The extensive desert and semi-desert areas in the southwestern part of southern Africa, south and west of the Zambezian Domain, house a rich and distinct flora and are classified as the Karoo-Namib Region. North of the coastal mountain ranges parallel to the Indian Ocean, these dry lands cover an area wider than 1000 km from the Atlantic coast to the centre of the plateau at about °E. Northwards this arid area gradually becomes narrower, following more or less the escarpment in the northern half of South West Africa until it tapers off on the Atlantic coast about 150 km south of Lobito in Angola (Chapter 7, Fig. 12). In the Kalahari sand area on the high plateau of southeastern South West Africa, southwestern Botswana and the northern Cape Province, the boundary with the adjacent Sudano-Zambezian Region is not clear cut (see Section 2.1). The two floras interdigitate here to a certain extent, but the 250 mm isohyet coincides satisfactorily with a chorological boundary and is here taken as the borderline. Further south on the loamier substrates of the Karoo and South African Highveld the chorological borderline coincides more closely with the 400 mm isohyet. Towards the Atlantic coast the annual precipitation rapidly decreases to less than 100 mm, while off the escarpment, in a narrow coastal belt, mist and fog constitute an important source of moisture (Walter 1936, 1973, Stengel 1971).