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| The genera of Cactaceae |
The plants succulent; opuntioid; low and very compacted in their entirety, or not ‘low and very compacted’. The stems spiny; globose, or shortly cylindric. The plants terrestrial and self supporting, or geophytic (often, almost); branched and offsetting (with tuberous roots, and one or more underground stems ascending to ground level, giving rise to several ephemeral aerial stems); prostrate to erect; dwarf shrubby; clustering; to 0.03–0.2 m high. The main stem more or less cylindrical. The branches cylindrical; to 4–20 cm long; to 2 cm in diameter. The stems not segmented (at least, in the normal sense), or segmented; not ribbed and grooved. The plants conspicuously tuberculate (if the commonly papillate stems/segments be so interpreted), or not conspicuously tuberculate. The tubercles not connected by ribs; spirally disposed. The areoles associated with tubercles (or papillae), or not tubercle-associated; distant; spirally disposed; simple; with glochids; with spines (variously hair-like, setaceous, acicular, awl-like or papery). The spines clustered; 6–30; 0.1–5 cm long; with radials and centrals differentiated, or showing little or no difference between radials and centrals. Central spines when determinable, 1–5. Radial spines 10–20. The spines without sheaths; not booked. The mature stems with much reduced leaves. Leaves of mature stems caducous, minute; fleshy (cylindric or subulate); terete.
Flowering during the day. The flowers truly terminal (terminating stems, sometimes ostensibly continuous with them); one per areole; rotate; sessile; medium-sized; 2–5 cm in diameter; regular. The receptacle not produced beyond the ovary; not naked; with spines. The pericarpel slightly tuberculate, areolate, with glochids and tufts of small spines. The perianth green, or yellow, or pink, or red. The perianth segments somewhat spreading, or more or less erect (exceeding the style and stamens); relatively short, broad. Stamens touch sensitive. The funicles circinate.
The mature fruit 2–3 cm long; depressed; probably naked ("the areoles falling away when ripe"), or not naked (?); probably without spines; apically depressed, non-fleshy when mature; dehiscent; dehiscing transversely near the top. The seeds pale brown, or white; more or less flat (papery); flattened; characteristically unevenly, broadly winged (via the encircling aril); encased in their bony arils. Cotyledons fleshy, foliaceous.
Natural Distribution. Argentina, Patagonia.
Classification. About 9 species. Subfamily Opuntioideae. Tribe Pterocacteae.
Cf. Hunt (1967).
Images. • Pterocactus hickenii and P. fischeri: Britton & Rose (1919).
We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa. See also Guidelines for using data taken from Web publications.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2018 onwards. The genera of Cactaceae: descriptions, illustrations, identification, and information retrieval. Version: 14th November 2021. delta-intkey.com’.
