It is believed that from the vast amount of watery juice in the cactus tribe, joined to the fact that most of them, and exactly those richest in sap, vegetate on dry sand, almost wholly devoid of vegetable mould, where they are besides exposed often three-fourths of the year to the parching sunbeams of an eternally serene sky; from this combination of circumstances, even, it is thought that we may the more safely conclude that these plants draw their nourishment from the air, since in our own hothouses also it has been observed that the branches of cactus stems cut off and left forgotten in a corner without further care, far from dying, have frequently grown on and made shoots three feet long or more. De Candolle first found the right path when he weighed such cactus shoots which had grown without soil, and found that the plant, though larger, was always lighter, therefore, instead of abstracting anything from the atmosphere, must rather have given up something to it. All the growth takes place, in such cases, at the expense of the nutritive matter previously accumulated in the juicy tissue, and it generally exhausts the plant to such a degree that it is no longer worth preserving. It is that succulent tissue which enables the cactus plants--one might compare them with the camels--to provide themselves beforehand with fluid, and thus to brave the rainless season. Their anatomical structure also assists them in this respect in a peculiar manner. We know from the experiments of Hales that plants chiefly evaporate the water they contain through their leaves, and the cactus tribe have none. Their stem, too, unlike that of all other plants, is clothed with a peculiar leathery membrane, which wholly prevents evaporation. This membrane is composed of very strange, almost cartilaginous, cells, the walls of which are often traversed by elegant little canals. Its thickness varies in different species, and it is thickest, and therefore most impenetrable, in the Melocacti, which grow in the driest and hottest regions, while it is least remarkable in the species of Rhipsalis, which are parasites on the trees of the damp Brazilian forests. Another striking point about this group is the formation of an extraordinary quantity of oxalic acid. If this acid were collected in large amount in the plant, it must necessarily be dead to it. The plant, therefore, takes up from the soil on which it grows a proportionate quantity of lime, which combines with the oxalic acid, forming insoluble crystals, which occur in abundance in all the Cactaceæ.