TRANSPIRATION FROM PLANTS 175 
Of one thing we are fairly sure: if the upward stream 
of water is wholly stopped for even a few hours, the 
plant is likely to be so severely injured as to be greatly 
handicapped in its future growth. 
Botanical authorities agree that transpiration is 
of value to plant growth, first, because it helps to dis- 
tribute the mineral nutrients necessary for plant 
growth uniformly throughout the plant; secondly, 
because it permits an active assimilation of the car- 
bon by the leaves; thirdly, because it is not unlikely 
that the heat required to evaporate water, in large 
part taken from the plant itself, prevents the plant 
from being overheated. This last mentioned value of 
transpiration is especially important in dry-farm 
districts, where, during the summer, the heat is often 
intense. Fourthly, transpiration ‘apparently influ- 
ences plant growth and development in a number of 
wavs not yet clearly understood. 
Conditions influencing transpiration 
In general, the conditions that determine the 
evaporation of water from the leaves are the same 
as those that favor the direct evaporation of water 
from soils, although there seems to be something in 
the life process of the plant, a physiological factor, 
which permits or prevents the ordinary water-dis- 
sipating factors from exercising their full powers. 
That the evaporation of water from the soil or from 
