HISTORY AND PROBLEMS . 
The estimated area of arid land is 43% if based on pedocals, 
36% if based on climate, and 35% if based on the natural vegeta- 
tion, and about the same area is marked by interior drainage. 
Here then is an area of about one third of the land area of the 
earth in which moisture is the chief limiting factor in the pro- 
duction of plant growth and the dependent animal and human 
populations. 
Plant Adjustments to Arid Conditions 
Since both plant life and animal life originated in an aquatic 
_ environment great adjustments were necessary to enable them to 
survive in the air and still greater adjustments to enable them to 
live in the arid zone. 
Water 
Soil 
moisture per cent 
Figure 1. To illustrate relation of hydrophytes, mesophytes, and 
xerophytes to moisture supply in a loam soil: hydrophytes @ to c; meso- 
phytes and xerophytes ¢ to d. Mesophytes die below d and xerophytes 
become dormant. Soil under desert drought sinks to about 5% or e. 
Rainfall at f raises water content to about field carrying capacity, 19.6 % 
at g. 
