568 RELATION TO ENVIRONMENT. 



II. The Sonora-Nevada Desert. 



1055. In the valley between the Sierra Nevada and Rocky 

 Mountains, including the Great Basin Ranges of Arizona, Utah, 

 southern Nevada, and the southern part of California, etc., which 

 is a part of the Sonoran arid area, a true desert condition exists, 

 shown by the low annual precipitation, as well as by the dry 

 hot climate and the great amount of water evaporated from the 

 ground, though parts of it are intensified by the salt or alkaline 

 basins, which would enforce a xerophytic habit irrespective 

 of the dry hot climate. The desert areas are not determined 

 alone by the low annual precipitation. A hot dry climate during 

 a large portion of the year is an equally great factor. A district 

 in the cooler portions of the north temperate zone with a low 

 annual rainfall distributed through the growing season may 

 furnish a luxuriant vegetation with no true desert vegetation. 

 On the other hand, a district with an equal annual rainfall farther 

 south, where the climate is hot and dry for a large portion of the 

 year, and the rainfall is principally during the cold winter season, 

 may have a true desert vegetation. In the dry hot season evapo- 

 ration of water from the soil is very great. The Desert Botanical 

 Laboratory of the Carnegie Institution, established near Tucson, 

 Arizona, will give an opportunity, long needed, for more careful 

 studies of the desert flora. 



1056. In the " Death's Valley" (within the area of this desert) 

 the true desert portions are quite distinct in vegetation from 

 the areas bordering on ponds or streams, or from the basins 

 where there is such an accumulation of water as to provide a 

 sufficient amount of ground-water (hydrostatic water) to support 

 a more luxuriant vegetation even through the dry season. These 

 true desert portions are on the higher area, dry ground, and 

 according to Coville are entirely devoid of hydrostatic moisture. 

 All the water they receive is the storm -water during the rainy 

 season which occurs roughly from December to March. The 

 rains during this period amount to about 10 cm. (4 inches). The. 



