Studies in African Rubiaceae: Vanguerieae: A New Circumscription of Pyrostria and a New Subgenus, Canthium subgen. Bullockia

Abstract

In two preceding papers (Bridson 1985 & 1986), concerned with Vanguerieae, Psydrax Gaertn. and Keetia E. P. Phillips, both chiefly characterised by narrowly cylindrical stigmatic knobs borne on long styles, were separated from Canthium sensu lato. Most taxa remaining in Canthium sensu lato have the stigmatic knob + as wide as long and the style sub-equal to or slightly longer than the corolla-tube; however, considerable diversity is still present, species, for the most part, forming natural groups (either generic or infrageneric). The African representatives of two such groups are revised here, one, Pyrostria, at generic rank, and the second as a subgenus of Canthium. Originally the genus Pyrostria, restricted to the Mascarenes, accommodated species with umbellate inflorescences enclosed in bud by persistent paired connate bracts, unisexual flowers and plurilocular ovaries. Subsequently, however, many 2-locular Madagascan species have been placed in Pyrostria. Species from the African mainland show little significant difference from Madagascan Pyrostria, apart from including some species with hermaphrodite flowers. The African species are currently dispersed amongst Canthium (including Plectronia sensu auctt.), Rytigynia and Dinocanthium, and although the combination in Pyrostria has been made for the well known Afro-Madagascan species, Canthium bibracteatum (Cavaco 1968), it has not hitherto been adopted in Africa.