A review of the genus Rhipidoglossum Schltr

Abstract

The large number of African orchids belonging to the group Monopodiales, the so-called ”Angraecoid Orchids“, constitute a puzzling assemblage of which the main lines of classification are still uncertain. Several well-defined minor groups can, however, be readily distinguished, the most striking of which are such genera as Cyrtorchis Schltr., Aerangis Schltr. (sensu stricto) and Tridactyle Schltr. Among the less welldefined but nevertheless reasonably natural groupings is the genus Rhipidoglossum. This genus was described by Schlechter in his comprehensive treatment and revision of the Angraecoid orchids in 1918. He separated four genera, including Rhipidoglossum, from the remainder on account of the presence of a foot to the column. This character, which appears to be of value in the delimitation of Asiatic genera belonging to the Monopodiales, is, however, of less use in classifying the African genera. Several pairs of closely allied species occur in which one species is with and the other without a foot to the column. On the whole those genera constantly possessing a column-foot can he easily characterised by other more obvious features. Rhipidoglossum, on the other hand, is clearly very closely allied to Diaphananthe in which the column foot is absent or very weakly developed. Indeed the theoretical delimitation of these two genera rests on the presence or absence of a callus in the mouth of the spur, the callus being absent in Rhipidoglossum. At the same time the two genera show different ranges of variation in habit and in floral structure, although the species at the various points of contact closely resemble some of those in the other genus. For instance, the stem is usually elongated in Rhipidoglossum whereas it is short with a rosette type of growth in Diaphananthe. D. bidens, however, which is typical in other respects, has much elongated stems. There is, on the other hand, a tendency towards shortening of the stem in some species of Rhipidoglossum. Secondly, in Diaphananthe the pollinia, although always provided with distinct stipites, usually share a common viscidium. There are also, however, a number of species in which two separate viscidia are found, this feature being general in Rhipidoglossum. The column in the two genera is very similar, and the rostellum is of the same general type