HYDRATURE AND PLANT PRODUCTION 273 



a certain temperature). From, hy we can compute the osmotic value* (W) : 



.^. 1000. RTs, p 



W= — In — 



M Pq 



(R= gas constant, T= absolute temperature, 5 = specific weight of water at 

 20°C = o-9982, M=molecular weight of water=i8.) After introducing 

 the constants we have : 



W= — I334ln.-^ = —3071 log ^ 

 Po Po 



Because of this relation between the osmotic value of a solution and its 

 relative humidity (one may imagine that in closed containers there always 

 exists equilibrium between the osmotic value of the solution and the 

 relative humidity in the air space above it) we can use either the relative 

 humidity or the osmotic value to indicate hydrature. 



For the determination of hydrature in plants we generally use the osmotic 

 value of the cell sap. From the equation : S= H^— Pit may be concluded that 

 the suction tension (5) might be the better expression for the hydrature of 

 plants (see Renner, 1933a and b; Huber, 1932; Fukuda, 1935; Stocker, 

 1954). But it can be answered that the suction tension represents only the 

 hydrature outside a cell, which must be considered as responsible for the 

 change of water from cell to cell. The osmotic value represents the hydra- 



* In order to avoid confusion we shall list here abbreviations used regarding osmotic 

 terms : 



DPD (diffusion pressure deficit) = 5 (suction tension) 

 OP (osmotic pressure) = W (osmotic value) 



TP (turgor pressure) = P (turgor pressure) 



Of the expressions mentioned above, the most erroneous is 'osmotic pressure,' as in 

 reality we are unable to measure such a pressure in solutions. The 'real' osmotic pressure 

 (in German 'osmotischer Druck') wliich actually exists, develops as hydrostatic pressure 

 in an osmometer for instance. Its equivalent in the cell is the turgor pressure. Both the 

 actual osmotic pressure and the turgor pressure act as back pressure which determines 

 together with the osmotic value the resulting suction tension in an osmotic system. In 

 order to distinguish exactly between the meanings of the equivocal term 'osmotic 

 pressure' (in the sense of osmotic value and actual osmotic pressure = hydrostatic 

 pressure) we suggest using principally the term 'osmotic value' (meaning the chem- 

 ical potential) instead of osmotic pressure. And because it is physically more useful 

 to compare only measurable quantities (diffusion pressure cannot be directly deter- 

 mined, see Walter, 1955), it should be obviously better to write the osmotic equation 

 not as: DPD = [OP — TP], but in the manner: 



5= W-P 



In this paper we shall use only these abbreviations and terms. 



