102 BULLETIN" 1059, U. S. DEPARTMENT OF AGRICULTURE. 



between the osmotic pressure within the plant and the antiosmotic 

 pressure exhibited by the soil moisture, with an allowance for the 

 distance through which this force must operate. This same basis 

 was used in the preceding section as a rough means of showing 

 changes in the soil' condition, but without any allowance for changes 

 in the absorbing power of the plant which occur with its loss or 

 gain of water, or without considering the factor of height and dis- 

 tance as it may affect tall trees. 



In attempting thus to express the availability of water to the 

 plant, in precise terms or osmotic pressures, currently for any con- 

 dition that may be encountered in the soil or plant, it is necessary 

 to determine the osmotic pressure of the soil or plant quickly and 

 accurately. 



The osmotic pressure of an aqueous solution is determined by the 

 increase in its boiling point over that of pure water ; by the depression 

 of its freezing point ; by the decrease in the vapor pressure over the 

 solution ; and, possibly, by the increase in the latent' heat of vapori- 

 zation. It is only recently that investigations of the last have been 

 made, so that there is no known formula which would make this 

 process available. 



Within the limits of so-called dilute solutions a rise of 1° C. in the 

 boiling point represents an osmotic pressure of about 57 atmos- 

 pheres; a depression of 11° C. in the freezing point indicates P= 

 12.05 atmospheres, and a depression of 1 per cent in the saturated 

 vapor pressure over the solution, the temperature being the same, 

 indicates about 12 atmospheres pressure. These approximate figures 

 permit us to judge of the practical utility and accuracy of different 

 methods. 



It may also be useful at this point to refer to the fact that in pure 

 solutions, such as may be used in the vapor-transfer method or in 

 plasmolytic tests on tissues, the osmotic pressure is very readily de- 

 termined by the concentration of the solution, in terms of the molec- 

 ular weight of the solute, provided the solute is chemically pure and 

 anhydrous. According to Nernst (134) the "molecular lowering 

 of the freezing point" for water is 18.4° C., 12 or 1.84° C. when 1 

 gram molecule of the substance is dissolved in a liter of water. A 1- 

 molecule solution, therefore, stands for 22.12 atmospheres osmotic 

 pressure. 



From these data it would seem that the boiling-point method 

 would insure the greatest precision in osmotic pressure determina- 



12 More recent investigations reported by Jones 128 show that the molecular lower- 

 ing may be twice this amount in the case of salts which are dissociated by water into 

 two ions. Freezing-point determinations should quickly decide this, in case of doubt. 



