Chap. IX] DESERTS 633 



in length. The flowers afford nothing that is worthy of note ; the fruits resemble 

 large oranges, or rather pomelloes, in size and form. 



The plant is perfectly adapted to an unfavourable environment of the kind in 

 which it grows. The roots grow down, until they reach the subterranean water, 

 which is absolutely indispensable to the existence of the plant : the rich nocturnal 

 dew, which renders possible the existence of many small herbs, is, however, useless 

 to Acanthosicyos, for the dew moistens the ground merely superficial^. When once 

 a connexion with the subterranean water has been secured, the plant exhibits vigorous 

 growth and great tenacity of life. Growing in windy localities on loose sand, it is 

 often completely buried, but soon its branches once more project freely and thus it 

 gradually becomes the centre of a sandy hillock, the top of which is crowned by the 

 green naras-bush. 



The branches are very completely protected against excessive transpiration by 

 various contrivances, such as a thick ring of sclerenchyma — which also affords the 

 necessary resistance to bending, — reduction in the intercellular spaces, water-storing 

 hypoderm, sunken position of the stomata, thick cuticle, and a coating of wax. The 

 very juicy fruits grow and ripen at a time when the subterranean water is most 

 abundant. Intensely bitter substances protect all parts of the plant against the keen 

 hunger of animals, but disappear in the ripe aromatic fruits, the seeds of which are 

 distributed by jackals. 



iv. THE DESERT IN AUSTRALIA. 



Regarding the formations composing the Australian deserts no serviceable 

 memoranda are available. The poorly clad or bare tracts appear to be 

 much more limited than those occupied by ' scrub.' Species of Triodia, 

 also, in South Australia, Spinifex hirsutus (Fig. 369), form on sandy soil 

 the chief constituents of the vegetation, while stunted Eucalypti appear 

 here and there, and saline localities are occupied by Chenopodiaceae and 

 Zygophyllaceae. 



2. THE DESERTS OF THE WESTERN HEMISPHERE. 



i. THE DESERT IN NORTH AMERICA. 



The broad valley between the Sierra Nevada and the Rocky Mountains, 

 and in particular its southern extensions, in which lie the States of Nevada, 

 Utah, the west of Arizona, and the south of California (Fig. 373), possesses 

 a typical desert climate with an annual rainfall of less than 20 mm. In the 

 southern part the rain falls chiefly at midsummer, but in its central and 

 northern parts more during winter, apparently however without exhibiting 

 any great regularity 1 . The atmosphere is very dry. The winter tempera- 

 ture in the northern elevated part is cold, but is mild in the low-lying 

 southern part ; the summer temperatures are very high, especially in the 



1 See Hann, Handb., Ill, p. 289, and Atlas XI and XII. 



