POLLEN MORPHOLOGY OF CANTHIUM, KEETIA AND PSYDRAX (RUBIACEAE, VANGUERIEAE) IN SOUTHERN AFRICA

Pollen of all the southern African members of Canthium, Keetia and Psy drax was studied by means of LM, SEM and TEM. Palynologically these th ree genera can easily be distinguished from one another, exine structu re and sculpturing being the most useful characters. The sexine is ess entially perforate with short columellae not usually distinguishable i n SEM in the Canthium type, coarsely reticulate with long columellae i n the Keetia type and more finely reticulate with short columellae in the Psydrax type. Palynologically Canthium sensu stricto (used here to refer to the Canthium complex excluding Psydrax, Keetia and Pyrostria ) is clearly distinct from Keetia and Psydrax, but similar to the othe r members of the Vanguerieae. This supports the proposed subdivision o f Canthium sensu late in southern Africa into these three genera. Diff erences amongst the various Canthium sensu stricto species suggest at least three different pollen sub-types. Bridson's placement of C. iner me and C. suberosum in the subgenus Lycioserissa is supported by the p ollen morphology. It is suggested that C. ciliatum, C. kuntzeanum, C. spinosum and C. vanwykii may also belong to this subgenus. The followi ng placements are supported by palynology: C. gilfillanii and C. mundi anum in the subgenus Afrocanthium and C. setiflorum in Bullockia. The presence of intine protruding from the apertures is shown not to be an artefact, but a phenomenon characteristic of many Rubiaceae. The term ''protruding oncus'' is proposed for these structures.