a ball or thick block of dough and roughly measure its surface, then roll it out till it is fairly thin and measure it again; you will see that the thinner you roll it the more surface there is; all the time, of course, the amount of actual dough remains the same. So that of two plants of the same bulk, the one with broad, thin leaves will expose the most surface to the air, and so lose more water than one with very thick leaves or none at all. The latter would therefore be better fitted to live under dry conditions.

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.

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.

Many plants roll up their leaves when it is dry, so that the surface with the transpiring pores is on the inside, and protected by the outer side with its hard skin (~see~ fig. 100). In damp weather these leaves unroll, and do all the work they can. Leaves like this are to be seen in many of the grasses, particularly those growing on sand dunes and moorland; while a number of the heaths and heather do the same thing to protect their transpiring surfaces.

~You will find that in nature, water is one of the most important things in the surroundings of plants~, and in their struggles to get it and keep it they have changed their forms in many ways, and in some cases have become extraordinary-looking creatures as a result.