Aa

Aa Reich. is a genus in the orchid family (Orchidaceae). It is classified in the subfamily Spiranthoideae, tribe Cranichideae, subtribe Cranichidinae. One explanation for the name apparently was rendered by the author (Reichenbach) to always appear first in alphabetical listings. Another - disputed - explanation, is that Reichenbach named this genus after Pieter van der Aa, the printer of the Dutch botanist Paul Herman's 'Paradisus Batavus'.

These species can be found in cold habitats, growing near the snowline in the Andes and also in Costa Rica. It usually appears close to small streams. The elongated inflorescence grows from a basal rosette of leaves, terminating in a small white upside down flower with the lip at the top. This lip is fringed and hood-shaped. The flower gives off a pungent smell that attracts flies.

There are more than 25 known species, endemic to the Andes. This genus has often been confused with the orchid genus Altensteinia.