Perennial, succulent plants, various in habit, mostly very spiny, characterized by specialized organs termed areoles. Leaves usually none, except in Pereskia and Pereskiopsis, where they, are large and flat but fleshy, and in Opuntia and its relatives, where they are usually much reduced and mostly caducous, terete, or subulate. Spines very various in size, form, arrangement, and color, sometimes with definite sheaths. The areoles are peculiar and complex organs, situated in the axils of leaves when leaves are present, and bearing the branches, flowers, spines, glochids, hairs, or glands; in some genera two kinds of areoles occur, either distinct or united by a groove. Flowers usually perfect, either regular or irregular, usually solitary but sometimes clustered, sometimes borne in a specialized terminal dense inflorescence called a cephalium; perianth-tube none, or large and long, the limb spreading or erect, short or elongated, the lobes few or numerous, often intergrading in shape and color, but sometimes sharply differentiated into sepals and petals; stamens commonly numerous, elongated or short, sometimes clustered in series, the filaments usually borne on the throat of the perianth, the small oblong anthers 2-celled; style one, terminal, short or elongated; stigma-lobes 2 to many, usually slender; ovary 1-celled, distinct, or immersed in a branch or forming a part of a branch; ovules numerous. Fruit a berry, often juicy and sometimes edible, sometimes dry, in one species described as capsular and dehiscing by an operculum, in others opening by a basal pore. Seeds various; cotyledons two, accumbent, sometimes minute knobs, often broad or elongated; endosperm little or copious; radicle terete.
The order consists of the following family only:
Family CACTACEAE Lindley, Nat. Syst. ed. 2. 53. 1836.
Characters of the order as given above. The family is composed of three tribes.
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