Crafts et al. 



190 



Water in Plants 



the capacity of a given volume of air to hold water is greatly afifected by 

 the temperature. For example, as shown in Figure 52, a change in tem- 

 perature from 40° F. to 50° F. may cause a change in relative humidity 

 of from 76 to 54 per cent, or a change in DPD of from 350 to 810 at- 

 mospheres when the air contains 0.004 pound of water per pound of dry 

 air. 



JOO 



30 -40 SO 60 70 SO SO 100 110 IZO 



Temperature -^F 



Fig. 52. — Relation between atmospheric temperature 

 and relative humidity for an atmosphere containing .004 

 pounds of water per pound of dry air. 



As a result of humidity and temperature changes there is a diurnal fluc- 

 tuation in DPD of the atmosphere, as shown in Figure 53. Coupled with 

 this fluctuation is the influence of light upon transpiration. This may act 

 either directly to increase the temperature of the leaves above that of the 

 atmosphere or physiologically to alter the permeability of the protoplasm 

 or the movement of stomata. Diurnal fluctuations in transpiration rates 



I 



J 



