A systematic study of the genus Acianthus (Orchidaceae: Diurideae)
Description
The genus Acianthus (Orchidaceae, tribe Diurideae) was studied using a combination of morphological, anatomical, molecular and cladistic methods in order to resolve its phylogeny and taxonomy. One new species is described. Two subgenera and five sections are recognized. The relationship of Acianthus to the other genera within the subtribe Acianthinae was investigated by comparing the anatomy and morphology of Acianthus species with representatives from Corybas, Stigmatodactylus, and Townsonia. Character evolution and geographical distributions are discussed for the subtribe Acianthinae and the genus Acianthus, and keys are provided to all infrageneric taxa within Acianthus. Species of Acianthus are terrestrial herbs that arise from small subglobose subterranean storage organs. The genus is characterized by its single, well developed, broad, membranaceous leaf blade with the veins anastomosing at the apex, terminal, racemose inflorescence, labellum disk with one or two calli at the base and a suprabasal, finely reticulate glandular region, and a pollinarium with either a single common viscidium or a pair of viscidia. Acianthus contains 22 species distributed throughout portions of Australia, New Zealand, New Caledonia and the Ile des Pins. A cladistic study of the subtribe Acianthinae suggests a close relationship between Acianthus and Stigmatodactylus and a more distant affinity with Townsonia. Cyrtostylis, which has been regarded as a segregate genus by some authors, is given sectional rank in the current treatment