H.G. Reichenbach, in Xenia Orchidaceae, described the unique genus Aa. Pronounced "ah ah" this genus of 25 species occurs in remote grasslands at high altitudes far above the tree line (3000–4400 m) in Andean South America and the paramo of Costa Rica. The genus is not normally found in cultivation because of its preference for a cold high-altitude habitat and also because of its the small dirty-white flowers. Not far below the snowline, the plants are terrestrial and characterized by a basal rosette of leaves with an elongated inflorescence that originates in a lateral position. The flowers, non-resupinate and surrounded by papery bracts, have an odd unpleasant sent that may attract a tiny fly. The lip is hood-shaped with a fimbriate margin. The glabrous column has large stigmas and four pollinia (see illustration).
Even though some orchid taxonomists have reduced Aa from a separate genus to simply a member of Altensteinia , most taxonomists accept it as a valid genus. The Lectotype for the genus is Ophrys paleacea Kunth (1806). Smithsonian orchid expert Bob Dressler classifies the genus in subfamily Spiranthoideae, tribe Cranichideae, subtribe Cranichidinae. The origin of the generic name is unclear. Some believe that it was contrived to occur first in a generic list while others believe it may be named for the Dutch artist Pieter van der Aa. F.C. Hoehne suggest the name was related to the words "agua corrente", a flow of water, since Aa is generally found near small streams. Aa is surely one of the oddest names in taxonomy. Other families also have unusual names, such as the wasp Lala palusa or the clam Abra cadabara.
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