294 Dandeno : Osmotic theories 



of ionization at all In fact, Jost admits this in the following 

 quotation : '' However, botanical investigations on the amount of 

 osmotic pressure are entirely independent of this physical theory." 

 The absorption of soil water by plants is a function which cannot 

 be explained on the gas-pressure theory, but is capable of reason- 

 able explanation on the theory of attraction. 



With regard to solutions of two or more substances, the state- 

 ment is generally made that osmotic pressure is the sum of the 

 individual pressures ; quoting from Livingston, p. 29, ''The total 

 osmotic pressure of a dilute solution of mixed solutes is the sum 

 of the partial osmotic pressures of the component solutes." But, 

 in some cases, this is not true. If we measure the osmotic pres- 

 sure of a dilute NaCl solution, and then add some CaClg to this 

 solution, there is no increase in osmotic pressure. This has been 

 borne out by Osterhout * in his experiments on plasmolysis which 

 he performed upon certain algae. He applied a NaCI solution to 

 the alga, and brought about plasmolysis. He then added some 

 CaCl2 to a similar solution of NaCl and applied this mixed solu- 

 tion to a similar alga. In this case, the plasmolysis did not occur, 

 although the solution was stronger osmotically. In the matter 

 of determining the osmotic pressure of mixed solutions, very 

 little has so far been done. What has been done, however, seems 



to show 



/ 



pressure of mixed solutions will be. As far as plasmolytic opera- 

 tions have been used to demonstrate relative osmotic pressure, it 

 may be inferred that the specific characters of the substances In 

 solution are the determining factors; and that these may differ 

 with different plants. If, however, we assume the theory pro- 

 posed here — that of specific attraction — these phenomena are 

 easily explained. TiTmlScea' solutions,*^ the various molecules of 

 solutes quite probably exert an attraction upon one another, as 

 well as upon the solvent ; and, to some extent at least, it may be 



F 



supposed that whatever attraction a substance might have when 

 alone in solution, would be modified by the presence of mole- 

 cules of other solutes. Hence, it Is not difficult to account for 

 the modified osmotic pressure of such mixed solutions. 



In plant and animal functions, it is mixed solutions mainly 



*Bot. Gaz. 46 : 54. 190S. 



