DANGERS RESULTING FROM TRANSPIRATION 265 
brings about the osmotic force by which the cells of the leaf draw 
inthe water columns. The energy contributed by transpiration is 
really the heat-energy involved in changing water into vapor, in 
which form the water escapes from the plant. Such seems to be 
the relation of transpiration to the ascent of sap, but what other 
factors are involved and to what extent we have no definite knowl- 
edge, and, therefore, we may attribute too much to transpiration. 
It was once generally believed that the flow of water through 
the plant is necessary to transport the mineral elements of the 
soil to the different regions of the shoot, and that the amount of 
the mineral elements reaching the leaves and other parts of the 
shoot is directly related to the amount of water flowing through 
the plant and, therefore, to transpiration. But some experiments 
indicate that in some cases, at least, the process of diffusion by 
which the mineral elements and other substances in solution pass 
to those regions where they are less concentrated, regardless of 
the movement of the water in which they are dissolved, can supply 
the mineral elements to different parts of the shoot.as rapidly as 
needed. In fact, in case of Tobacco plants, analyses have shown 
that plants grown in the shade may have a higher mineral content 
than plants grown exposed to excessive transpiration. In other 
words, the plants through which the least water flows may take the 
most mineral from the soil. However, since the water carries the 
dissolved substances along in its current, the movement of water 
through the plant tends to aid diffusion in the distribution of the 
elements in solution. 
Since transpiration, like evaporation, is a cooling process, it 
often prevents leaves from becoming overheated. Sometimes 
bright sunshine, following a summer shower which has filled the 
air with moisture, results in the leaf injury known as scalding. 
Under these conditions, transpiration is checked and the tempera- 
ture of the leaf becomes too high. As a large part of the sunlight 
is changed into heat by the leaf, the heat accumulates very 
rapidly in bright sunshine. It has been found in the case of 
some leaves that the excess of heat, if transpiration be stopped, 
may raise the internal temperature of the leaf to the death point 
in a few minutes. Transpiration, therefore, rids the leaf of the 
dangerous excess of heat. 
Dangers Resulting from Transpiration. — So long as water 
from the roots can be supplied as rapidly as water is lost by 
