250 PLANT PHYSIOLOGY 



as the plant would probably get enough salts without this, it is 

 reasonable to expect that the entrance of more salts will be a 

 result of transpiration. 



As has been shown (Chap. IV), only a small amount of the 

 energy which is absorbed from the sunlight is used to manufac- 

 ture food. The rest, which is absorbed in the form of heat, would 

 soon raise the leaf to so high a temperature that the protoplasm 

 would be coagulated and killed if this surplus heat were not ab- 

 sorbed by the water present and evaporated. This results in a 

 lowering of the temperature and, instead of being much warmer 

 than the surrounding air, the leaf has about the same temperature 

 or even a lower one. Although Clum (1926) reports the tempera- 

 ture always higher than the surrounding air, this may have been 

 partly due to the use of the thermocouple which slightly wounds 

 the tissue. Miller and Saunders (1923) found that in bright sun- 

 light most leaves kept about the same temperature as the surround- 

 ing air, but alfalfa leaves even here were a trifle cooler. In the 

 shade, however, the leaves were as much as 3° C. cooler than the 

 surrounding air; and it has been noted also that leaves which are 

 beginning to wilt may be as much as 6° C. warmer than normally 

 turgid ones. When the sun shines brightly after a shower, leaves 

 are commonly scalded for two reasons: first, the transpiration is 

 checked by the high humidity of the air after the rain and, secondly, 

 the little drops of water remaining on the leaves act like convex 

 lenses of a burning glass and focus the light even more strongly 

 than ever upon the leaf. The lowering of the temperature by 

 transpiration, if such a lowering actually occurs, is of genuine 

 benefit and, from the functional point of view, in this respect only 

 can transpiration be called a function. Even here, as pointed out 

 by Curtis and Clum (1926), the benefit is very doubtful. 



Evaporation from the plant takes place mostly through the 

 stomata of the leaves and the lenticels of the young bark. Wherever 

 there are openings which permit the exchange of gases, through 

 these water vapor is destined to pass out. A small percentage of 

 water (5-10%) also evaporates from the cuticle, but this cuticular 

 transpiration is nearly negligible in comparison with the amounts 

 which pass off through the stomata. 



Determining Factors. — The factors which determine tran- 

 spiration are both external (outside the plant) and internal. The 

 external conditions which affect transpiration are those which 



