PERMEABILITY OF PLANT CELLS 



133 



Instead of waiting for the recovery of a plasmolyzed cell we 

 may determine the comparative rate of penetration of a sub- 

 stance by observing the osmotic pressure of the plasmolyzing 

 solution and comparing this with the osmotic pressure of a bal- 

 anced solution which plasmolyzes to the same extent in the same 

 time (and at the same temperature) . It is evident that the more 

 rapidly a substance penetrates the higher the osmotic pressure 

 of its solution must be in order to plasmolyze.^ The results of this 

 method agree with those of the method of recovery and of the 

 electrical method to be described later. 



It is a noteworthy fact that in Overton's list inorganic salts are 

 the only substances which are set down as wholly incapable of 

 penetrating the protoplasm. From the standpoint of Overton's 

 theory this is a necessary conclusion because salts are for the 

 most part insoluble in lipoid. But from the standpoint of plant 

 nutrition it is quite inconceivable. In order to put the matter to 

 a decisive test the experiments which Overton had made on 

 Spirogyra to test its permeability to inorganic salts, were repeated 

 with the improvements on Overton's method which have just 

 been described. 



The results were in all cases quite the opposite of those ob- 

 tained by Overton. In salts of NH4, Rb, Na, Cs, K, Li, Mg, Ca, 

 Sr and Al, the protoplast which was plasmolyzed and left in the 

 solution expanded again to its normal size, showing that all these 

 salts readily penetrate the protoplasm. 



As an illustration, an experiment with NaCl is described, for 

 the reason that this is very generally employed as a plasmolyzing 

 agent. Filaments of Spirogyra were placed in a 0.4M NaCl 



^ The comparison with the standard solution may be made bj' other means. 

 The loss of volume may be measured in a pycnometer (or unicellular organisms 

 may be centrifuged) and in some cases (where there is a constant amount of gas 

 or none at all in the intercellular spaces) loss of weight may serve as a criterion. 

 These determinations depend upon the principle that hj-pertonic solutions of 

 substances which do not penetrate will cause a more rapid loss of water than 

 corresponding solutions of substances which penetrate. But emphasis should 

 be placed on the fact that loss of weight or volume may also be caused by false 

 plasmolysis, and that unless it is possible to distinguish between true and false 

 plasmolysis, loss of weight or of volume or change of specific gravity can not be 

 relied upon as a criterion of penetration. 



