6io 



ZONES AND REGIONS [Pt. Ill, Sect. II 



richly peopled are the wadis traversing typical stony deserts of this type, 



and in particular the immediate banks of the water- courses, which are 



usually dry. 



The sandy desert (Areg, Fig. 339) has a less scanty vegetation, except on 



the dunes (Fig. 345), which are often entirely devoid of plants. 



The sharpest contrast to the veritable bare desert tracts, provided only 

 with scanty, low, pale green plants, is afforded by 

 the deepest valleys and depressions containing 

 springs, the oases (Fig. 340). Usually so sharply 

 bounded by the desert that their contour can be 

 defined by a line, they nourish a luxuriant vege- 

 tation of trees and herbs, which however are all 

 artificially grown, for the oases are entirely under 

 cultivation. 



The distribution of desert plants appears to be 

 dependent far more on subterranean water, retained 

 in the depths of the soil, than on the direct 

 moistening of the soil by rain. Yet an ephemeral 

 rain-flora seems to be present in all deserts. In 

 spite of their short duration and scantiness, the 

 spring rains evoke numerous annual plants, which 

 for the most part completely disappear immediately 

 after the rainy season, so that the previously green 

 surface then appears once more just as free of 

 vegetation as before the rain. The spring rains are 

 also important to perennial plants, often however 

 rather by reducing transpiration than by directly 

 supplying moisture to the deeply-seated roots. 

 Many species bear leaves and flowers only during 

 this period. Others however blossom during the 

 dry, cool winter. We have consequently to dis- 

 tinguish between two oecological groups of desert 

 plants, the one the existence of which depends 

 directly upon rain, and the other in which it depends 

 upon the presence of subterranean ivater. 



P la tits of the desert dependent directly upon 

 rain 1 . These are partly annuals, which germinate 

 at the commencement of the rain, ripen their seed 



when it terminates, and die forthwith ; partly, though in smaller numbers, 



plants, the underground portion of which is perennial, but which after the 



rain disappear from the surface just like annuals. 



My own observations on the flora of the Sahara were confined to the 

 1 See particularly Volkens, II. 



Fig. 341. Flora of theSahara. 

 Genista (Retama) Raetam, 

 Forsk. Natural size. After 

 Taubert in Engler und 

 Prantl, Die nattirlichen 

 Pflanzenfamilien. 



