Another cause, which serves to counteract the want or scarcity of rain, is the admirable foresight of Nature in providing these arid lands with plants suited to their peculiar climate. Thus creepers abound which, having their roots buried far beneath the soil, feel but little the effects of the scorching sun. The number of these which have tuberous roots is very great--a structure evidently intended to supply nutriment and moisture when, during the long droughts, they can be obtained nowhere else. One of these blessings to the inhabitants of the desert is the Leroshua, a small plant with linear leaves, and a stalk not thicker than a crow’s quill; but on digging down a foot or eighteen inches beneath, the root enlarges to a tuber, often as big as the head of a young child, which, on the rind being removed, is found to be a mass of cellular tissue, filled with fluid much like that in a young turnip. Owing to the depth beneath the surface at which it is found, it is generally deliciously cool and refreshing. Another kind, named _mokuri_, is seen in other parts of the country, where long-continued heat parches the soil. This plant is an herbaceous creeper, and deposits under ground a number of tubers, some as large as a man’s head, often in a circle, a yard or more horizontally from the stem. The natives strike the ground on the circumference of the circle with stones, till, by hearing a difference of sound, they know the water-bearing tuber to be beneath. They then dig down a foot or so and find it. But the most wonderful plant of the desert is the Kengwe, the water-melon of the Caffres. In years when more than the usual quantity of rain falls, vast tracts of the country are literally covered with these juicy gourds, and then animals of every sort and name, including man, rejoice in the rich supply. The creeping plants of the desert serve, moreover, a double purpose; for, besides their use as food, they fix, by means of their extensive ramifications, the constantly shifting sands--thus rendering similar services to those of the sand-reed (_Ammophila arundinacea_) on the dunes along the sandy coasts of the North Sea. The Mesembryanthemums are another family of plants admirably adapted to the Kalahari, as their seed-vessels remain firmly shut while the soil is hot and dry, and thus preserve the vegetative power intact during the highest heat of the torrid sun; but when rain falls, the seed-vessel opens and sheds its contents, just when there is the greatest likelihood of their vegetating. This is the more wonderful, as in other plants heat and drought cause the seed-vessels to burst and shed their charge. One of this family possesses a tuberous root, which may be eaten raw; and all are furnished with thick, fleshy leaves, with pores capable of imbibing and retaining moisture from a very dry atmosphere and soil; so that if a leaf is broken during the greatest drought it shows abundant circulating sap.