Chap. IX] 



DESERTS 



617 



of the vegetation is different, and on the whole less meagre than in the north of the 

 Sahara. The hills, it is true, are entirely devoid of plant-life, but in the wadis, 

 where subterranean water accumulates, the trees attain large dimensions. They 

 are, however, usually small-leaved and thorny, therefore of a xerophilous stamp. 

 To them there belong, in particular, Acacia Seyal, Del., Maerua rigida, R. Br., 

 Zizyphus Spina-Christi, Wild., Balanites aegyptica, Del., and the palm, Hyphaene 

 thebaica. A species of Stapelia grows on the rocks. The granitic sand of the 

 former water-courses in the wadis is overgrown by Panicum turgidum, Forsk., 

 but grasses are otherwise rare. 



The east coast of Africa along the southern half of the Red Sea and as 

 far as the equator, although less poor in rain than the Sahara, is, owing to 



Fig. 351. Semi-desert with succulent plants near Kihuiro, at the base of Kilimanjaro. 

 Reduced from one of Volkens' figures. 



the great heat, dry enough to assume the character of semi-desert or desert 

 in places where the soil is more permeable. Such desert and semi-desert, 

 due to a combination of climatic and edaphic influences, alternate with less 

 pronounced % xerophilous formations, for example, in Equatorial East Africa, 

 between Kilimanjaro and the coast. 



Volkens 1 picturesquely describes such a formation (Fig. 351): 



' It is a pure steppe of succulent plants 2 , the driest and most unfertile district one 

 can imagine, but just for that reason covered by a vegetation, the like poverty ot 

 which I have observed only in the driest desert tracts of Egypt. Every plant is really 



1 Volkens, IV, pp. 17, 18. 2 Not a steppe according to our nomenclature. 



