Types of Desert Vegetation.—The vegetation of deserts varies from region to region as much as does that of the forests. For ex ample, on the borders of the Transcaspian Desert beyond the Caspian Sea and in parts of the Arabian Desert the whole country seems to consist of nothing but bare sand dunes. Then comes a heavy rain, and within a week or two the sand is covered with a short, sweet growth of grass which makes it look as fertile as the prairie. Yet in a few more weeks the grass has ripened its seeds and dried up, and soon the sand is blowing freely as before. Farther out in the desert grass almost never appears, but the sand is dotted here and there with tough little bushes three or four feet high which seem leafless until one notices the little scales pressed tight against the stems. In some of the greatest deserts where sand dunes rise to heights of several hundred feet the space between the dunes is gouged out by the wind to such a depth that the water table is almost reached. Here one sometimes finds beds of reeds or patches of the feathery shrub called tamarisk. As the dunes slowly advance they may kill such plants, but sometimes, if the dunes are small, the plants may shoot up fast enough to keep their heads above the sand. Occasionally, after the dunes have gone on and again left them uncovered one finds reeds and bushes curiously elongated as if on stilts.
In the gravel deserts the vegetation is even less abundant than in the sand, for there are few favored spots and even when rain comes the plants have hard work to grow. In some of the vaster deserts of Persia and Central Asia the gravel at the foot of the mountains forms expanses like huge beaches 10, 20, or even 40 miles wide. Here the gravel is cemented together by a slightly salty calcareous deposit. Sometimes after this has been soaked by one of the occasional rains it splits into irregular polygons 5 to 12 feet in diameter, and cracks several inches deep are formed along the edges. In these the wind deposits sand, and later tiny plants take root so that sometimes the polygons are outlined in green, like little gardens where the plants occupy the paths instead of the beds. In deserts where gravel is mixed with soil an unusual rain may sometimes cause the desert' to blossom as the rose. One would have to travel far to find any scene more beautiful than the Mohave Desert of California at such a time when it is carpeted for miles with the loveliest flowers, white, yellow, orange, and blue.
In deserts like most of those in the United States where some rain falls in both summer and winter, bushy vegetation is the pre dominant type. The newcomer who sees the abundant sage brush of Utah and Nevada, or the larger greasewood and mesquite of Arizona with the accompanying cactus and the grasses that spring up after rains, can scarcely believe that in a region which looks so fertile agriculture is impossible. Many a tenderfoot from the East has
thought to his cost that the westerners were lying when they told him not to take up land of that kind. He found, however, that the types of vegetation which grow in the desert can subsist on an amount of moisture which will not possibly support crops. That is the great outstanding feature of every desert.
Oases.—The spots called oases are places where desert vegetation gives place to that of well-watered regions. Although they are in the desert, they are not of it. Yet they must be considered, since they occur in every desert, and support far more people than the vast surrounding areas. Moreover, the true desert people, the nomads, have much the same relation to the oases that country people have to cities. The larger oases are places where streams from the mountains spread out upon the desert plain and serve for irrigation. The moun tains and the oases may be as far apart as the snowy heights of the Himalayas and the hot sunny delta of the Indus. The whole of cultivated Egypt indeed, with its 11,000,000 people, is a great oasis watered by the rains that fall on the mountains of Central Africa. In such oases crops like millet, wheat, barley, grapes, and many other fruits are raised. The houses are generally made of sun-dried bricks called adobe. Sometimes, where there are trees enough, the houses have wooden frames, but in the oases of the driest deserts liLe eastern Persia even the roofs are made of adobe bricks forming small domes.
In books we hear much of the beauty of such oases as Damascus, but generally the descriptions are exaggerated. Nevertheless, when a traveler on a camel, the ship of the desert, comes to an oasis where he can rest and supply his needs it seems very beautiful to him just as any port seems a haven of rest to a storm-tossed mariner.
Palm Oases.—In the smaller oases of the driest, hottest deserts such as those of Arabia and northern Africa, palm trees are almost the only kind of vegetation. We are apt to think that this kind of oasis is typical, but really it is comparatively unimportant. Such oases are located where little springs bubble out of the earth or in depressions where the ground is slightly moist and wells can be dug. The palms are often planted in pits 5 or 6 feet deep where the soil is moist. At much greater depths water can be obtained in wells. It is raised by hand or by camels drawing buckets at the end of long ropes and is conducted with great care to the pits where it waters the trees.