TRANSPIRATION 179 
also is an interesting subject that should be more 
fully investigated in the future. 
Ninth, the vigor of growth of a crop appears to 
have a strong influence on transpiration. It does not 
follow, however, that the more vigorously a crop 
grows, the more rapidly does it transpire water, for 
it is well known that the most luxuriant plant growth 
occurs in the tropics, where the transpiration is exceed- 
ingly low. It seems to be true that under the same 
conditions, plants that grow most vigorously tend to 
use proportionately the smallest amount of water. 
Tenth, the root system — its depth and manner of 
growth — influences the rate of transpiration. The 
more vigorous and extensive the root system, the 
more rapidly can water be secured from the soil by 
the plant. 
The conditions above enumerated as influencing 
transpiration are nearly all of a physical character, and 
it must not be forgotten that they may all be annulled 
or changed by a physiological regulation. It must 
be admitted that the subject of transpiration is yet 
poorly understood, though it is one of the most im- 
portant subjects in its applications to plant produc- 
tion in localities where water is scarce. It should 
also be noted that nearly all of the above conditions 
influencing transpiration are beyond the control of the 
farmer. The one that seems most readily controlled 
in ordinary agricultural practice will be discussed in 
the following section. 
