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..
Succulent plants, with jointed branching stems, the joints flat, or cylindric, and small mostly subulate deciduous spirally arranged leaves, the areolae axillary, often spine-bearing and almost always with barbed bristles (glochides). Flowers usually lateral. Calyx-tube not prolonged beyond the ovary, its lobes numerous, spreading. Petals numerous, slightly united at the base. Stamens very numerous, arranged in several rows; filaments distinct or slightly united. Ovary cylindric, exserted; style cylindric, longer than the stamens; stigma 2-7-rayed. Berry pear-shaped, often spiny. [Named from a town in Greece.]
About 200 species, natives of America. Besides the following, some 90 or more others occur in the western and southwestern States. Type species: Cactus Opuntia L.
Joints flattened, oval, oblong, obovate or orbicular; stems prostrate or ascending.
Fruit fleshy, juicy, spineless or sparingly spiny.
Joints spineless, or with solitary stout spines.
Joints spiny (no. 2 sometimes unarmed), the spines 1 - 15 at each areola.
Joints 3'-5' long; longer spines 1/2'-1 1/2' long.
Joints 6'-8' long; longer spines 1'-2 1/2' long.
Spines reddish brown to black; j'oints 6'-8' long.
Fruit dry, with spine-bearing areolae.
Joints orbicular or broadly obovate, flat.
Joints little flattened, ovoid, or subglobose.
Joints cylindric, or nearly so; stem erect.
Fig. Fig. 2986
Cactus Opuntia L. Sp. Pl. 468. 1753.
O. vulgaris Mill. Gard. Dict. Ed. 8, no. 1. 1768.
O. Opuntia Coult. Contr. Nat. Herb. 3: 432. 1896.
Prostrate, or ascending, joints obovate, oval or orbicular, 2'-5' long. Leaves subulate, appressed or somewhat spreading, 2"-4" long, usually early deciduous; bristles greenish or yellowish brown; spines, when present, solitary, grayish or variegated, stout, not deflexed, 3"-1 1/2' long, often wanting; flowers yellow, sometimes with a reddish center, 2'-3' broad; petals 8-10; fruit obovoid, fleshy, edible, 1'-1 1/2' long, red.