570 RADIATION BIOLOGY 



tion or production of a substance with resulting changes in concentration. 

 Such processes give no support to the assumption that changes in perme- 

 abihty have occurred. In the great majority of experiments, however, 

 the conditions have been such that the effects of Ught indicated a direct 

 association between this factor and the permeabiUty of the plasma. 



A number of workers have put forward hypotheses or assumptions 

 regarding the manner in which the effect of light occurs. Systematic 

 experiments to determine the nature of the mechanism have, however, 

 been made in only a few instances. 



According to Lepeschkin (1930, 1938), the action of light can be 

 explained by the supposition that the change in permeability produced 

 by it is a photochemical process, and that the cells are able to restore 

 some substances important for the selective permeability of protoplasm 

 and destroyed by light. Both processes may proceed in light, hut if the 

 cells are transferred to the dark, only a restoration takes place. 



As support for his hypothesis, Lepeschkin (1938) referred to the changes 

 undergone by leukocytes on exposure to light. These were manifested, 

 for example, in their reaction to staining of the protoplasm. He also 

 mentioned the hemolysis taking place in red corpuscles in light. 



Lepeschkin also considered his theory to be substantiated by other 

 experiments. He found (1940) that light influenced the movements 

 of salts from potato discs immersed in water toward their outer side 

 ("exosmosis"). The net exit or the net accumulation of salts was deter- 

 mined by the method of the electrical conductivity of weak salt solutions 

 in which the objects were immersed. The exosmosis was increased by 

 direct sunlight and by the rays of a quartz-mercury-vapor lamp, i.e., 

 chiefly by ultraviolet rays, whereas electric incandescent light (a 200-watt 

 lamp) did not influence it noticeably. Lepeschkin assumed that light 

 probably destroys the most unstable lipoprotein complexes ("vitaids") 

 participating in the maintenance of the semipermeability of the cell 

 protoplasm and that these destroyed vitaids are replaced in the dark 

 by synthetic processes in the cells. In this connection, Jarvenkyla's 

 hypothesis (1937), mentioned earlier, i.e., that light loosens the lipoids 

 in the plasmatic membranes, may be recalled. 



Thorough studies of the photic mechanism were made by Brauner and 

 Brauner (1937). They noted that asymmetric illumination gave rise to 

 electrical potentials and changes in the potentials in Elodea densa, the 

 illuminated part, or that most illuminated, being positive as compared 

 with the unilluminated or less illuminated part. In order to elucidate 

 the mechanism, they (1938) arranged special experiments of the photo- 

 electric reactions in models of membranes. These led to the conclusion 

 that the mobility of cations and of at least one voluminous anion (P04~) 

 seems to be considerably decreased within the illuminated boundary 

 layers of these systems. 



