556 RADIATION BIOLOGY 



In these experiments the influence of light on the time of centrifugation 

 has been interpreted as due to its effect on the resistance of the protoplasm 

 through which the chloroplasts move, i.e., on the viscosity of the proto- 

 plasm. It cannot be connected with photosynthetic processes, because 

 the plasmatic changes are induced by light quantities insufficient to reach 

 the compensation point of photosynthesis, and of such short duration that 

 the photosynthetic production would be practically nonexistent. 



The mechanism of the influence of light is as yet unknown, but there 

 are two possible explanations: 



1. If the dispersion viscosity is involved, it is conceivable that light 

 influences the electric charge of the particles, thus affecting their hydra- 

 tion and thereby their volume and friction. Support is lent to this 

 hypothesis by the investigations of L. Brauner (1935, 1937). He found 

 that the charge of membranes is changed by light and that this change 

 has a rapid onset. Pincussen (1930), who made experiments with acidoid 

 gels, stated that light caused a reduction in the charge and a slight shrink- 

 ing of the gel. Pincussen also recalled Young's studies (1922), which 

 showed that the greater the amount of protein solutions freed from the 

 electrolytes, the greater is their sensitivity even for long-wave-length 

 light. Pyrkosch (1936) and others (see Lepeschkin, 1938, p. 192) later 

 obtained similar results. Pyrkosch found that both the light from a 

 200-watt bulb and monochromatic light caused a change in the colloidal 

 state of milk. Ultraviolet light, however, had a considerably more 

 marked effect. 



The same similarity in the effect of visible light and ultraviolet light 

 appears in the case of the viscosity. In visible light the effects are reversi- 

 ble and form part of the normal processes occurring in the protoplasm, 

 but ultraviolet light — at any rate, in large doses- -has a destructive effect 

 in that concentration continues and coagulation occurs (Gibbs, 1926; 

 Heilbrunn and Young, 1930). The similarity between the influences of 

 the respective kinds of light indicates that the same processes or states 

 are affected and that the difference in the manner of action is only of a 

 quantitative nature. In long-wave-length light the changes are less 

 marked or only moderate and therefore reversible, whereas in short-wave- 

 length light they are intensified and exceed the border line to irreversible 

 changes. As will be shown in the following, the same difference between 

 the two kinds of light is also noticeable in their effect on the permeability. 



2. On the other hand, if the structural viscosity is altered, it might be 

 supposed that light exerts its influence through the mechanism of the 

 bonds by which the elements of the plasmatic framework are joined. The 

 more the bonds are broken, the more fluid will the protoplasm become. 



It is also conceivable that more special processes are involved in the 

 mechanism. As mentioned earlier, Alsup (1942) found that calcium is 

 essential for the action of light. He therefore made an attempt to bring 



