From the singular form and succulent nature of the whole of the Cactus family, it might be inferred that, in these characters alone, we have reliable marks of relationship, and that it would be safe to call all those plants Cactuses in which such characters are manifest. A glance at some members of other families will, however, soon show how easily one might thus be mistaken. In the Euphorbias we find a number of kinds, especially amongst those which inhabit the dry, sandy plains of South Africa, which bear a striking resemblance to many of the Cactuses, particularly the columnar ones and the Rhipsalis. (The Euphorbias all have milk-like sap, which, on pricking their stems or leaves, at once exudes and thus reveals their true character. The sap of the Cactuses is watery). Amongst Stapelias, too, we meet with plants which mimic the stem characters of some of the smaller kinds of Cactus. Again, in the Cactuses themselves we have curious cases of plant mimicry; as, for instance, the Rhipsalis, which looks like a bunch of Mistletoe, and the Pereskia, the leaves and habit of which are more like what belong to, say, the Gooseberry family than to a form of Cactus. From this it will be seen that although these plants are almost all succulent, and curiously formed, they are by no means singular in this respect.
The characters of the order are thus defined by botanists: Cactuses are either herbs, shrubs, or trees, with soft flesh and copious watery juice. Root woody, branching, with soft bark. Stem branching or simple, round, angular, channelled, winged, flattened, or cylindrical; sometimes clothed with numerous tufts of spines which vary in texture, size, and form very considerably; or, when spineless, the stems bear numerous dot-like scars, termed areoles. Leaves very minute, or entirely absent, falling off very early, except in the Pereskia and several of the Opuntias, in which they are large, fleshy, and persistent. Flowers solitary, except in the Pereskia, and borne on the top or side of the stem; they are composed of numerous parts or segments; the sepals and petals are not easily distinguished from each other; the calyx tube is joined to, or combined, with the ovary, and is often covered with scale-like sepals and hairs or spines; the calyx is sometimes partly united so as to form a tube, and the petals are spread in regular whorls, except in the Epiphyllum. Stamens many, springing from the side of the tube or throat of the calyx, sometimes joined to the petals, generally equal in length; anthers small and oblong. Ovary smooth, or covered with scales and spines, or woolly, one-celled; style simple, filiform or cylindrical, with a stigma of two or more spreading rays, upon which are small papillae. Fruit pulpy, smooth, scaly, or spiny, the pulp soft and juicy, sweet or acid, and full of numerous small, usually black, seeds.
Tribe I.—Calyx tube produced beyond the Ovary. Stem covered with Tubercles, or Ribs, bearing Spines.