Dandeno : Osmotic theories 295 



with which we are concerned. The *' gas-pressure " theory 

 affords no explanation, while the theory of attraction gives a 

 foundation for reasonable explanations of all such phenomena. 

 Root absorption, as a function of plants, is one which, as far as 

 the text-books are concerned, is explained by the statement that 

 the force causing absorption is osmotic pressure. To apply van't 

 Hoff's '' gas-pressure " theory with a view to an explanation of 

 the process is next to impossible, for the osmotic pressure is 

 within the cell of the plant, and it is not easy to see how such 

 pressure acting otttzvard could cause an inflow of water. It should 

 also be considered that we have to account for, not only the inflow 

 of water, but also the inflow of substances in solution. The theory 

 of attraction is quite applicable to these activities, and it furnishes 

 a reasonable explanation from the fact that a disturbance of the 

 content of soil water, by the addition of another solute, may render 

 what the soil contained previously to this addition more (or less) 

 available to the plant ; because in the theory here advanced, it is 

 assumed that solutes have an attraction for one another as well as 

 for the solvent When a solution is saturated, this attraction Is 

 practically satisfied ; or more explicitly, it is counteracted by the 

 attraction of one molecule for another of its kind in the process 

 of crystallization. 



With regard to the relatively high osmotic pressure said to be 

 produced in plant cells, it may be said that the method is founded 

 entirely upon van't Hoff's theory and Pfeffer's calculations ; and 

 as employed to estimate such pressure, it is open to serious objec- 

 tion. The method of finding the osmotic pressure in a given cell 

 is to use a solution of known osmotic strength (?), and if this just 

 induces plasmolysis, the cell sap of the plant is said to be of equal 

 osmotic pressure. But it has been shown by Osterhout, previously 

 cited, that distilled water may, under certain conditions, bring about 

 plasmolysis ; also that the addition of CaCl^ to a solution of NaCl 

 which has produced plasmolysis, will actually restore turgor, 

 although the external solution is now stronger osmotically. 

 Moreover, it is not yet known experimentally just what is the 

 magnitude of the osmotic pressure of most of the solutions which 

 have been used in plasmolysis, hence the conclusions are open to 



objection. 



