distribution of the great regional types of desert vegetation is 

 determined by the limited amount of precipitation which falls in 

 a given region. 



The effect of environment on the distribution of plants is 

 well recognized, those best adapted to resist the unfavorable con- 

 ditions of a region being the most successful in surviving. As 

 stated by Weaver (37) "The natural vegetation, for many centuries, 

 has been sorted out by climate as well as by soil in the process 

 of development. The various species of plants have usually in- 

 habited a region so long that they are now quite definitely dis- 

 tributed with relation to the environmental complex, species well 

 adapted to a given environment now occurring in abundance. Thus, 

 the growth of native vegetation becomes a measure of the effects 

 of all the conditions which are favorable or unfavorable for plant 

 production." This is particularly true in arid regions where high 

 rates of evaporation and transpiration, and a very limited water 

 supply, have reduced vegetation to a veritable struggle for exis- 

 tence . 



Again, according to Warming (36), "No other influence ex- 

 presses its mark to such a degree upon the internal and external 

 structures of the plant as does the amount of water present in the 

 air and soil." It appears, then, that authorities are in accord 

 on the soil-moisture plant distribution relation. Contributing 

 factors, as temperature, altitude, humidity, and evaporation also 

 must be considered, but these are so interrelated with water 

 supply that the effects are not always discernible. 



Natural vegetation grows under moisture conditions that are 

 always changing. Plants that do not subsist on ground water but 

 depend upon moisture held by the soil particles may have an abun- 

 dant supply at one time and suffer a scarcity at another. Ground 

 water fluctuates and roots in contact with it are alternately wet 

 and dry. Soil moisture is dependent upon precipitation, but evap- 

 oration, transpiration, percolation, and run-off cause its uneven 

 distribution in the soil. 



