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Дата написания1970-01-01
Язык книгиen
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зарубежная образовательная литературавысокие технологии

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It is interesting to note, too, how much alike in build and gait are these three thoroughbred desert roamers, the giraffe, the ostrich, and the camel or dromedary. In their long legs, their stalking march, their tall necks, and their ungainly appearance they all betoken their common adaptation to the needs and demands of a special environment. Since food is scarce and shelter rare, they have to run about much over large spaces in search of a livelihood or to escape their enemies. Then the burning nature of the sand as well as the need for speed compels them to have long legs which in turn necessitate equally long necks, if they are to reach the ground or the trees overhead for food and drink. Their feet have to be soft and padded to enable them to run over the sand with ease; and hard horny patches must protect their knees and all other portions of the body liable to touch the sweltering surface when they lie down to rest themselves. Finally, they can all endure thirst for long periods together; and the camel, the most inveterate desert-haunter of the trio, is even provided with a special stomach to take in water for several days at a stretch, besides having a peculiarly tough skin in which perspiration is reduced to a minimum. He carries his own water-supply internally, and wastes as little of it by the way as possible.

What the camel is among animals that is the cactus among plants—the most confirmed and specialised of desert-haunting organisms. It has been wholly developed in, by, and for the desert. I don't mean merely to say that cactuses resemble camels because they are clumsy, ungainly, awkward, and paradoxical; that would be a point of view almost as far beneath the dignity of science (which in spite of occasional lapses into the sin of levity I endeavour as a rule piously to uphold) as the old and fallacious reason 'because there's a B in both.' But cactuses, like camels, take in their water supply whenever they can get it, and never waste any of it on the way by needless evaporation. As they form the perfect central type of desert vegetation, and are also familiar plants to everyone, they may be taken as a good illustrative example of the effect that desert conditions inevitably produce upon vegetable evolution.

Quaint, shapeless, succulent, jointed, the cactuses look at first sight as if they were all leaves, and had no stem or trunk worth mentioning. Of course, therefore, the exact opposite is really the case; for, as a late lamented poet has assured us in mournful numbers, things (generally speaking) are not what they seem. The true truth about the cactuses runs just the other way; they are all stem and no leaves; what look like leaves being really joints of the trunk or branches, and the foliage being all dwarfed and stunted into the prickly hairs that dot and encumber the surface. All plants of very arid soils—for example, our common English stonecrops—tend to be thick, jointed, and succulent; the distinction between stem and leaves tends to disappear; and the whole weed, accustomed at times to long drought, acquires the habit of drinking in water greedily at its rootlets after every rain, and storing it away for future use in its thick, sponge-like, and water-tight tissues. To prevent undue evaporation, the surface also is covered with a thick, shiny skin—a sort of vegetable macintosh, which effectually checks all unnecessary transpiration. Of this desert type, then, the cactus is the furthest possible term. It has no flat leaves with expanded blades, to wither and die in the scorching desert air; but in their stead the thick and jointed stems do the same work—absorb carbon from the carbonic acid of the air, and store up water in the driest of seasons. Then, to repel the attacks of herbivores, who would gladly get at the juicy morsel if they could, the foliage has been turned into sharp defensive spines and prickles. The cactus is tenacious of life to a wonderful degree; and for reproduction it trusts not merely to its brilliant flowers, fertilised for the most part by desert moths or butterflies, and to its juicy fruit, of which the common prickly pear is a familiar instance, but it has the special property of springing afresh from any stray bit or fragment of the stem that happens to fall upon the dry ground anywhere.

True cactuses (in the native state) are confined to America; but the unhappy naturalist who ventures to say so in mixed society is sure to get sat upon (without due cause) by numberless people who have seen 'the cactus' wild all the world over. For one thing, the prickly pear and a few other common American species, have been naturalised and run wild throughout North Africa, the Mediterranean shores, and a great part of India, Arabia, and Persia. But what is more interesting and more confusing still, other desert plants which are not cactuses, living in South Africa, Sind, Rajputana, and elsewhere unspecified, have been driven by the nature of their circumstances and the dryness of the soil to adopt precisely the same tactics, and therefore unconsciously to mimic or imitate the cactus tribe in the minutest details of their personal appearance. Most of these fallacious pseudo-cactuses are really spurges or euphorbias by family. They resemble the true Mexican type in externals only; that is to say, their stems are thick, jointed, and leaf-like, and they grow with clumsy and awkward angularity; but in the flower, fruit, seed, and in short in all structural peculiarities whatsoever, they differ utterly from the genuine cactus, and closely resemble all their spurge relations. Adaptive likenesses of this sort, due to mere stress of local conditions, have no more weight as indications of real relationship than the wings of the bat or the nippers of the seal, which don't make the one into a skylark, or the other into a mackerel.

In Sahara, on the other hand, the prevailing type of vegetation (wherever there is any) belongs to the kind playfully described by Sir Lambert Playfair as 'salsolaceous,' that is to say, in plainer English, it consists of plants like the glass-wort and the kali-weed, which are commonly burnt to make soda. These fleshy weeds resemble the cactuses in being succulent and thick-skinned but they differ from them in their curious ability to live upon very salt and soda-laden water. All through the great African desert region, in fact, most of the water is more or less brackish; 'bitter lakes' are common, and gypsum often covers the ground over immense areas. These districts occupy the beds of vast ancient lakes, now almost dry, of which the existing chotts, or very salt pools, are the last shrunken and evanescent relics.

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