268 



THE PLANT WORLD 



Structure occur, the diurnal changes of temperature are of 

 wide amplitude, the effects of wind erosion are very marked, 

 the precipitation sustains a very low ratio to the possible 

 evaporation, as a consequence of which all areas under 4000 

 to 6000 feet are strongly desert in character. Within the 

 limited areas offered by the higher mountain slopes, much 



NEV. 



SONORA 



V.r:. 3. —-Map oi' Arizona, 'the Dcscit Laboratory is situated in tlie eastern 



■ part of Pima county. 



more mesophytic conditions prevail by reason of the greater 

 precipitation and decreased evaporation. In such compara- 

 tively humid localities a vegetation of mesophytic forms is 

 encountered, forming islands in this great desert. Also at 

 lower levels there occur a large number of species which 

 carry out their entire cycle of activity during the brief rainy 

 seasons or periods of greatest precipitation, and these do 



