A BIOSYSTEMATIC APPROACH TO GENERIC DELIMITATION IN RUELLIA (ACANTHACEAE)

Ruelliais a large, pantropical group of herbs and shrubs that has undergone a long series of changes in classification. Soon after Linnaeus's original description the genus was divided into numerous segregate genera by Nees von Esenbeck and by others. Later Bentham sunk 18 Neesian genera in his expanded concept of Ruellia, and this circumscription has come to be the accepted definition for the plants. Recently Bremekamp revived a number of old segregate groups and described several new ones. He believes Ruellia sensu latois an unwieldy and unnatural assemblage of taxa. Experimental hybridizations have established the existence of species‐complexes (ecospecies) and species‐groups (comparia) in Ruellia. The latter are genetically isolated from other species‐groups, and taxonomically they correspond to certain segregate genera. There is evidence that North American taxa are closely related to tropical species. Since numerous species can be artificially hybridized, application of the ecotype concept may provide a practical means of identifying natural species‐groups. Present evidence suggests that Ruellia, as generally understood, is not a natural, phyletic, unit.