Chapter X 



189 — 



Loss and Retention 



resistance of the cuticle, protoplasmic resistance, self-generated convection 

 currents, air humidity, leaf cooling, and wind velocity. 



Fluctuations in the Rate of Transpiration: — The rate at which 

 water escapes from the colloidal walls of the mesophyll is a function of the 

 complex interaction of internal and external forces and their influence upon 

 the DPD of the water in the wall and in the intercellular spaces. Under 

 conditions of adequate water supply the normal daily fluctuations in me- 

 teorological conditions are reflected in the daily march of transpiration. 



lOO 90 ao 70 60 ,50 40 30 10 10 Q 



Relative Jiumidity in percent 



Fig. 51.— DPD of the atmosphere at 20° C as re- 

 lated to relative humidity. Data of table 18, Chapter V. 



For, although equilibrium is never attained, the movement of water into, 

 through, and out of the plant responds to the tendency to establish equilib- 

 rium of water in the soil, plant, and atmosphere. In the final analysis, it is 

 the diffusion pressure deficit of the water vapor in the atmosphere which 

 most profoundly affects this movement. In Table 18 of Chapter V, fig- 

 ures are given for the DPD of the atmosphere at constant temperature and 

 changing relative humidity. Figure 51 shows a graph of this relation. 

 Shull (1939) has pointed out that the influence of temperature at con- 

 stant relative humidity is very small. He did not, however, point out that 



