176 DRY-FARMING 
a free water surface is not the same as that from 
plant leaves may be shown in a general way from the 
fact that the amount of water transpired from a 
given area of leaf surface may be very much larger 
or very much smaller than that evaporated from an 
equal surface of free water exposed to the same con- 
ditions. It is further shown by the fact that whereas 
evaporation from a free water surface goes on with 
little or no interruption throughout the twenty-four 
hours of the day, transpiration is virtually at a stand- 
still at night even though the conditions for the rapid 
evaporation from a free water surface are present. 
Some of the conditions influencing the transpira- 
tion may be enumerated as follows: — 
First, transpiration is influenced by the relative 
humidity. In dry air, under otherwise similar con- 
ditions, plants transpire more water than in moist air, 
though it is to be noted that even when the atmos- 
phere is fully saturated, so that no water evaporates 
from a free water surface, the transpiration of plants 
still continues in a small degree. This is explained 
by the observation that since the life process of a 
plant produces a certain amount of heat, the plant 
is always warmer than the surrounding air and that 
transpiration into an atmosphere fully charged with 
water vapor is consequently made possible. The 
fact that transpiration is greater under a low relative 
humidity is of greatest importance to the dry-farmer, 
who has to contend with the dry atmosphere. 
