572 RADIATION BIOLOGY 



the tissue was unaffected by illumination. It was concluded that strong 

 osmotic agents destroy the sensitivity of the system to light. 



Hypotonic electrolyte solutions affected the photoreaction of the water 

 intake in a characteristic way. It could best be explained by the assump- 

 tion that the electrical diffusion potential between tissue and medium 

 plays a decisive role in the process. Comparing these results and their 

 previous experience with the photoelectric effect in organic membranes, 

 the authors found it most probable that the light-sensitive factor of the 

 water intake is its electroosmotic component. 



Thus it was necessary to attribute the surplus water intake observed 

 in illuminated samples to the appearance of an additional motive force, 

 rather than to an increase in the water permeability itself. At any rate, 

 the part played by the latter factor remained unsubstantiated. 



It has been shown that an antagonism is to be expected between the 

 electrostatic field effect of the membrane charge (primary photoeffect) 

 and the mechanical filter effect of the pores, depending on the degree of 

 hydration of the membrane substance (secondary photoeffect). A loss 

 in charge would facilitate the transport of water by reducing the electric 

 braking force of the membrane, but at the same time it would have a 

 delaying effect by a constriction of the pore diameter, owing to a decrease 

 in the swelling power of the micellar structure. 



If this view of the mechanism is correct, the velocity of the water flow 

 should be controlled mainly by the electrostatic factor when the inter- 

 micellar spaces are wide, but by the mechanical effect when they are 

 narrow. 



Brauner and Brauner (1943) also made an attempt to investigate this 

 important question. They kept the ion concentration uniform through- 

 out the system by using a homogeneous membrane (cellophane type) 

 instead of a living cellular structure. They thus avoided the develop- 

 ment of an electroosmotic component. The electric charge of the dia- 

 phragm was influenced by varying the pH of the medium and not by 

 irradiation, thus making it possible to investigate a much wider range of 

 charge conditions and, moreover, to test membranes insensitive to light 

 as well. Instead of osmosis, a fixed hydraulic pressure was used to force 

 the liquids through the membrane. 



In a second part of the investigation a study was also made of the 

 relation between the water permeability of living plant cells (storage 

 tissue of potato and red beet root and epidermal cells of Allium cepa) 

 and the pH of the ambient medium. 



It was concluded from these experiments that the water permeability 

 of plant cells in a state of normal hydration is regulated mainly by the 

 electrostatic valve effect of the micellar charge, and that the mechanical 

 filter control becomes operative only after artificial densification of the 

 protoplasm, e.g., after previous plasmolysis. 



