Cactus, the general name in popular use for the 800 species of the order Cactaceae, which are now referred to 18 genera. They are a somewhat isolated group of calycifloral dicotyledons, almost all natives of America, inhabiting the dry regions of the south-western United States, Mexico, Peru, and the Andean plateaux. They are succulent shrubs with stems either flattened and leaf-like, spherical, or polygonal and columnar; and their leaves are represented by spines grouped in clusters or undeveloped branches. They have a watery juice, in which they differ from "the milky spinous Euphorbias that occupy similar situations in Africa. Cacti have large sessile flowers with indefinite sepals graduating into the petals, which are also numerous, as are the stamens. The ovary is inferior and one-chambered, with numerous seeds on parietal placentas, and forms a succulent fruit. Several species have been introduced into Southern Europe and the East, especially the prickly pear (Opuntia vulgaris) and the nopal (Nopalea coccinellifera), the food of the cochineal insect (Coccus cacti).