Rydberg, speaking of the Big Bad Lands in South Dakota, says that there are in some places great stretches of land consisting of cañons separated by small ridges, in which not a speck of green is visible over several sections.[62] (A section is more than a square mile.) [62] Schimper, _l.c._, p. 674. But though Aden looks exactly like "a barrack stove that no one's lit for years and years," plants grow there. Even in Egypt, when one has left the Nile inundation limit, a botanical eye very seldom fails to detect plants of one sort or another even in a dangerous and thorough-going desert. Plants are almost as hardy as men; they can adapt themselves to almost any climate. In some curious and inexplicable way the very dangers of the climate seem to produce automatically a means of resisting it. The chief peril, of course, is a loss of the precious water through the leaves. When the skin or epidermis of a plant is being formed, the walls of its cells are laid down, layer by layer, one inside the other, by the secretion of the living matter inside. In a dry desert the loss of water by evaporation will be so rapid that these layers of cell-wall are much thicker than in ordinary plants. The very fact that they are thicker and less penetrable tends to prevent any further loss of water.[63] [63] Schimper, _l.c._, p. 653. So that plants in a dry climate have the power of altering themselves to resist its dangers. Another author found that, in Scandinavia, plants of the same species can acclimatize themselves if necessary. Sheep's Sorrel which had grown on dry, droughty gravel banks only lost 10 per cent. of its water in the first two days, when it was artificially dried. Other Sheep's Sorrels, which had been luxuriating in meadows where they had no lack of moisture, lost no less than one third (33 per cent.) of their water when dried in the same way. That is interesting, because very likely our readers might in crossing a desert be perishing of thirst when a Bedouin Arab would be perfectly happy. The plants have learnt to do without water just in the same way as the Arab has done. Of the many interesting desert plants, the Succulents, Cacti, Euphorbias, and others of the same extraordinary, fleshy, dropsical appearance, come first. When a Cereus plant (one of the American Giant Cacti) was dried, it did not lose the whole of its water for 576 days. That is probably the longest time "between drinks" on record. A Houseleek (_Sempervivum_), which has to grow on dry rocks where it has no water for days together, remained quite fresh for 165 days. [Illustration: GIANT CACTUS NEAR ACONCAGUA VALLEY, CHILE This plant was about 8 feet high. The darker part on the tallest branch is the dark red flower of the parasitic horanthus. The thorns covering the branches are quite distinct.]