Tribe 2 Opuntieae
Plants usually very fleshy, never epiphytic, branched (usually much branched), one to many-jointed; joints diverse in structure, terete, compressed, or much flattened, with irregularly scattered areoles, ribless, except one species; leaves usually caducous, but in some species more or less persistent, small or minute, subulate or cylindric, in one genus broad and flat; areoles usually glochidiferous (except in Maihuenia; in Grusonia only those of the ovary), mostly spine-bearing; spines usually slender, straight or nearly so, sometimes sheathed; corolla mostly rotate (sepals and petals in Nopalea erect); flowers sessile, diurnal, one from an areole; fruit usually a fleshy berry, sometimes dry, rarely capsular; seed white or black, globular, flattened or even winged, with a thin or hard testa; cotyledons large, elongated.
This tribe contains 7 genera and at least 300 species, various in habit, flower, fruit, and seeds. It is more closely related to the Pereskieae than to the Cereeae. The following characters possessed by some or all genera of the Opuntieae are wanting in the Cereeae:
Leaves on the stem (see also Harrisia and Hylocereus); glochids in the areoles; sensitive stamens; sheathed spines; winged, white, and bony-covered seeds; the separation of withering calyx, stamens, and style from the ovary; areoles irregularly distributed over the stem in all the genera except Grusonia, in which they are arranged on ribs as in many of the Cereeae.
The tribe is distributed throughout the cactus regions of the Americas, but the genera, except Opuntia, are localized.
Continue reading here: Key to Genera
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