336 Part III. Chapter 4. 
xerophilous Sonoran elements occupy the arid mountain slopes and high 
plateaus of the transition and upper Sonoran climatic zones, extending also 
into the deserts of the lower Sonoran climatic zone. Its southward distribution 
3 
S | N NEN IE 
EI \ REN: a 
3 r AI Bo 
N ER il 
ls NT | 
Sy 5 EN ETTRRL 
ri 
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Fig. 21. Gilia congesta Hook., a perennial, wooly plant with densely clustered white flowers, 
native to the Sierra Nevada and Rocky Mountains at an elevation of 10,000 feet. After Pflanzen- 
reich Abtlg. IV 250, p. 122. 
has been favored by the existence of an arid zone comprising the moistureless 
west slopes and enclosed plateaus of the Mexican and tropic Andes, Iying 
mostly below the altitudes of alpine conditions. Both the aridity and continuity 
of this zone have been varied with the changes in elevation, and in all 
