THE DAILY PERIODICITY OF TRANSPIRATION 207 



Decrease in the water content of the leaf cells also results in a rise in 

 their diffusion pressure deficit, and a correlated decrease in their vapor pres- 

 sure. A fully turgid cell {i.e. one with a diffusion pressure deficit of zero) 

 has an equilibrium vapor pressure equivalent to that of pure water regardless 

 of its osmotic pressure. At 20° C. this is 17.54 mm- Hg. If the diffusion 

 pressure deficit of the mesophyll cells of a leaf at this temperature were 

 raised to 100 atmos., a value which can be attained in only a very few species 

 of plants, their vapor pressure would decrease by only about 1.27 mm. Hg 

 (Table 24). Since the vapor pressure of the cell walls, with which the 

 vapor pressure of the intercellular spaces tends to be in equilibrium, in turn 

 tends to be in equilibrium with that of the cell sap, it would appear that an 

 increase in the diffusion pressure deficit of the mesophyll cells has only a very 

 slight influence upon vapor pressure conditions in the intercellular spaces. 



TABLE 24 RELATION BETWEEN DIFFUSION PRESSURE DEFICIT AND VAPOR PRESSURE OF WATER 



AT 20° C. 



However, certain other factors must be taken into consideration in evaluat- 

 ing the effect of a diminished leaf water content upon the vapor pressure 

 of the intercellular spaces. Decrease in the water content of the leaf cells 

 involves a reduction in the quantity of water in the cell walls as well as in 

 other parts of the cell. Water undoubtedly passes through cell walls prin- 

 cipally through the intermicellar material. The principal effect of a decrease 

 in the quantity of water in the walls is a shrinkage in the volume of the 

 hydrophilic intermicellar material and a resulting contraction in diameter of 

 the intermicellar capillaries. This probably results in a decrease in the per- 

 meability of the walls to water. Hence, under such conditions, water often 



