Cactaceæ, an order of calycifloral dicotyledons, consisting of succulent shrubs of very singular appearance. Linnaeus included all the Cactaceæ in the single genus Cactus, which is now divided into a number of genera, with about 1000 species; the name Cactus, however, still continuing in popular use, common to the whole order. The cactuses are almost without exception natives of America, and their extraordinary forms give a remarkable character to the vegetation of its warmer regions. One is found in Ceylon, and one is a probable native of West Africa. Most of them are leafless, but Pereskia, the tree-cactus of the West Indies and Central America, has in some of its species large oblong elliptical deciduous leaves, somewhat succulent, but essentially similar to those of ordinary plants. But in adaptation to the excessive poverty and dryness of the soils in which they flourish, and especially to the protracted droughts and torrential rains which characterise the dry and the rainy season, the leaves are frequently reduced to spines, or mere abortive scales, with their axillant buds developed merely as a wart bearing a tuft of spines, or even reduced altogether. The vegetative functions are thus thrown entirely upon the stem, of which the rind-parenchyma or cellular envelope (see BARK) becomes of great succulence, often forming the greater thickness of the stem, although a small central woody bundle can of course always be distinguished. The stem may become flattened, as in the Prickly Pear (Opuntia), or more frequently be thrown into ridges and furrows, so as to increase the vegetative surface. Protected by its dense and continuous epidermis, the bulky stem of the cactus is thus enabled to store a vast supply of water in the spongy bark and pith, and thus withstand the drought and sun of the most rocky and desert situations, of which they often cover large tracts. On this account they are often of great value as sources of water-supply to man and animals; and travellers upon the Mexican plateau have described how the herds of half-wild horses break down branches of Cereus with their hoofs, or kick open the spheroidal Melocactus, to quench their thirst. Hence such forms have been called 'springs in the desert.' Some are copiously branched (Phyllocactus), and sometimes spreading or trailing. Usually, however, the branches are rarer and inserted with peculiar stiffness (Opuntia). In yet more specialised forms the checking of the vegetative buds reduces branching still further, or even stops it altogether, as in the tall upright pillar-like and generally unbranched Cereus giganteus, which gives such a strangely monotonous, almost architectural rather than vegetative character to the deserts of New Mexico. Finally, even upward growth ceases, and we have merely the short, thick, spheroidal stems, like those of Echinocactus, Melocactus, Mammillaria, &c., which of course furnish the ultimate term far up mountain slopes, some Mexican forms almost reaching the snow-line. The peculiar vegetative character, which constitutes the most striking peculiarity of the order, leads to the employment of species of Cereus and Opuntia, &c., in the formation of hedges of great strength and impenetrability, especially in Mexico and the West Indies. Hence the Opuntia has been introduced into the Mediterranean countries, where, in conjunction with the Agave and the native Fan Palm (Chamaerops), it frequently gives a curiously American character to the landscape. The stems of similar species are used for posts and spars, as also for fuel, especially in districts of Peru and Chili, where timber is scarce; and the wood of old Opuntia stems has been used by cabinetmakers.
The cultivation of the cactus family has long been increasingly popular, not so much on account of the frequent beauty or rarity of their flowers as the bizarre and often grotesque vegetative forms, of which the effect is heightened by the spines, which often densely clothe the whole plant, especially in the spheroidal forms, where they may also be replaced or supplemented by a thick growth of woolly or silky hairs.
Some species are easily grown, but their extensive cultivation requires a separate cactus-house, in which the natural conditions of extreme drought during great portion of the year can be imitated. Most of them are easily propagated by branches, which are usually allowed to dry a little before being planted. Those like Melocactus, which does not readily produce branches, can be made to do so by cutting or burning out the solitary apical bud.
The flowers are often of great size and beauty, and are remarkable for their floral envelopes, of which the constituent leaves run in spirals instead of whorls, and are thus insensibly graduated from small green foliage leaves with axillant spiny buds into large and splendid petals. The stamens are also indefinite, the style single, and the stigma three-lobed. The flowers are usually very short-lived; in some night-flowering species, such as the well-known Cereus grandiflorus, common in hot-houses, the flower only opens after dark, and perishes before morning.
The fruit is sometimes termed a berry, but not with justice; since the presence of leaves and spine-tufts over its whole outer surface clearly shows we have to do, as in the rose, with a succulent axis, into which the ovary is depressed. The fruit of many species, especially of Opuntia, is edible and wholesome (see PRICKLY PEAR), and the same genus is further of importance as the food-plant of the cochineal insect. See COCHINEAL.