Most of the smaller genera of the subfamily Opuntioideae have received little attention from researchers and are often rare in cultivation. Such is not the case with Pterocactus, however, at least with respect to research. Roberto Kiesling (1982) published a comprehensive study of the genus, his research clarifying the taxonomy of the species and the relationship of Pterocactus to other cacti. A question remained, however, about the type species of the genus. Nigel Taylor
Pseudorhipsalis lankesteri, photograph by Wilhelm Barthlott
Pseudorhipsalis lankesteri, photograph by Wilhelm Barthlott and James Uiff (1996) analyzed correspondence and other materials to sort out the answer. In 1837 Ludwig Pfeiffer described Opuntia tuberosa (= P. tuberosus) from plant material collected by John Gillies near Mendoza, Argentina. In 1897 Karl Schumann described P. kutitzei (= P. tuberosus) in the same publication in which he described the genus Pterocactus, the plant material also coming from near Mendoza. Taylor and Iliff presented persuasive evidence that the material described in both publications is from the same plants and kept nomenclatural matters as simple as possible by neo-typifying O. tuberosa with the lectotype of P. kuntzei (- P. tuberosus), thus making P. tuberosus the type species.
The name Pterocactus is derived from the Greek pteron, wing, thus winged cactus, referring to the distinctive winged seeds. The nine species have large tuberous roots, underground shoots, short-lived aerial stems, flowers immersed in the shoots, glochids present, unique dehiscent fruits, and arils that develop into broad wings surrounding the seeds. They flower during the day in summer. Pterocactus is a highly specialized member of the subfamily Opuntioideae.
Subfamily Opuntioideae. Plants dwarf, nearly geophytic shrubs. Roots tuberous, producing one or more slender underground stems or necks that ascend to ground level, where several ephemeral aerial stems arise. Stems small, globose to club shaped or cylindrical, green to brown to purple, to 10 cm (3.9 in) longand 2 cm (0.8 in) in diameter, often with papillate surfaces. Leaves small, awl shaped, soon falling away. Areoles with fine spinesand glochids. Spines few, needlelike, awl-like, orpapery. Flowers truly terminal, immersed in the tips of the stem segments, yellow to reddish, rotate; pericarpels slightly tu-berculate and bearingtufts of small spines; stamens touch sensitive. Fruits dry, depressed apically, dehiscing transversely near the tops. Seeds winged, more or less circular, flat, pale beige, papery, surrounded by the arils that forms broad wings. Distribution: Argentina, throughout Patagonia and into the northwest as far as Salta.
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