478 GENERAL MORPHOLOGY OF PLANTS 



with thick, narrow and long leaves with a firm and thick cuticle; 

 small shrubs or herbs with compact rounded habit and small 

 thick gray leaves. All of these structures conserve moisture. 

 The mesquite tree is one of the common trees in portions of the 

 Sonora Nevada desert. Besides the true desert plants, desert 

 societies have a rainy-season flora consisting of annuals, which 

 can germinate, vegetate, flower and seed during the period of 

 rain and before the ground moisture has largely disappeared, 

 and then pass the resting period in seed. 



650. Conditions with which desert plants contend. The 

 conditions of the desert are very austere, so that plant life comes 

 in sharp competition with the climate. The principal factors 

 are: 



First, the very low rainfall, varying from 8 to 10 inches; in some 

 deserts, to 4 inches, or even less than i inch in some areas. 



Second, the great amount of evaporation during the long, dry, 

 hot season. 



Some of the minor factors which might be mentioned are as 

 follows : 



First, the strong light (solar radiation), especially during the 

 warm season. This is due to the absence of clouds which form 

 a blanket over the earth, not only cutting off direct solar radia- 

 tion during the day, but which also check evaporation of moisture 

 from the soil both during the day and night. 



Second, high winds, which often sweep over desert areas, 

 increasing the drying effect of the air on vegetation. 



Third, the physical or chemical character of the soil often is 

 such as to enforce a xerophytic habit for vegetation even if rain- 

 fall were greater; for example, salty or alkaline condition of the 

 soil, calcareous soils in some desert or semidesert areas, the loose 

 and crumbling condition of soil in some regions which permits 

 the rapid filtering away of storm- and ground-water; the topog- 

 raphy of the region also, when very rolling or hilly, permits 

 rapid run-off of storm-water. 



651. How desert plants meet these conditions. This they 

 do by provision for: First, reduction of transpiration; second, 



