PLANTS THAT MUST COPE WITH A WATER SHORTAGE

THE Scots pine, though capable of producing successful forest on poor, ill-watered soils in a climate where the si n-supply is very meagre, lives in comparative luxury compared with some other species. The available light seems to suffice and is very rarely too much. Where there is superabundance of sun and the most meagre water-supply imaginable, plants are in poor case indeed. We cannot find the extreme of these conditions in the British Isles. If we want to see what plants can do in such circumstances we shall have to go abroad. Yet we should remind ourselves that the problem remains the same in essentials : how to get and keep enough water to carry on the life-processes and at the same time to evaporate enough at the exposed surfaces to keep the plant-body cool and leave behind enough mineral salts. It should be reasonably apparent that the greater bulk of plant-life on this earth does not find itself in the most favourable conditions all of the time. The actual amount of land areas upon which plant-life is quite impossible is probably much smaller than most of us imagine. In other words, an absolute desert in which there is no life whatever is a very rare thing.

The next thing to it, in which the living organisms are just, and only just, above the level of subsistence, is pretty common. A glance at a map of the world will show enormous areas marked ‘desert’ in nearly every large land-mass. Most of these deserts, however, have enough of the essential supplies to support some kind of plant-life. When the water-supply fails completely for a long time that is an end of the matter as far as the plants are concerned. They are accustomed to suspend operations until there is sufficient improvement in conditions to make possible a small amount of constructive or reconstructive work, but if the improvement is too long delayed, even the resources of the seed become exhausted, and when the improved conditions do come there is nothing left alive to take advantage of them.

The conditions of desert life have been considerably investigated at the Desert Laboratory of the Carnegie Institution of Washington at Tucson in the State of Arizona. Conditions are not pleasant enough to make such studies really popular, even among scientific workers. On the other hand there is great fascination in finding out as exactly as possible how plants and animals manage to survive such rigorous

conditions. One of the first things to try to know accurately is the annual rainfall. For this purpose a special rain-gauge had to be invented. Obviously in such arid conditions any rain that does fall is very quickly evaporated unless means are devised to prevent it. This is achieved by allowing the rain-water to be delivered into a collecting vessel under a layer of heavy oil. Other meteorological data are collected in much about the ordinary way, and so we have a fairly good knowledge of the climatic factors.

The plants of this region are of two main types; those which are equipped with water-storing tissues and those without any notable storage but with such modifications of the plant-body as reduce the evaporation to a minimum. Chief among the water-storers are the cacti—familiar to many people because their bizarre forms have caused them to be objects of cultivation, though the really magnificent members of the family are too large and slow-growing to be common pets. It may be noted in passing that the two thousand or more known species of the cactus family are, with one possible exception, natives of the American continent, and they are to be found chiefly in the arid regions of the tropical belt.

The giant cactus of Arizona and Texas is a superb example of what plants can do even in the most extreme conditions of heat and drought. In about two hundred years it can grow to a height of thirty-five feet with a diameter of about two feet. The whole plant-body has a thick waterproof covering, and the only communication through it to the dry, water-hungry atmosphere outside is by means of the stomata (or breathing pores) which are sunk in deep, longitudinal furrows. Water is stored in the bulk of the inner tissues, and breach of the water-controlling outer tissues by animals is prevented by a plentiful equipment of spines.

HITHERTO we have been concerned with actual shortage of water. Now we may pass on to consider what we may call virtual shortage; a state, let us say, when there is plenty of water about, but the plants are for some reason unable to make use of it. It has been explained elsewhere * that the passage of water into a plant-cell depends on the difference between the amount of dissolved substances in the cell-sap

and that of the fluid outside the cell. When the available water has a large quantity of substances dissolved in it, the passage of water from the roots of a plant through the plant-body to the atmosphere is difficult from the start. We might very properly say, on seeing a tropical mangrove swamp for the first time, that we had never seen anything wetter in oui lives. We might, therefore, be justifiably astonished, not to say incredulous, if we were told by a plant-physiologist that the soil was ‘physiologically dry ‘. Yet this is a very good way of expressing what we know of such places.

So it is that the plants of mangrove swamps are those which have the means of rigid control of such water as the roots can take in. The same rigid control will be necessary if any plant is to have its roots in highly saline water and the rest of its body in the atmosphere. The trees of the mangrove swamp do this by having thick, leathery, well-waterproofed leaves. In spite of these restrictions some species are able to colonise coral reefs ten miles off-shore and maintain themselves as a dense forest of magnificent sixty-foot trees. The probable course of such a colonisation is well worth considering.

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