THE PERMEABILITY OF CELLS 527 



exclude sources of error and make sure that the permeabihty of 

 living cells is in reality the problem studied, a point not always 

 carefully guarded against. For this reason the observations and 

 experiments which follow are recorded in considerable detail. 



2. Objections to the use of an indicator within the cell 



Despite the fact that indicators offer the most delicate means 

 of detecting alkalies (and neutral red is exceedingly sensitive in 

 this respect, reacting to Na2HP04, there are four possible com- 

 plications arising from its use in the cell, all of which would tend 

 to make the amount of alkali entering, appear less than that which 

 actually entered. Or, to put it in another way, more alkali would 

 enter than we should calculate, judging from the concentration at 

 which the color change takes place in pure water. 



1. In determining alkalinity in a test tube we have only a very 

 weak solution of the indicator ; in the leaf cells of Elodea, neutral 

 red is in concentrated solution (otherwise it would be quite 

 invisible in so narrow a space) and also in combination with some 

 complex organic acid. Enough alkali must enter to convert it 

 all into the free base (ROH) before the color change occurs. 



2. In Spirogyra neutral red is accumulated as a tannin com- 

 pound which becomes straw color in the presence of alkali and 

 red again on adding acid and begins to dissolve. This compound 

 as well as the combination formed by neutral red with the granu- 

 ules of animal cells require a greater concentration of alkali or a 

 longer time, to turn them yellow than does the pure dye in dis- 

 tilled water. By treating cells with chloroform or by rupturing 

 their surface, as described under each organism investigated, it is 

 possible to exclude the complication due to the above mentioned 

 conditions. The concentration of alkali may be found which 

 brings about a color change instantaneously. 



3. The most serious objection to the use of neutral red as an 

 indicator within the cell is, as I have determined experimentally, 

 that in the presence of proteids like egg albumen, the dye (in 

 solution) is unaffected bv a certain amount of added alkali which 



