8 



requirements, but generally one doainant species is best adapted 

 to the prevailing conditions of rainfall. Generally these species 

 will be found in widely scattered regions where much the same con- 

 dition of rainfall exists. In other areas, the dominant species 

 may be in association with others which in turn will dominate as 

 moisture conditions become more favorable to their growth. 

 Changes such as these are shown in Table 1, which indicates the 

 relation of temperatures and rainfall to some of the prevailing 

 types. 



Although rainfall is a matter of record in most localities, 

 but little is known regarding the limits of soil moisture upon 

 which the flora of the desert survive. Statements of the moisture 

 percentages in a given soil are of little value unless the physi- 

 cal properties of the soil are likewise known, as the percentage 

 alone does not indicate the quantity of water available for plant 

 use. For this purpose moisture in excess of the wilting percent- 

 age is a better indicator of the quantity that the plant may ex- 

 tract from the soil. "Permanent wilting percentage" has been 

 defined by Veihmeyer and Hendrickson (35) as "the lower limit of 

 readily available soil moisture." In agriculture this is the 

 soil-moisture condition that limits the activities of plants. In 

 the case of desert growth, the plant survives, but remains dor- 

 mant during long periods of deficiency and resumes growth when 

 new moisture is received. 



Limited investigations in the Coachella Valley, Calif. , 

 (26) found creosote bush, chamiso ( Atriplex canescens ) and desert 

 sage growing in areas having extremely little available moisture. 

 In the spring, moisture in the soil ranged from 2.2 per cent below 

 the wilting point to 3-5 per cent above, but by the following 

 autumn these amounts had decreased to as much as 8.0 per cent 

 below the wilting point. In the Gila Valley, Ariz., (26) much 

 the same conditions were observed. Investigations in the Tooele 

 Valley, Utah, (15) showed big sagebrush ( Artemisia tridentata ), 

 shadscale ( Atriplex confertif olia ) , kochia ( Kochia vestita ) , and 



