Taxonomic revision of Saxicolella (Podostemaceae), African waterfall plants highly threatened by Hydro-Electric projects.

Species of the genus Engl. (Podostemaceae) are African rheophytes, restricted to rapids and waterfalls as are all members of the family. Previously, was shown to be polyphyletic with two separate clades. The name Engl. was recently resurrected for one clade that is sister to the American genera (Tul.) Wedd., Michx. and all Old World Podostemoideae. has distichous phyllotaxy, bilocular ovaries, filiform roots with paired holdfasts, and rootcaps. The second clade, , including the type of the generic name, has spiral phyllotaxy, unilocular ovaries, ribbon-like or crustose roots that lack both holdfasts and rootcaps. , sampled from the type species, Engl. of Cameroon, is embedded within and near the base of the major clade of African podostemoids and is sister to all other African genera apart from R.E.Fr. and Engl. Recently reduced to three species in Cameroon and SE Nigeria by the resurrection of is expanded to eight species in this monograph by description of five new taxa. Cheek and Cheek are newly described from Guinea, Cheek from Cameroon, the informally named sp. A from Gabon, and Cheek from Angola. The known geographic range of the genus is thus expanded c. 2,500 km westwards to Guinea from eastern Nigeria and c.1,500 km southeastwards from near Yaoundé to Cuanza do Sul, Angola. The greatest concentration of species occurs in the Cross-Sanaga interval of western Cameroon and eastern Nigeria, with three species. Cameroon (3 species) followed by Nigeria and Guinea (2 species each) are the countries with highest species diversity. A classification is proposed grouping the species into three subgenera ( (G.Taylor) Cheek comb. et stat. nov. and Cheek subgen. nov.) based on root morphology and shoot position and morphology. The discovery, morphology, circumscription, distribution and ecology of is reviewed, an identification key to the species is presented, together with descriptions, synonymy and links to illustrations. All of the species are provisionally assessed as either Endangered or Critically Endangered using the IUCN 2012 Red List Criteria. The major threats, above all, are hydro-electric projects. may already be globally extinct, and two of the four known locations of appear lost, sp. A of Gabon is threatened at at least one of its three locations, while is threatened at all three locations. Contamination of watercourses by increased turbidity from silt-load due to anthropic changes and by eutrophication from pollution are also threats for the majority of the species.