By far the greater part of the species of Cactuses belong to the group with tall or elongated stems. “It is worthy of remark that as the stems advance in age the angles fill up, or the articulations disappear, in consequence of the slow growth of the woody axis and the gradual development of the cellular substance; so that, at the end of a number of years, all the branches of Cactuses, however angular or compressed they originally may have been, become trunks that are either perfectly cylindrical, or which have scarcely any visible angles.”
A second large group is that of which the Melon and Hedgehog Cactuses are good representatives, which have sphere-shaped stems, covered with stout spines. We have hitherto spoken of the Cactuses as being without leaves, but this is only true of them when in an old or fully-developed state. On many of the stems we find upon their surface, or angles, small tubercles, which, when young, bear tiny scale-like leaves. These, however, soon wither and fall off, so that, to all appearance, leaves are never present on these plants. There is one exception, however, in the Barbadoes Gooseberry (Pereskia), which bears true and persistent leaves; but these may be considered anomalous in the order.
The term “succulent” is applied to Cactuses because of the large proportion of cellular tissue, i.e., flesh, of their stems, as compared with the woody portion. In some of them, when young, the woody system appears to be altogether absent, and they have the appearance of a mass of fleshy matter, like a vegetable marrow. This succulent mass is protected by a tough skin, often of leather-like firmness, and almost without the little perforations called breathing and evaporating pores, which in other plants are very numerous. This enables the Cactuses to sustain without suffering the full ardour of the burning sun and parched-up nature of the soil peculiar to the countries where they are native. Nature has endowed Cactuses with a skin similar to what she clothes many succulent fruits with, such as the Apple, Plum, Peach, &c., to which the sun’s powerful rays are necessary for their growth and ripening.