TPIE FREEZING POINTS OF TOTTINGHAM'S NUTRIENT y^^S:^ 



SOLUTIONS! /^^«^^ 



JOHN W. SHIVE I^JLIBf^AfJY 



The Johns Hopkins University, Baltimore, Maryland * -^V '^VO^^ 



In Tottingham's^ recent work upon the properties of nutrient ' ■i-^j^^J^!' ^ 

 solutions for plant cultures, the osmotic concentrations or diffu- -^ • r 

 sion tensions^ of the solutions used were approximated by a 

 method of calculation, which considered that each salt of a com- 

 plex mixture ionizes to the same degree as though the other 

 salts were not present. Tottingham calls attention to the pos- 

 sibility of considerable error in the making of this assumption 

 that other salts in the same solution are without influence upon 

 the ionization of each particular salt, but notes that the assump- 

 tion leads to a closer approximation of the conditions actually 

 existing than can any other method of calculation, considering 

 our present ignorance of ionization phenomena in complex salt 

 mixture. The author just mentioned employed three series of 

 84 solutions each, all of the solutions of each series having the 

 same calculated diffusion tension but differing in their proportions 

 of the four nutrient salts. The same salts occurred in all the 



^ Botanical contribution from the Johns Hopkins University No. 42. 



2 Tottingham, W. E., A quantitative chemical and physiological study of 

 nutrient solutions for plant cultures. Physiol. Res. 1: 133-245. 1914. 



' As has been pointed out by Livingston (The role of diffusion and osmotic 

 pressure in plants. Chicago, 1903, pp. 30-31) and later by Findlay (Osmotic 

 pressure. London, 1913, pp. 3-4) a solution does not, in itself, exert osmotic 

 pressure. Such pressure results only when a solution is separated from the 

 pure solvent, or from a weaker solution, by a membrane permeable only to the 

 solvent, or at least more permeable to it than to the solute. The term diffusion 

 tension as used by Livingston seems suitable to denote the condition existing 

 in the solution itself. This tension is numerically equal to the osmotic pressure 

 which would result if a solution were separated from the pure solvent by a mem- 

 brane perfectly permeable to the solvent and not at all to the solute. In the 

 present paper the term osmotic pressure will be avoided, since no membranes 

 are here to be considered. 



345 



THE PLANT WORLD, VOL. 17, NO. 12, 1914 



