14 CELLULAR 



rather than the permeability. Some cells contain 

 substances which combine with the dye so that 

 it becomes far more concentrated within the cell 

 than in the external solution. Unless the cell has 

 this power it often fails to appear colored even 

 though it may contain the dye in the same con- 

 centration in which it exists outside. In such cases 

 it may sometimes be detected by plasmolyzing 

 the cell and thus concentrating the dye. A fur- 

 ther complication is that a cell may appear to 

 have taken the dye into its interior when in re- 

 ality only the surface or the cell wall is stained. 

 There are many other difficulties, which need 

 not be discussed here, such as toxic action and 

 changes in the dye (including decolorization as 

 it enters the cell). 



In some cases the penetration of acids and al- 

 kalies has been studied by means of organisms 

 containing natural indicators or by introducing 

 indicators into the cell. Use has also been made 

 1 of the fact that the penetrating substance may 

 cause a visible precipitate within the cellj this is 

 especially the case with alkaloids. Furthermore 

 the absorption of calcium has been detected by 

 V 



