CONDUCTIVITY AND PERMEABILITY 211 



The penetration of acids and alkalies has been studied 

 by employing organisms containing natural indicators, 

 or by introducing indicators into the cell." In some cases 

 the penetrating substance may cause a visible precipitate 

 with the cell: this is especially the case with alkaloids.'® 

 The absorption of Ca" has been detected by observing 

 the formation of crystals of Ca-oxalate within the cell. 

 It is evident, however, that these methods have but 

 limited application and that in many cases they are open 

 to the objection that the penetrating substance injures 

 the cell. 



The penetration of a substance may often be demon- 

 strated by observing its effect upon metabolism, but this 

 method is unsatisfactory from a quantitative standpoint. 

 Some investigators state that substances may produce 

 effects on metabolism by their action at the surface, with- 

 out actually penetrating the cell. 



It is evident that the most satisfactory method would 

 be to place the cell in a solution containing the substance 

 whose penetration was to be investigated, and, after a 

 definite time of exposure, to obtain the cell sap without 

 contamination and test it for the presence of the sub- 

 stance. Experiments of this sort have apparently not 

 been carried out, though interesting results have been 

 obtained by Meyer,^'* Hansen,^^ Wodehouse,'^^ and 

 Crozier,^^ by comparing the cell sap of Valonia (which 

 can be obtained without contamination) with the sea 



"For the literature see Haas, A. R. C. (1916, B), Crozier (1916, A, 

 B; 1918). 



"For the literature see Czapek (1911). 



"Osterhout (1909). 



"Meyer (1891). 



» Hansen (1893). 



"Wodehouse (1917). 



^Crozier (1919). 



