276 DETERMINATIONS OF PERMEABILITY 



The taking up of dyes has been extensively investigated but this 

 method is beset by many pitfalls^ and the results hitherto obtained 

 are confusing. 



In some cases the penetration of acids and alkalies has been studied 

 by means of organisms containing natural indicators, or by intro- 

 ducing indicators into the cell.^ Use has also been made of the fact 

 that the penetrating substance may cause a visible precipitate within 

 the cell; this is especially the case with alkaloids.* Furthermore the 

 absorption of Ca^ has been detected by observing the formation of 

 crystals of Ca oxalate within the cell. It is evident, however, that 

 these methods have but limited application, and that in many cases 

 they are open to the objection that the penetrating substance injures 

 the cell. 



The penetration of a substance may sometimes be demonstrated 

 by observing its effect upon metabolism, but this method is 

 inadequate from a quantitative standpoint. Some investigators 

 contend that substances may produce effects on metabolism by their 

 action at the surface, without actually penetrating the cell. 



^ To a great extent the coloration of the cell by a dye shows the extent to which 

 the dye can combine with the substances within the cell rather than the rate at 

 which the dye penetrates. Thus many cells contain si^bstances which combine 

 with methylene blue so that it becomes far more concentrated within the cell 

 than in the external solution (PfeSer, W., Physiology of plants, 2nd edition, 

 Oxford, 1897, i, 96). Unless the cell has this power it often fails to appear colored 

 even though it may contain the dye in the same concentration in which it exists 

 outside. In such cases it may sometimes be detected by plasmolyzing the cell 

 and thus concentrating the dye. A further complication is that a cell may appear 

 to have taken the dye into its interior when in reality only the surface or the cell 

 wall is stained. There are many other difficulties, which need not be discussed 

 here, such as toxic action of the dye and changes in the dye (decolorization, etc.) 

 as it enters the cell. A very serious objection to this method is that it does not 

 give quantitative results. A review of the literature wiU be found in Hober, R., 

 Physikalsche Chemie der Zelle und der Gewebe, 4th edition, Leipsic and Berlin, 

 1914. 



^ For the literature see Haas, A. R., /. Biol. Chem., 1916, xxvii, 233; Crozier, 

 W. J., /. Biol. Chem., 1916, xxiv, 255; xxvi, 225; 1918, xxxiii, 463. 



*For the literature see Czapek, F., Ueber eine Methode zur direkten Bestim- 

 mung der Oberliachenspannung der Plasmahaut von Pflanzenzellen, Jena, 1911. 



^ Osterhout, W. J. V., Z. physik. Chem., 1910, Lxx, 408. 



