516 EDMUND NEWTON HARVEY 



rotation goes on and when placed in A they were able to collect 

 the dyestuff from the surrounding solution. The stain may be 

 'adsorbed' by the cellulose cell walls from acid solutions just as 

 it is adsorbed by glass only when in the acid condition. But 

 this phenomenon has nothing to do with the question of the per- 

 meability of the cells for the dye, although on casual observation 

 the appearance of dye accumulation may be given. Elodea cells 

 fail also to stain in 2/00 acetic acid, yet accumulate the dye if 

 transferred to a neutral solution.^ 



c. Theory of indicators applied: Indicators as neutral red or 

 litmus are very weak bases or acids. According to Ostwald, the 

 color change is due to the transformation of the acid or base into 

 a salt which is very highly ionized. The ions give the color to 

 the solution and their color is different from that of the undis- 

 sociated molecule of the free color base or acid, only slightly 

 ionized in solution. Neutral red has the following structure : 



(CHs)^ N = CeHaC I > CeHs (CH3) NH2 . HCl or RCl 



A small amount of acid converts all of the neutral red into dis- 

 sociated R + and CI — . A small amount of alkali forms ROH, 

 undissociated, and it is the undissociated color base w^hich may 

 enter cells. In the neutral condition a small proportion of free 

 base ROH is present, due to hydrolytic dissociation. Conse- 

 quently cells may be stained in neutral solution. 



d. Action of the acid is on the dye: That the effect of the acid 

 is on the dye and not on the plasma membrane of the cells, de- 

 creasing its permeability, is made probable by the following facts. 



1. Certain acid dyes (eosin) enter only in the presence of 

 dilute acid and fail to enter in alkaline solution. 



2. The presence of dilute HCl does not prevent the toxic 

 effect of heavy metal salts like CUCI2. 



* The observations refer only to the normal rotating cells and not to certain 

 large cells filled with a mass of white granular matter, which stains in acid solu- 

 tion. 



