6 3 8 



ZONES AND REGIONS [Pt. Ill, Sect. II 



stony or even covered with larger blocks, while flat areas more distant 

 from the mountains exhibit a soil of uniformly fine-grained consistency. 

 The soil is calcareous, clayey, or silicious according to the nature of the 

 rock from which the soil derives its origin. 



Woody plants are numerous, but all are shrubby, except the species of 

 Yucca, Yucca brevifolia (Fig. 370), for which Lorrey's name, Y. arborescens, 

 is more descriptive, and Y. macrocarpa, and both of these plants are con- 

 fined to the higher parts of the region. 



Of the shrubs the following are very common : — 



Tetradymia comosa (Compositae), Acamptopappus sphaerocephalus (Compositae), 

 Amphiachyris Fremontii (Compositae), Aster mohavensis, Atriplex confertifolia, A. 

 hymenelytra, Bebbia juncea aspera (Compositae), Cassia armata, Cereus Engelmanni, 



Fig. 374. North American desert. Banks of the Great Salt Lake, Utah. From a photograph 



by Soennecken. 



Echinocactus polycephalus, Ephedra californica, Hymenoclea Salsola (Compositae), 

 Krameria parvifolia, Lyeium Andersonii, Opuntia basilaris, O. echinocarpa, Salazaria 

 mexicana (Labiatae). Many of these species, like those of Yucca, are confined to 

 higher regions ; some appear to be dependent on edaphic influences : thus Atriplex 

 hymenelytra grows only on silicious soil rich in alkalis, Cassia armata on dry sand. 



The commonest herbaceous perennials of the mesas are : — 

 Cladothrix oblongifolia (Amarantaceae), Euphorbia polycarpa, Lepidium Fre- 

 montii, and Mirabilis laevis ; whilst among the numerous annuals the following 

 species are especially common : Atrichoseris platyphylla (Compositae), Chorizanthe 

 rigida (Polygonaceae), Cleomella obtusifolia (Capparidaceae), Encelia eriocephala 

 (Compositae), Eschscholtzia minutiflora, Gilia floccosa, Leptosyne Bigelovii (Com- 

 positae), Plantago patagonica, P. gnaphaloides, and Sisymbrium canescens. 



