z6 CELLULAR 



mechanism is, it is necesary to experiment with a 

 substance which accumulates and to analyze the 

 process step by step. 



The first of such quantitative investigations 

 on Nitella were made by Miss Irwin, using a 

 basic dye, brilliant cresyl blue. The technique is 

 very simple: after suitable exposure of the cell 

 to the dye the sap is drawn up into capillary 

 glass tubes whose color is compared with that of 

 similar tubes filled with dye of known concen- 

 tration. The accuracy of the method is satis- 

 factory, since it is possible to distinguish between 

 concentrations of .000017 and .000024 molar. 

 This procedure avoids all the difficulties pre- 

 viously mentioned in connection with experi- 

 ments on dyes. 



The penetration of the dye follows the time 

 curve of a reversible monomolecular reaction. It 

 is possible that the dye combines with some con- 

 stituent of the cell, giving a reversible reaction 

 which appears to be monomolecular because the 

 concentration of the uncombined dye is constant. 



The idea of chemical combination is sup- 



