A taxonomic revision of 51 species of Riccia, nearly three quarters of which are probably endemic to southern Africa, as well as the cosmopolitan, mono typic species, Ricciocarpos natans, is presented. The systematic treatment includes keys to the genera, subgenera, sections, groups and species. Each species is described in detail, accompanied by its nomenclature, as well as information on its geographical distribution and ecology. Several thousand specimens have been collected by the author and by others, notably O.H. Yolk, over the last 10 years or more, and are listed, together with older collections by Amell, Duthie and Garside and by Sim, to name but a few. Illustrations include line drawings of the thalli of every species, plus photographs of the thalli of some and spore micrographs of all. The phylogeny is briefly considered with reference to the following criteria: palaeobotanical, phytogeographical, morphological, cytological and biochemical. A hypothetical ancestor is postulated and primitive character states as opposed to advanced ones are examined. An extensive bibliography is provided. / Thesis (PhD)--University of Pretoria, 1991. / Plant Science / unrestricted