THE INTAKE AND UTILIZATION OF WATER 



27 



creased wind velocity, but in both cases the saving of water is great 

 enough to be of real importance in plant production. 



Table 16. — The Influence of Wind Velocity Upon Rate of Transpiration 



(After EberfW) 



Light. — In its lower ranges increased illumination has been found to cause 

 increased transpiration irrespective of the action of the guard cells already- 

 discussed. '*'' The effect of light may be due to absorption of radiant energy or to 

 increased permeability of the membranes. The heat set free by chemical proc- 

 esses or received by radiation may pass directly into the latent form without 

 effecting a rise in the tempeature of the leaf: that is, it may be used completely 

 in vaporizing water. Protoplasmic membranes are more permeable in light 

 than in darkness and the same seems to hold for the non-cutinized cell wall.^^ 



This increased rate of transpiration produced by exposure to hght probably 

 accounts for the characteristic action of illumination in retarding the rate of 

 growth and the dependence of green plants upon hght for the differentiation of 

 tissues. ^^ The gradual reduction in the osmotic concentration of the stomatal 

 guard cells found by Wiggins^^^ may also be attributed to increased permeabihty 

 after exposure to hght. In the afternoon the manufacture of soluble carbohy- 

 drates is apparently more than offset by the increased permeabihty. Hence, 

 the osmotically active substances in the guard cells diffuse into the adjoining 

 cells, the guard cells lose their turgidity and the stomata close soon after dark- 

 ness sets in. 



Temperature. Slope of Ground. — The rate of transpiration increases 

 with rising temperature. It is one of the reasons, though probably not 

 the main reason, why north slopes may be preferable to south slopes 

 when moisture is a limiting factor. It likewise furnishes the explanation 

 of most of the phenomena connected with the temporary wilting and later 

 recovery of turgidity in plants. Of particular interest is the effect of 

 temperatures below 0°C. on the transpiration from twigs. The data 

 presented in Table 17 on the transpiration from a branch of Taxus haccata 

 from which the leaves had been removed are interesting not only in show- 

 ing the influence upon transpiration of an increase in temperature, but 

 also in showing that transpiration takes place at temperatures consider- 

 ably below the freezing point. Water loss under these conditions is 

 associated with certain types of winter injury, a matter discussed in 

 more detail in another section. 



