DESERT VEGEl^ATION. $&7 



1054a. Conditions of environment in the desert. We under- 

 stand that the conditions of environment are very austere, so 

 that plant life comes in sharp competition with the climate. 



The principal factors are: 



ist, 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 

 (see paragraphs 926, 928, 933, 1060). 



2d, the great amount of evaporation during the long dry hot 

 season; the amount of evaporation from the soil will depend 

 on the character of the soil and the amount of ground-water, 

 some soils conserving water more than others. 



3d, the alternation of rainy and dry seasons, the rainy season 

 in the temperate regions usually occurring during the cold part 

 of the year, when plants cannot make use of it. 



Some of the minor factors which might be mentioned are 

 as follows: 



ist, 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 radiation 

 during the day, but which also check radiation of moisture from 

 the soil both during day and night. 



3d, high winds, which often sweep over desert areas, increas- 

 ing the drying effect of the air on vegetation. 



4th, 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 semi-desert areas, the 

 loose and crumbling condition of soil in some regions which 

 permits the rapid filtering away of storm- and ground-water, 

 the topography of the region; also when very rolling or hilly, 

 permits rapid run-off of storm-water. 



