Grafts et al. 



— 56 



Water in Plants 



be in the neighborhood of 17,000 atmospheres, is the limiting factor in 

 determining the pull which the molecules will develop as a result of evapora- 

 tion from the surface. In practice tensions of from 100 to 300 atmospheres 

 have actually been demonstrated. This is the mechanism which the plant 

 utilizes to supply water to its uppermost parts. It is a practical demon- 

 stration of the diffusion pressure of the water molecules. 



The equilibrium vapor pressure of the water molecules above the mem- 

 brane constitutes a measure of the diffusion pressure of the water inside. 

 The relation between the diffusion pressure deficit of the water inside the 

 membrane and the vapor pressure lowering is given by the equation 



Po 



RT In p 



{10) 



DPD = 



V 



If the base line for establishing DPD values is obtained by setting the 

 diffusion pressure of pure water at atmospheric pressure equal to zero, 

 DPD values for water may be calculated. Homan, Young, and Shull 

 (1934) give a series of such values terming them "stromogenic tensions." 

 Meyer and Anderson (1939) give a more detailed series (p. 207, Table 

 24). Table 18 presents values at 20° C. calculated by the above equation. 

 Each atmosphere of diffusion pressure deficit shown in the fourth column is 

 equivalent to a column of water over 30 feet in height. This indicates that 

 in the case of the experiment mentioned above, if air having a relative 

 humidity of 99 per cent were used in contact with the membrane of Figure 

 14 evaporation would support a column of water approximately 400 feet 

 in height ; an atmosphere at 98 per cent would maintain a column 800 feet 

 high and lower humidities would support columns of proportionate height. 

 This concept of the diffusion pressure or driving force of water is extremely 

 useful in problems of plant physiology as it provides a basis for the en- 

 trance of water into plants, its movement to the leaves, and its evaporation 

 into the air. It should also prove valuable to the animal physiologist and 

 soil scientist. 



Table 18. — Relation between relative humidity, vapor pressure, and diffusion pressure 

 deficit of atmospheric moisture at 20° C. : — 



In spite of the arbitrary assignment of zero for the base line of water, 

 it is evident that pure water has a very high diffusion pressure. It is this 



