Studies on the origin, evolution, and distribution of the Gramineae. III. The tribes of the subfamily Eragrostoideae

Abstract

In further studies of grass distribution, maps are presented showing the world distribution of the Eragrosteae (sensu str.) and of the subfamily Eragrostoideae within which it is included. Both taxa show centres of high relative specific differentiation in inland Australia and in South West Africa, but in addition, the subfamily has centres of differentiation in the Sahara rekion, northern Mexico, and north-western India. The centres of differentiation are all in regions of hot, arid climate near the tropics of Cancer and Capricorn. The close relationship between climate and distribution is most apparent in the case of the subfamily Eragrostoideae, species of which are relatively abundant in the grass flora of all arid regions with high winter temperatures and summer or non-seasonal rainfall incidence. The distributions of most of the other tribes and subtribes which have been included in the Eragrostoideae show a similar relationship to climate. Some apparent exceptions to this are discussed, and it is shown that the geographical evidence supports conclusions from recent cytotaxonomic and anatomical studies that the taxa concerned should be removed from the subfamily. The very wide distribution of the subfamily and of its constituent taxa, as well as the close relationship between the distribution pattern and climate, suggests that the subfamily is a very old one. Geographical and taxonomic evidence indicates that it may have originated in tropical or subtropical Africa at least as early as the Oligocene.