The Cacti may be described in general terms as plants having a woody axis, overlaid with thick masses of cellular tissue forming the fleshy stems. These are extremely various in character and form, being globose, cylindrical, columnar or flattened into leafy expansions or thick joint-like divisions, the surface being either ribbed like a melon, or developed into nipple-like protuberances, or variously angular, but in the greater number of the species furnished copiously with tufts of horny spines, some of which are exceedingly keen and powerful. These tufts show the position of buds, of which, however, comparatively few are developed. The stems are in most cases leafless, using the term in a popular sense; the leaves, if present at all, being generally reduced to minute scales. In one genus, however, Peireskia, the stems are less succulent, and the leaves, though rather fleshy, are developed in the usual form. The flowers are frequently large and showy, and are generally attractive from their high colouring. In one group, represented by Cereus, they consist of a tube, more or less elongated, on the outer surface of which, towards the base, are developed small and at first inconspicuous scales, which gradually increase in size upwards, and at length become crowded, numerous and petaloid, forming a funnel-shaped blossom, the beauty of which is much enhanced by the multitude of conspicuous stamens which with the pistil occupy the centre. In another group, represented by Opuntia (fig. 1), the flowers are rotate, that is to say, the long tube is replaced by a very short one. At the base of the tube, in both groups, the ovary becomes developed into a fleshy (often edible) fruit, that produced by the Opuntia being known as the prickly pear or Indian fig.
The principal modern genera are grouped by the differences in the flower-tube just explained. Those with long-tubed flowers comprise the genera Melocactus, Mammillaria, Echinocactus, Cereus, Pilocereus, Echinopsis, Phyllocactus, Epiphyllum, &c.; while those with short-tubed flowers are Rhipsalis, Opuntia, Peireskia, and one or two of minor importance. Cactaceae belong almost entirely to the New World; but some of the Opuntias have been so long distributed over certain parts of Europe, especially on the shores of the Mediterranean and the volcanic soil of Italy, that they appear in some places to have taken possession of the soil, and to be distinguished with difficulty from the aboriginal vegetation. The habitats which they affect are the hot, dry regions of tropical America, the aridity of which they are enabled to withstand in consequence of the thickness of their skin and the paucity of evaporating pores or stomata with which they are furnished,—these conditions not permitting the moisture they contain to be carried off too rapidly; the thick fleshy stems and branches contain a store of water. The succulent fruits are not only edible but agreeable, and in fevers are freely administered as a cooling drink. The Spanish Americans plant the Opuntias around their houses, where they serve as impenetrable fences.
Melocactus, the genus of melon-thistle or Turk's-cap cactuses, contains, according to a recent estimate, about 90 species, which inhabit chiefly the West Indies, Mexico and Brazil, a few extending into New Granada. The typical species, M. communis, forms a succulent mass of roundish or ovate form, from 1 ft. to 2 ft. high, the surface divided into numerous furrows like the ribs of a melon, with projecting angles, which are set with a regular series of stellated spines—each bundle consisting of about five larger spines, accompanied by smaller but sharp bristles—and the tip of the plant being surmounted by a cylindrical crown 3 to 5 in. high, composed of reddish-brown, needle-like bristles, closely packed with cottony wool. At the summit of this crown the small rosy-pink flowers are produced, half protruding from the mass of wool, and these are succeeded by small red berries. These strange plants usually grow in rocky places with little or no earth to support them; and it is said that in times of drought the cattle resort to them to allay their thirst, first ripping them up with their horns and tearing off the outer skin, and then devouring the moist succulent parts. The fruit, which has an agreeably acid flavour, is frequently eaten in the West Indies. The Melocacti are distinguished by the distinct cephalium or crown which bears the flowers.
