Taxonomic revision of the Cyathula achyranthoides group (Amaranthaceae, Achyranthoids) in continental Africa and Madagascar

Desmochaeta (now Cyathula) achyranthoides was described from South America and reported to be a widespread tropical plant in both Africa and the Americas. A revision of herbarium material revealed that inter alia leaf shape differs between the populations of the Eastern and Western Hemispheres. Therefore, we maintain the name C. achyranthoides s. str. for the American populations and re-instate the name C. geminata for most of the African plants.

Both species are found in tropical evergreen forests, mainly at low altitudes. Furthermore, two mountain species, C. brevispicata from Madagascar and C. aethiopica from east tropical Africa, which were previously identified as C. achyranthoides, are described as new to science. Compared to both C. achyranthoides and C. geminata, these new species have short inflorescences and longer, recurved or uncinate perianths in the fertile flowers and morphologically resemble C. fernando-poensis; the latter is only known from the mountains of Equatorial Guinea (Bioko Island), south-west and (newly recorded here) North-West Regions of Cameroon.

The species under study are compared with one another and with the related, pantropically distributed species C. prostrata; their synonymy is verified and typifications are established. The fine-level partial florescence (cyme) structure of each species is also studied, with further taxonomic implications. Cyathula geminata seems to be restricted to west and central tropical Africa, with its range replaced eastwards by C. aethiopica and in Madagascar by C. brevispicata.