8 THE FUTURE OF ARID LANDS 
The adaptive features by which plants were able to move from 
a hydrophytic environment to a mesophytic and then to a xero- 
phytic environment have been studied for many years (10). Still 
there is much misunderstanding with respect to these adjustments. 
Plants are separated on a very broad basis into hydrophytes, 
water plants, mesophytes, moist soil plants, and xerophytes 
plants characteristic of drought regions and which many people 
still think can grow in dry soil. These groups are not sharply 
separated and merge into each other, but they serve as most con- 
venient groups in discussing the many adjustments. These groups 
are found under all climates, and their presence is determined by 
the condition of the substratum. 
To make this clear a simple diagram is used (Figure 1). A loam 
soil is chosen to illustrate the relationships of the moisture retain- 
ing power of the soil. If the soil chosen were sand the values would 
still have the same definite relation, but the values would be 
lower and if a heavier soil were chosen the values would be higher. 
In order to simplify the graph only a few of the laboratory and 
field determinations of the moisture holding capacity are entered 
here. 
In Figure 1: 
Dry soil is at o% (based on dry weight) 
Desert dry soil under desert vegetation 5% (16) 
Hygroscopic coefficient 6.8% (5) 
Wilting coefficient 10% (2) 
Field carrying capacity 19.6% (3) 
Moisture holding capacity 50% (6) 
Saturated soil 50% 
Water table at 50% 
In free water from a to 4 on the graph we would find floating, 
submerged, and amphibious hydrophytes. In soil conditions be- 
tween 4 and c only hydrophytes would grow. Most of these hydro- 
phytes would continue alive and grow it the soil moisture were 
lowered from c to d but would die below this point. At water con- 
tents between c and d mesophytes and xerophytes would grow, 
but below d the mesophytes would die and the xerophytes become 
semi-dormant or dormant. The soil moisture below d would be 
slowly absorbed and lost to the air and reduced slowly to about 
