Sematophyllum is a pantropical genus of approximately 170 species. The genus has never been revised on a global scale and is poorly known in many areas of the world. This is perhaps especially true in Africa where 60 accepted species are recorded, many seemingly endemic to the eastern and central tropical areas. Further taxonomic work will enable better understanding of the genus, its biogeography and regional patterns of plant diversity; help guide conservation efforts; and facilitate a

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... understanding of the evolution of the Afrotropical flora.Objectives: The objective of this study is to describe a new species of Sematophyllum from Rwanda, part of the mega-diverse Albertine Rift system.Methods: Light microscopy was used to compare anatomical and morphological details of the putative new species with specimens of other members of the genus with which it could potentially be conspecific.Results: Sematophyllum rheophyticum W.R. Buck & Hedd. was described as new and is currently known only from the type locality at Gisakura, Nyungwe National Park, Rwanda. It is distinguishable from all congeners by a unique combination of characters including the concave, abruptly acuminate leaves, the alar areolation and the rheophytic habitat.Conclusion: Sematophyllum rdicum is a new species, defined by a combination of several anatomical characters, known currently only from the type locality. Furthermore, it is one of only a few species in the genus that occurs in rheophytic habitats. Its phylogenetic relationships are obscure and will probably need to be evaluated with molecular evidence.