302 PLANT PHYSIOLOGY 



There is no direct proportionality between the velocity of the 

 wind and the magnitude of transpiration. A strong wind 

 increases transpiration not much more than a breeze does; 

 for evaporation, i.e., the change of water from a liquid to the 

 gaseous state, actually occurs in the intercellular spaces of the 

 leaf, which are protected from the direct influence of the wind. 



Light has a very important function in the process of trans- 

 piration. Since chlorophyll absorbs certain wave lengths of 

 radiant energy, the temperature of the leaf is increased per- 

 ceptibly in light. This immediately increases the difference 

 between the factors F and / and consequently intensifies trans- 

 piration, which in turn lowers the temperature of the leaf. 

 Hence, plants that have a high power of evaporation are heated 

 hardly at all in light, but their transpiration increases con- 

 siderably. It was stated in the chapter on assimilation (Art. 42) 

 that usually only 1 to 5 per cent of the radiant energy absorbed 

 by the plant is used in photosynthesis, while over 80 per cent is 

 spent in transpiration. Even diffuse light increases transpiration 

 30 to 40 per cent, while in direct sunlight it may be several times 

 greater. This accounts for the marked difference between day 

 and night transpiration, which is many times greater than 

 differences in evaporation produced by any other factors. 



Besides the direct heating effect, light increases transpiration 

 indirectly also. It favors the opening of stomata and increases 

 the water permeability of the protoplasm of the evaporating 

 cells. Both of these circumstances favor the loss of water from 

 plants. 



The influence of en\dronmental factors controls the daily 

 change>s in transpiration. Transpiration is low in the early 

 morning hours and increases rapidly with the rise of the sun, 

 w^hich is followed by a rise in temperature and an increase of the 

 saturation deficit. It attains its maximum in the early afternoon 

 and again falls rapidly with the setting of the sun. If curves are 

 plotted for the diurnal changes in transpiration, solar radiation, 

 the saturation deficit, and temperature, it will be noted that 

 these curves will almost coincide. Radiation, however, reaches 

 its maximum slightly ahead of transpiration, while the highest 

 point in the saturation deficit is attained somewhat later (Fig. 

 95). This shows that sunlight is the chief factor determining 

 the intensity of transpiration. 



