Crafts et al. — 44 — Water in Plants 



would be a measure of the tendency of water to cross the membrane into 

 the glycerine. Thus the diffusion pressure of pure solute or pure solvent 

 may be pictured as the driving force with which each would diffuse into 

 the other in pure phase and would be equal and opposite to the force that 

 would have to be applied on either pure phase to prevent entry of the other. 

 This latter force on a unit area basis is equal to the osmotic pressure of the 

 solution as its composition approaches pure solute. 

 From the formula 



PV = RT In — (1) 



it is obvious that the osmotic pressure of a solution approaches infinity 

 as the concentration approaches 100 per cent solute. The same is true for 

 the substance designated as the solvent if the differential permeability of 

 the membrane be reversed. It seems therefore that the diffusion pressure 

 of pure solute or pure solvent approximates infinity. 



Ordinarily, diffusion pressures are not measured ; only differences are 

 of value in consideration of osmotic systems. Thus diffusion pressure 

 requires mathematical treatment in a manner similar to that of free energy 

 where only differences are measured. Furthermore, like free energy, 

 diffusion pressure may be influenced by any force tending to restrict the 

 activity of the molecules. Among such influences are osmotic pressure, 

 hydrostatic pressure, and the action of adsorptive and electrostatic force 

 fields. In a simple osmometer, osmotic pressure is the predominating force 

 in determining the diffusion pressure differences of solvent and solute in 

 the system. Cells in plants may also be under the influence of hydrostatic 

 pressure other than atmospheric (usually subatmospheric) and adsorptive 

 forces of colloidal hydration may also be involved. 



Eyster (1940) would identify osmotic pressure with the diffusion pres- 

 sure of the solvent in an osmotic system. The distinction which must be 

 made between these terms for a clear analysis of the subject should be ap- 

 parent from the discussion that follows. We are in agreement with the 

 definitions of Hall (1940) but would prefer the term "diffusion pressure" 

 to "escaping tendency," which he favors. 



The term diffusion pressure has been used for many years in publica- 

 tions on osmosis and diffusion. Van Laar used it in his contribution to 

 the Faraday Society symposium (1917); Haldane used it in his paper 

 (1918) ; Meyer and Anderson use it in their Textbook of Plant Physiology 

 (1939), to mention only a few. 



"The diffusion pressure deficit of water [in a solution] .... is the 

 amount by which its diffusion pressure is less than that of pure water at the 

 same temperature and under atmospheric pressure" (Meyer, 1945). 



This definition is adequate for the simple osmometer in contact with 

 pure water at atmospheric pressure. For cells in plants where the diffusion 

 pressure of water external to the cells may be above or below that of water 

 at atmospheric pressure the DPD of water in a cell due to solute should 

 be defined as the amount by which its diffusion pressure is less than that 

 of pure water under the same external pressure (stage A of Figure 15). 

 This DPD may not correspond to the DPD of water in the cell referred to 

 pure water at atmospheric pressure. For example, the DP of water in the 

 xylem conductors of plants may be lowered much more by transpiration 

 pull than by the solutes dissolved in it. 



Turgor may be defined as the state of an osmotic system such as a cell, 



