208 AFRICA 



the hamadas and the regs. The oases, due to the 

 accumulation of underground water or to the short 

 mountain streams which lose themselves in the sands, 

 are found in depressions or along the foot of the heights 

 and tablelands. The well-known date-palm, which is 

 the feature of their vegetation, is useful alike for its 

 protection and for its products and by-products, just 

 as the camel is valuable in so many ways. The date- 

 palm in the oases and the camel on the road are the two 

 essentials of desert life ; and the very simple existence 

 of the nomads and of the few settled communities is 

 practically based on them. 



Sudan Seini-Desert. The transition from the desert 

 to the less arid southern regions is very gradual. A 

 certain amount of rain falls between July and September, 

 but this is very irregular, and some years may be 

 practically rainless. 



The semi-desert naturally offers a more varied land- 

 scape than the desert, on account of a more severe erosion. 

 Small trees and a dearth of grass appear to be the most 

 prominent features of the vegetation. First, small bushes 

 and low, gnarled trees, either solitary or in thin groves, 

 make their appearance. They no longer belong to the 

 mediterranean flora, but consist largely of various species 

 of acacias and acacia-like plants, often thorny or prickly, 

 with small, deciduous leaves ; other plants are swollen 

 into water reservoirs ; the majority are resinous or waxy. 

 Presently, the isolated bushes cluster in loose scrub ; on 

 some of the tablelands, besides entirely bare stretches, 

 sprinklings or even thickets of acacias occur occasion- 

 ally, on others, sparse but regular deciduous woodlands 

 are formed 'by a relative of the balsam-trees, commiphora 

 the aderas, on the naked soil. Grass is confined to 

 the river-beds and to a few slopes. In depressions with 



