560 RADIATION BIOLOGY 



cell, its rate would be an indicator of the rate of permeation, i.e., of the 

 permeabihty. Fitting (1915) found a change in permeability in the 

 course of the experiments, it being possible to record changes within 

 15 min. He therefore concluded that the plasmolytic substance either 

 affected the plasmatic membranes, causing a change in their properties, 

 or gave rise to changes in the content of osmotically active substances in 

 the cells. Because these processes affect the values of the isotonic coef- 

 ficients, he criticized the method used bj^ Lepeschkin and by Trondle. 

 He also raised the objection that these authors had not taken into con- 

 sideration the possibility of an exit of osmotically active substances from 

 the cells. Lepeschkin (1923) countered this objection by stating that 

 such deficiencies are common to all plasmolytic methods and were there- 

 fore also applicable to those used by Fitting. Moreover, as pointed out 

 by Trondle (1918), Fitting used a relatively unsuitable test object, 

 Tradescantia. The permeability of the leaves of this plant is so sUght 

 that the experimental errors will be relatively large in proportion to the 

 results. 



Zycha (1928), using the same test object as did Lepeschkin and 

 Trondle, i.e., Rhoeo and Buxus, made a critical study of the methods 

 in Fitting's own laboratory. He found that the errors of the method 

 could be considerable, as much as ±32.5 per cent. He was also unable 

 to establish any definite effect of light on the permeability. 



Zycha's work has been criticized by Lepeschkin (1930), who pointed 

 out that Zycha did not use the same slice of epidermis for the determi- 

 nation of the isotonic coefficients as Lepeschkin had done, but used two 

 parallel rows of slices. He considered the method to be insufficientlj'' 

 accurate, since the concentration of the cell sap of epidermal cells is com- 

 paratively small and varies too much, whereas the permeability of proto- 

 plasm is not great enough. He contended that Zycha was therefore 

 unable to observe the difference between the isotonic coefficients in light 

 and in darkness. 



The effect of Ught on the plasmatic permeability has subsequently 

 been the object of a number of investigations. In some of them, how- 

 ever, this question has only been a detail in a larger problem. Thus, 

 for example, studies of the stomata mechanism have often included an 

 investigation of the association between light and permeability, on the 

 grounds that the guard cells of the stomata are sensitive to light. The 

 physiological processes in such highly specialized cells are, however, com- 

 plicated and far from fully elucidated. It is therefore practically impossi- 

 ble to determine whether an individual reaction can be considered as a 

 cause or as an effect of permeability or whether it has any association 

 whatsoever with this factor. No account of such investigations will 

 therefore be given in the following. 



Ruhland (1912) and Ruhland and Hoffmann (1925) made use of the 