Mammillaria.—This genus, which comprises nearly 300 species, mostly Mexican, with a few Brazilian and West Indian, is called nipple cactus, and consists of globular or cylindrical succulent plants, whose surface instead of being cut up into ridges with alternate furrows, as in Melocactus, is broken up into teat-like cylindrical or angular tubercles, spirally arranged, and terminating in a radiating tuft of spines which spring from a little woolly cushion. The flowers issue from between the mammillae, towards the upper part of the stem, often disposed in a zone just below the apex, and are either purple, rose-pink, white or yellow, and of moderate size. The spines are variously coloured, white and yellow tints predominating, and from the symmetrical arrangement of the areolae or tufts of spines they are very pretty objects, and are hence frequently kept in drawing-room plant cases. They grow freely in a cool greenhouse.
Fig. 2.—Echinocactus much reduced; the flowers are several inches in diameter.
Echinocactus (fig. 2) is the name given to the genus bearing the popular name of hedgehog cactus. It comprises some 200 species, distributed from the south-west United States to Brazil and Chile. They have the fleshy stems characteristic of the order, these being either globose, oblong or cylindrical, and either ribbed as in Melocactus, or broken up into distinct tubercles, and most of them armed with stiff sharp pines, set in little woolly cushions occupying the place of the buds. The flowers, produced near the apex of the plant, are generally large and showy, yellow and rose being the prevailing colours. They are succeeded by succulent fruits, which are exserted, and frequently scaly or spiny, in which respects this genus differs both from Melocactus and Mamrmllaria, which have the fruits immersed and smooth. One of the most interesting species is the E. ingens, of which some very large plants have been from time to time imported. These large plants have from 40 to 50 ridges, on which the buds and clusters of spines are sunk at intervals, the aggregate number of the spines having been in some cases computed at upwards of 50,000 on a single plant. These spines are used by the Mexicans as toothpicks. The plants are slow growers and must have plenty of sun heat; they require sandy loam with a mixture of sand and bricks finely broken and must be kept dry in winter.
Cereus.—This group bears the common name of torch thistle. It comprises about 100 species, largely Mexican but scattered through South America and the West Indies. The stems are columnar or elongated, some of the latter creeping on the ground or climbing up the trunks of trees, rooting as they grow. C. giganteus, the largest and most striking species of the genus, is a native of hot, arid, desert regions of New Mexico, growing there in rocky valleys and on mountain sides, where the tall stems with their erect branches have the appearance of telegraph poles. The stems grow to a height of from 50 ft. to 60 ft., and have a diameter of from 1 ft. to 2 ft., often unbranched, but sometimes furnished with branches
which grow out at right angles from the main stem, and then curve upwards and continue their growth parallel to it; these stems have from twelve to twenty ribs, on which at intervals of about an inch are the buds with their thick yellow cushions, from which issue five or six large and numerous smaller spines. The fruits of this plant, which are green oval bodies from 2 to 3 in. long, contain a crimson pulp from which the Pimos and Papagos Indians prepare an excellent preserve; and they also use the ripe fruit as an article of food, gathering it by means of a forked stick attached to a long pole. The Cereuses include some of our most interesting and beautiful hothouse plants. In the allied genus Echinocereus, with 25 to 30 species in North and South America, the stems are short, branched or simple, divided into few or many ridges all armed with sharp, formidable spines. E. pectinatus produces a purplish fruit resembling a gooseberry, which is very good eating; and the fleshy part of the stem itself, which is called cabeza del viego by the Mexicans, is eaten by them as a vegetable after removing the spines.
Pilocereus, the old man cactus, forms a small genus with tallish erect, fleshy, angulate stems, on which, with the tufts of spines, are developed hair-like bodies, which, though rather coarse, bear some resemblance to the hoary locks of an old man. The plants are nearly allied to Cereus, differing chiefly in the floriferous portion developing these longer and more attenuated hair-like spines, which surround the base of the flowers and form a dense woolly head or cephalium. The most familiar species is P. senilis, a Mexican plant, which though seldom seen more than a foot or two in height in greenhouses, reaches from 20 ft. to 30 ft. in its native country.