n6 



PHYSIOLOGY OF NUTRITION 



by a volume-molecular solution of potassium nitrate be considered as 3, then 

 the pressure developed by a volume-molecular solution of any other substance 

 not in the same group is 2, 4, or 5, according to the group in which the given 

 substance belongs. On this account deVries adopted as his unit of osmotic 

 pressure one-third of the pressure produced by a volume-molecular solution of 

 potassium nitrate, so that a volume-molecular solution of this salt, or of any 

 other salt belonging to the same group, always produced a pressure of 3, and the 

 three other groups of substances gave pressure of 2, 4, and 5, respectively. The 

 numbers 2, 3, 4 and 5 were termed isosmotic coefficients; they represent the 

 relative osmotic pressures developed by equimolecular solutions of the various 

 substances. 



The isosmotic coefficients were determined in the following manner. Three 

 cane sugar solutions, 0.20-, 0.22- and 0.24- volume-molecular, and three solu- 

 tions of potassium nitrate, 0.12-, 0.13- and 0.14- volume-molecular, were em- 

 ployed, for plasmolytic experiments with epidermal cells of Curcuma 

 rubricaulis. Each experiment lasted seven hours. The results obtained 

 in three such tests are given in the following table, where n denotes that 

 no plasmolysis occurred, hp denotes that about half of the cells were 

 plasmolyzed and p denotes that most of the cells were plasmolyzed. IC 

 denotes the isosmotic concentration, taken to be osmotically equal to the cell 

 sap. Volume-molecular concentration is denoted by m. 



Since the osmotic pressure produced by a volume-molecular potassium 

 nitrate solution is taken as 3, the numbers in the last column are to be multi- 

 plied by 3, and the average ratio thus becomes 1.8 1, which is the isosmotic 

 coefficient of saccharose when that of potassium nitrate is considered as 3. 



A list of substances thus tested by deVries is given in the next table, together 

 with their isosmotic coefficients, as actually derived from experiment and also 

 in round numbers. The next to the last column gives the percentage concen- 

 trations thus found to be isosmotic with a one-tenth volume-molecular solution 



