1 Leaves persistent, with broad, flat, pinnately-veined blades; flowers borne in many-flowered cymes; [FL peninsula and s. TX only]; [subfamily Pereskioideae]

1 Leaves absent, rudimentary (usually < 1 mm long), or caducous (lasting up to 3 months, up to 3 cm long, narrowly conical in shape, and terete or broadly elliptical in ×-section); flowers solitary; [collectively widespread in our area].

  2 Areoles with detachable glochids (retrorsely barbed bristles, finer than spines) (and also with or without spines), stems segmented into “pads” that are circular, ovate, obovate, or elliptical in outline and flattened in ×-section, at least apically (Consolea) or throughout (Opuntia), or into cylindric-clavate segments that are terete in ×-section (Cylindropuntia, Grusonia); [subfamily Opuntioideae].

    3 Stems segments terete in ×-section, spines bearing papery epidermal sheaths either deciduous for the whole length of the spine (Cylindropuntia), or deciduous only at apex of spine and exposing spine tips (Grusonia); [tribe Cylindropuntieae].

      4 Plants shrubs to small trees, spine sheaths deciduous for whole length of spine to expose the entire spine

    3 Stems segments flattened in ×-section to form pads, spines without papery epidermal sheaths; [tribe Opuntieae].

        5 Main axis of plant unsegmented, bearing ‘pads’ at its summit; fruits and flowers bearing spines; [FL peninsula only]

        5 Main axis of plant segmented throughout into ‘pads’; fruits and flowers lacking spines; [widespread in our area]

  2 Areoles lacking glochids (and also with or without spines); stems either cylindrical, or cylindrical but with 3-many ribs, or flattened with 2 wings and an apparent central thickened cord; [subfamily Cactoideae].

          6 Stems < 7 mm in diameter; plant a pendulous epiphyte; tepals <12; [tribe Rhipsalidae]

          6 Stems > 7 mm in diameter; plant either a tree, or an erect, ascending, to sprawling shrub, or an epiphyte with broadly winged stems; tepals > 12.

             7 Mature stems (2-) 3-8 angled or winged; [tribe Hylocereeae].

               8 Plants terrestrial, generally lacking adventitious roots

               8 Plants usually epiphytic or epipetric, conspicuously producing adventitious roots.

                 9 Ribs 2 [-3 on proximal portions of juvenile stems], the stem margins crenate-undulate to shallowly lobed, leaf-like

                 9 Ribs [3-]4-8[-12]; stems conspicuously producing adventitious roots

             7 Mature stems terete in general outline, tuberculate or with 4-31 rounded to keeled ribs.

                   10 Spines absent from all areoles of stems, areoles instead with dense tufts of trichomes; stems spheric-hemispheric and often depressed at apex, or rarely cylindric in deep shade (Lophophora); [s. TX only]

                     11 Stems speckled with tufts of whitish trichomes outside of areoles; ribs [4-]8-10; inner tepals bright yellow and proximally red

                     11 Stems not speckled with tufts of whitish trichomes; ribs 5-13; inner tepals white with pinkish midribs

                   10 Spines present on at least some areoles, stems various.

                       12 Plants with at least some central spines recurved to hooked apically

                          13 Stems hemispheric to oblate; plants often buried to near flush with soil surface

                          13 Stems spheric to cylindrical.

                            14 Longest central spines all strongly flattened; stems 7.5-20cm in diameter; tepals bright yellow.

                            14 Longest central spines terete or flattened only adaxially, or rarely some entirely flattened (Hamatocactus); stems 2-12cm in diameter; tepals bright yellow and proximally red-tinged or bright green to yellow-green (rarely reddish).

                              15 Stems with ribs thin & sharply keeled, radial spines 10-19 per areole; tepals bright yellow and proximally red-tinged.

                              15 Stems with ribs rounded or becoming tuberculate, radial spines 13-28 per areole, tepals bright green to yellow-green (rarely reddish).

                       12 Plants with central spines not apically-hooked, if not straight then only weakly curving downward (Coryphantha).

                                16 Long upper spines strongly flattened

                                16 Spines all terete.

                                  17 Stems strongly tuberculate.

                                    18 Flowers borne away from stem apex, areoles lacking linear grooves

                                    18 Flowers borne from stem apex, areoles with narrow, linear grooves on adaxial surfaces.

                                       19 Tepals with entire margins

                                       19 Tepals with fimbriate margins

                                  17 Stems ribbed; the ribs rounded to sharply keeled.

                                         20 Flower tube and fruit with prominent thick or bristle-like spines in addition to scales and hairs. [TX and OK only]

                                         20 Flower tube and fruit with soft hairs or glabrous with only scales.

                                           21 Stems < 7 cm in diameter, stems unbranched to sparsely branched, ascending to sprawling, often scrambling through and supported by other vegetation

                                           21 Stems > 7cm in diameter, stems unbranched to heavily branched, ascending to erect; plants tree-like.

                                             22 Ribs 4-7, spines of mature stems 8-10, flower tube 10 cm long

                                             22 Ribs 8-13, spines of mature stems 9-31, flower tube < 1.5 cm long