Taxonomic significance of trichomes in the genus Acanthopsis Harv. (Acanthaceae, tribe Acantheae)

ABSTRACT Hitherto characters derived mostly from macromorphology were used to distinguish among the members of Acanthopsis Harv., a taxonomically difficult group of plants endemic to arid parts of southern Africa. The taxonomic significance of leaf and bract indumentum in Acanthopsis was investigated using light, and scanning electron microscopy. Five non-glandular and three glandular trichome types were distinguished on the leaves and floral bracts. None of these trichome types are unique to the genus. Although the indumentum is variable in Acanthopsis, the density/dominance of specific trichome types was useful to distinguish among certain taxa, particularly on the abaxial surfaces of leaves and bracts. Each species of Acanthopsis has a characteristic trichome complement as far as dominant trichome types on both leaves and bracts are concerned. However, no obvious association between the type of trichomes and habitat (degree of aridity) or geographical distribution was evident.