628 BIOLOGICAL EFFECTS OF RADIATION 



meability under one condition or another. No aspect of cell physiology- 

 has been more frequently studied. By many authors increase or decrease 

 in permeability has been related to the essential phenomena in the life 

 and death of the cell. 



Numerous methods have been employed to test permeability. These 

 include staining methods, osmotic methods, chemical analyses of cells 

 and their surrounding fluid, measurements of electrical conductivity, etc. 

 No one of the methods is above criticism, although some are to be pre- 

 ferred to others. The problem is somewhat different in plant cells which, 

 unlike animal cells, are usually characterized by large vacuoles. In 

 the case of enormous cells like those of Valonia or Nitella the contents 

 of the vacuoles may be analyzed chemically. These vacuoles do not 

 represent protoplasm, and the entrance of substances into them implies 

 passage into the protoplasm and out of it again. 



In recent years there has been an increasing tendency to examine 

 results critically, to apply more certain methods, and to obtain quan- 

 titative results. On the whole, chemical and osmotic methods are now 

 preferred to staining and electrical techniques. The staining of a cell 

 depends on more than mere permeability, and when one observes increases 

 in the electrical resistance of a mass of cells, a host of factors other than 

 permeability are involved. Modern authors have studied the permeabil- 

 ity of the cell membrane to water as well as to dissolved substances. 

 (For recent reviews, see Jacobs, 172, Stiles, 349, Gellhorn, 1929, Homes, 

 164, Lucke and McCutcheon, 244.) 



In interpreting permeability studies each experiment should be 

 considered critically. In view of the fact that no method is entirely 

 satisfactory, agreement between results obtained by different methods 

 is greatly to be desired. In the case of irradiated cells the fact that the 

 rays cause a release of bound salts from the protoplasm (see below) is 

 likely to be a complicating factor. 



VISIBLE LIGHT 



The effect of visible light upon permeability has been studied mostly 

 by botanists. As early as 1879 Pringsheim (297), focusing a powerful 

 beam of white light upon Spirogyra filaments, noted that when the radia- 

 tion caused a breakdown of chlorophyll the cells became permeable to 

 aniline blue, which did not penetrate normal cells. The earliest "quan- 

 titative" permeability method, the method of isotonic coefficients, was 

 used in the study of the action of light by Lepeschkin (218) and Trondle 

 (361, 362). In such measurements, the difference between the concen- 

 tration of a penetrating substance, such as KNO3, required to cause 

 plasmolysis and the concentration of the same substance isotonic with a 

 solution of a nonpenetrating substance (such as sucrose) which will 

 cause the same degree of plasmolysis, is taken as the measure of the 



