The Colorado Desert lies at a low altitude, between and 

 about 500 feet (152 m). Characteristic species are members of 

 the Goosefoot family (Chenopodiaceae), a few low bushes, and 

 several herbs. Only a few species of trees occur, such as Desert 

 Ironwood (Olneya tesota), Mesquite {Prosopis juliflora var. tor- 

 reyana), and Screw Bean {P. pubescens), these being limited to 

 stream beds and the borders of springs or low lying valleys. 

 This desert passes gradually into the Mojave Desert. 



The Mojave Desert lies at a higher level than the Colorado 

 Desert, the altitudes ranging from about 2,000 to 5,000 feet 

 (610 to 1 ,524 m), and the rainfall is usually somewhat greater. 

 In other respects it has a desert climate similar to that of the 

 Colorado Desert, and its vegetation presents a similar desert as- 

 pect. Hundreds of square miles exhibit the dark green of Creosote 

 Bush {Larrea divaricatd), a shrub commonly 3 to 6 feet (0.9 to 

 1.8 m) high with very small resin-covered evergreen leaves, the 

 individuals widely but rather regularly spaced in response to the 

 meagreness of soil-water (or perhaps allelopathy). Low shrubs 

 or bushes of gray hue are abundant, and include various widely 

 distributed species of the desert valleys belonging to the salt- 

 bush %Qnu% A triplex, and several members of the Compositae. 

 Bladder Sage {Salazaria mexicana) and box thorn {Lycium ander- 

 sonii and L. cooperi) are roughish or spiny shrubs. Seep weed 

 (Suaeda spp.) is characteristic of alkaline valleys, while Turpen- 

 tine Broom (Thamnosma montand) is a small switch plant of the 

 arid slopes. There are several characteristic desert herbs as well. 

 Extensive groves of Joshua Tree {Yucca brevifolid), the individ- 

 uals 16 to 30 feet (5 to 9 m) high, lend an added touch of 

 strangeness to the xerophytic populations. Save for this one 

 species, true trees are mainly absent except that along stream 

 courses, about springs, or in low valleys where roots may go down 

 20 to 70 feet (6 to 21 m) to a low-lying water stratum, a few 

 species occur such as the mesquites Prosopis juliflora var. glandu- 

 losa and P. pubescens. 



The Valley Sonoran comprises the plain of the Central Valley 

 of California except the lower or central delta portion. It is a 

 grassland formation, varying in altitude from 10 to 500 feet (3 

 to 152 m), with less extremes of temperature than the desert 



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