This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Fleshy plants, with flattened terete ridged or tubercled, continuous or jointed stems, leafless, or with small leaves (only the tropical genera, Pereskia and Peres-kiopsis, with large flat leaves), generally abundantly spiny, the spines developed from cushions of hairs or bristles (areolae). Flowers mostly solitary, sessile, terminal or lateral, perfect, regular, showy. Calyx-tube adnate to the ovary, its limb many-lobed or with distinct sepals. Petals numerous, imbricated in several rows, mostly distinct. Stamens numerous, inserted on the throat of the calyx. Filaments filiform; anthers small. Ovary I-celled; ovules numerous, anatropous, borne on several parietal placentae. Style terminal, elongated; stigmas numerous. Fruit a berry, mostly fleshy, sometimes nearly dry. Seeds smooth, or tubercled, arillate, the testa usually crustaceous or bony; endosperm little, or copious.
About 40 genera and 1000 species, all natives of America, except some species of the epiphytic genus Rhipsalis in tropical Asia and Africa.
Stems subglobose, oval, ovoid or cylindric, tubercled, ribbed or angled; no proper leaves.
Flowers borne on the tubercles or ribs, at or near the areolae.
Flowers borne close to fully developed clusters of spines.
Flowers borne close to areolae, from which the spines subsequently develope
Flowers borne between the tubercles, distant from the areolae.
Plants jointed, the joints flattened, or cylindric; leaves present, terete and mostly early deciduous.