318 BOTANICAL GAZETTE [april 



determining the loss of hydrogen ions from the solution by means 

 of a hydrogen electrode. 



It may be observed in this connection that the absorption oi 

 dissolved substances by living cells has been employed extensively 

 as a criterion of permeability. The amount of absorption is 

 usually determined by analysis (of the solution or of the tissue) 

 before and after the organism is placed in the solution. A more 

 convenient method, which suffices in some cases, is to determine 

 the conductivity of the solution. Nephelometry is also useful. 

 Such methods may also be used to determine the excretion of sub- 

 stances by the organism. 



The results obtained by these methods have been so largely 

 misinterpreted that there is widespread confusion in regard to their 

 significance. This confusion is due in part to uncritical technique 

 and in part to overlooking some of the many variables involved in 

 such experiments; but the principal difficulty lies in confusing 

 permeability with absorption. 



The nature of this difficulty is evident from the following 

 illustration. Suppose a glass tube closed at one end by a membrane 

 in contact with a solution to which it is freely permeable. The 

 solution will pass through the membrane into the tube until 

 equilibrium is established. If, however, we place in the tube 

 something which precipitates the dissolved substance, more of the 

 latter will diffuse in, and this will go on as long as the precipitation 

 continues. It is not even necessary that the precipitation should 

 occur, since the result can be obtained by causing the dissolved 

 substance to unite with something within the tube so as to form a 

 compound which cannot pass out through the membrane. 4 The 

 dissolved substance will then continue to pass into the tube. 



It is evident that the permeability of the membrane remains 

 the same whether precipitation or other chemical action occurs or 

 not. But while the permeability remains the same, the amount of 

 adsorption will vary enormously. 



This may be observed with the living cell. When a cell is 

 placed in a dye which is precipitated within the cell (giving a 

 visible precipitate), the absorption of the dye goes on as long as 



4 Cf. Loeb, J., Dynamics of living matter. 1906 (p. 72). 



