640 



ZONES AND REGIONS [Pt. Ill, Sect. II 



fifth belt is formed of Atriplex polycarpa, the sixth chiefly of Larrea tridentata. 

 Then commences the usual mesa-vegetation. 



The oases around the springs and by the flowing waters possess a vegeta- 

 tion of tropophilous trees and shrubs, such as Populus Fremontii, Prosopis 

 pubescens, Salix longifolia, S. nigra, S. venulosa, and various herbaceous 

 perennials likewise differing from those of the desert soil. 



The parts of the desert lying 

 above 1,500 meters forming # the 

 Upper Son or an region are less 

 dry than the lower parts and 

 are characterized by the growth 

 of isolated trees of Pinus mono- 

 phylla (Fig. 375). Juniperus 

 californica, var. utahensis, is also 

 common here. The shrub-flora 

 is not less distinct from that of 

 the Lower Sonoran region. Here 

 the bushy grey Artemisia tri- 

 dentata is common (Figs. 376, 

 377,386), also Ceanothus Greg- 

 gii, Garrya Veatchii, var. flave- 

 scens, Kunzia glandulosa, Ribes 

 leptanthum, R. brachyanthum, 

 Salvia carnosa. The transition 

 belt between this region and 

 that of the Lower Sonoran is par- 

 ticularly characterized by Yucca 

 brevifolia. Still higher, corre- 

 sponding to the increasing hu- 

 midity, there appear on the 

 highest mountains of the desert 

 pine-forests, which agree closely 

 with those on the slopes facing 

 the coast. 



The adaptations to the cli- 

 matic conditions, in particular 

 to dryness, are very similar to 

 those which Volkens has so ac- 

 curately described in relation 

 to the Sahara ; water- reservoirs are however absent except in the case of 

 the succulent plants (Cactaceae, Fig. 372, Chenopodiaceae), which are 

 mainly confined to the Upper Sonoran region and to the saline localities 

 of the Lower Sonoran region. The shrubs are usually dense bushes of 



Fig. 377. North American desert in Arizona. 

 Artemisia tridentata. Natural size. 



