122 J. F. MCCLKXDOX. 



by their precipitation of the tannic acid in the cell sap. Most 

 alkaloids penetrate rapidly, but only in the form of the free 

 (undissociated) base produced by hydrolysis. Hence the pene- 

 tration (precipitation and toxic effect) may be prevented by 

 adding a little acid to the medium. 



Pfeffer had shown that methylene blue is precipitated by tannic 

 acid in the cell sap of certain plants. 



Some discussion has arisen as to whether the mechanism of 

 the entrance of dyes into plant cells is similar to that of alkaloids. 

 Overton showed that lipoid soluble basic dyes penetrate easily. 

 He at first supposed that only the free color base (undissociated) 

 is able to penetrate the cell. 1 Overton found, however, that 

 triphenylmethane and chinonimid dyes disprove his assumption, 

 showing that it is at least not general. This question was taken 

 up again by Harvey 2 who found that neutral red or methylene 

 blue, which stain Elodea leaves in tap water, do not do so if just 

 enough acid be added to the water to prevent any free color 

 base from forming. 



He observed that, although these dyes are not precipitated 

 in the cell sap of this plant, they become more concentrated in 

 the cell sap than in the medium. Neutral red is bright red in 

 the cell sap, indicating that the reaction is acid (no free color 

 base is present). He supposes that the absence of any of the 

 dye in the form of the free color base prevents it from diffusing 

 out of the cell, hence it becomes more concentrated within than 

 without. 



In using the plasmolytic method, if a cell does not recover 

 from plasmolysis in a solution" of a salt, it is said to be imperme- 

 able to that salt. However, the cell may recover, but may be 

 killed by penetration of the salt, and shrink again. It is possible 

 that Overton and others failed in some cases to note this transient 

 recovery. Contrary to Overton, Osterhout 3 found Spirogyra 

 permeable to alkali-salts and alkaline earth salts, but more 



1 In this connection it is interesting to note that Robertson observed that free 

 color bases, and to a less extent free color acids, are much more soluble in fats 

 than are their salts. This is what we should expect, since the salts dissociate in 

 water, and ions are insoluble in fats. 



* Science, 1910, n.s., XXX11., 565. 



3 Science, 1911, n. s., XXXIV., 187 ; XXXV., 112. 



