CHAPTER II 
THE THEORETICAL BASIS OF DRY-FARMING 
Tue confidence with which scientific investigators, 
familiar with the arid regions, have attacked the 
problems of dry-farming rests largely on the known 
relationship of the water requirements of plants to 
the natural precipitation of rain and snow. It is 
a most elementary fact of plant physiology that no 
plant can live and grow unless it has at its disposal 
a sufficient amount of water. 
The water used by plants is almost entirely taken 
from the soil by the minute root-hairs radiating 
from the roots. The water thus taken into the 
plants is passed upward through the stem to the 
leaves, where it is finally evaporated. There is, 
therefore, a more or less constant stream of water 
passing through the plant from the roots to the 
leaves. 
_ By various methods it is possible to measure the 
water thus taken from the soil. While this process 
of taking water from the soil is going on within the 
plant, a certain amount of soil-moisture is also lost 
by direct evaporation from the soil surface. In 
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