Pollen morphology of Canthium,Keetia and Psydrax (Rubiaceae: Vanguerieae) in southern Africa

Pollen of all the southern African members of Canthium, Keetia and Psydrax was studied by means of LM, SEM and TEM. Palynologically these three genera can easily be distinguished from one another, exine structure and sculpturing being the most useful characters. The sexine is essentially perforate with short columellae not usually distinguishable in SEM in the Canthium type, coarsely reticulate with long columellae in 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 other members of the Vanguerieae. This supports the proposed subdivision of Canthium sensu lato in southern Africa into these three genera. Differences amongst the various Canthium sensu stricto species suggest at least three different pollen sub‐types. Bridson's placement of C. inerme and C. suberosum in the subgenus Lycioserissa is supported by the pollen morphology. It is suggested that C. ciliatum, C. kuntzeanum, C. spinosum and C. vanwykii may also belong to this subgenus. The following placements are supported by palynology: C. gilfillanii and C. mundianum 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.