Systematics of the Genus Angraecum (Orchidaceae)

Abstract

Summary. The infrageneric classification of the genus Angraecum is reviewed. Included is a key to the I9 sections recognized by the writer, of which three are established for the first time, as well as a complete index to all species that have been included in Angraecum, indicating their correct taxonomic disposition. Currently there are approximately 550 binomials in the genus Angraecum and of these 206 are accepted to denote taxonomically as well as biologically distinct species. Such large numbers inherently reflect a complexity in which phyletic relationships are often obscured by the phenomena of convergent and parallel evolution. Hence, when an attempt is made to arrange the constituent taxa in a systematic order, the complexity of the genus becomes very real. Plagued by the varied and often contradicting generic delimitations of Angraecum found in the literature, Schlechter undertook a synoptic revision of all angraecoid orchids in 1918. At that time he distributed the known species in 32 genera, many of which were proposed by him for the first time. In the genus Angraecum he included all those plants in which the clinandrium is deeply divided into two broad lobes with a rudimentary, tooth-like rostellum in the centre. The two pollinia usually are attached through distinct but rudimentary stipites or stipe-like caudicles either to a single viscidium or each pollinium to one of the two separate viscidia. The lip is almost always undivided, rarely with a short anterior lobe, enveloping the column only at the base, but with a more or less concave and spreading lamina above. Armed with this circumscription, Schlechter divided the genus into six sections of which he typified only four. Soon after the publication of his revision, Schlechter received a very extensive collection of orchids from Perrier de la Bathie, collected in Madagascar, in which the angraecoid orchids were exceptionally well represented. In view of the information contained in this large collection Schlechter found his six sections of Angraecum woefully inadequate to accommodate the rich diversity that is so characteristic for the genus. Once again he started afresh to rework the genus Angraecum. The results of this new study were presented in his 'Orchidaceae Perrierianae' (1925). Although this new study was a great improvement over the former one, for it included a systematic key to the various sections which had grown to thirteen in number, it was primarily designed to accommodate species reported only from Madagascar and from the neighbouring islands. Again Schlechter failed to typify most of his sections. The obvious discrepancy between the two presentations of Schlechter, the manner in which he distributed the various species among the sections, and his lack of attention to the mainland species prompted Summerhayes to update Schlechter's work in 1958, i.e. so far as the continental African