DANGERS RESULTING FROM TRANSPIRATION 265 



brings about the osmotic force by which the cells of the leaf draw 

 in the 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 



