Grafts et al. 



54 — 



Water in Plants 



appreciate the significance of the range of states through which the osmom- 

 eter passes from "state A" to "state B." Both are right insofar as their 

 definitions go but both definitions are deficient. The only complete defini- 

 tion involves the concept expressed by the relation 



OP = DPD + TP (2) 



In words, osmotic pressure is a physical property of a solution. For 

 an osmotic system it measures the limiting value that the turgor pressure, 

 or the diffusion pressure deficit, or their sum, may attain. 



zoo 



5 



Ho 



(3 



160 



\ 



<nlOO 



'so 



Ho 

 O 



o'- «- — I 1 



100 zoo 300 ■400 JOO 600 700 300 



Crams sucrose per 1000 ml. of <2olution 



Fig. 16. — Calculated and observed values of osmotic pressures of cane 

 sugar solutions. A, observed values of Frazer and Myrick. B, values 

 calculated according to Morse. C, ideal values from van 't Hofif's law. 



The "solute pressure" theory attempts to explain turgor pressure at 

 water equilibrium ("state B") whereas the "solvent pressure" hypothesis 

 tries to rationalize osmotic pressure in terms of the diffusion pressure 

 deficit of the water in the solution ("state A"). The force causing osmosis 

 is obviously this excess diffusion pressure of the pure solvent. And at the 

 instant the differentially permeable membrane comes in contact with pure 

 solvent this force is numerically equal to the osmotic pressure of the solu- 

 tion. But it is directed inward and it is not the osmotic pressure of the 

 system as commonly defined by physicists. Only after the solution is com- 

 pressed until it is in water equilibrium across the membrane is osmotic 

 pressure in the classical sense manifested and that pressure — the hydro- 

 static or turgor pressure of the system — is an outwardly directed force of 

 the proper dimensions. 



Calculation by Indirect Methods : — Many methods have been used to avoid the 

 controversial aspects of osmosis. One has been to work in terms of vapor pressure. 

 Because the reversible work done in a change of state does not depend upon the process, 

 vapor pressure measurements may provide accurate data from which to calculate 

 osmotic pressure values, as shown by Table 13. They are, however, of little aid in 

 clarifying the mechanics of osmosis. Callendar (1908) has gone so far as to pro- 

 pose that osmosis takes place as passage of water vapor through minute capillaries in 

 the membrane. A vapor phase seems highly improbable as an essential feature of the 

 artificial membranes used to measure osmotic pressure (copper ferrocyanide in col- 

 loidal form), and of plant membranes in view of their hydrophilic nature. 



Another method is to calculate osmotic pressure values from indirect measurements 

 upon thermodynamic principles avoiding the question of mechanism. Although such 

 values may be very accurate, they do not give the answer that the biologist is seeking, 



