A POLLEN MORPHOLOGICAL-STUDY OF DYSCHORISTE NEES AND CHAETACANTHUS NEES (ACANTHACEAE, RUELLIEAE)

The pollen morphology of 31 species of Dyschoriste Nees and 3 species of Chaetacanthus Nees is described and illustrated using light, scanning and transmission electron microscopy of acetolyzed herbarium pollen. Most of the material examined is placed in one pollen type, which is heterocolpate, 3-colporate, with 4-13 pseudocolpi between adjacent colpori. Seven subtypes are recognized, based on variation in the number of pseudocolpi. These form a continuum of variation and on this basis the separation of Dyschoriste and Chaetacanthus is not justified. Uniformity in surface sculpturing and exine stratification also indicate that these two genera should be merged. This is supported by evidence from macromorphological characters. The pollen subtypes are fairly consistent within a species and may indicate groups of closely related species. Variation, however, occurs in two species, Dyschoriste erecta and D. hildebrandtii. There is no evidence to date that the number of pseudocolpi is correlated with ploidy level. The greatest diversity in pollen morphology occurs in South African species, and may indicate that this is a centre of diversity for Dychoriste and Chaetacanthus. The aperture structure, foot layer and endexine show similarities to Hygrophila and Brillantaisia, also in the Ruellieae. Dyschoriste and Chaetacanthus are, however, characterized by distinctive branched columellae. ''Abnormal'' pollen is found in D. hildebrandtii and C. costatus. In the latter, the pollen is so distinctive that it is placed in a separate pollen type. It is suggested that this deviation is due to ontogenetic factors and that ontogenetic plasticity may be a possible reason for the extreme variation found in the pollen of the Acanthaceae.