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, scanni ng 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 co lpori. 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. Uni formity 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 con sistent within a species and may indicate groups of closely related sp ecies. Variation, however, occurs in two species, Dyschoriste erecta a nd D. hildebrandtii. There is no evidence to date that the number of p seudocolpi is correlated with ploidy level. The greatest diversity in pollen morphology occurs in South African species, and may indicate th at this is a centre of diversity for Dychoriste and Chaetacanthus. The aperture structure, foot layer and endexine show similarities to Hygr ophila and Brillantaisia, also in the Ruellieae. Dyschoriste and Chaet acanthus are, however, characterized by distinctive branched columella e. ''Abnormal'' pollen is found in D. hildebrandtii and C. costatus. I n the latter, the pollen is so distinctive that it is placed in a sepa rate pollen type. It is suggested that this deviation is due to ontoge netic factors and that ontogenetic plasticity may be a possible reason for the extreme variation found in the pollen of the Acanthaceae.