VISCOSITY, PERMEABILITY, PROTOPLASMIC STREAMING 571 



Thus Brauner and Brauner (1940) referred to the main principles of 

 photochemistry. According to these, the primary effect of light on 

 photosensitive systems consists in a separation of electrons from the 

 absorbing surface, which in turn implies a loss of negative charge by 

 the irradiated body. If the system, like the membranes in the afore- 

 mentioned experiments, is originally negatively charged, then this loss 

 must mean a more or less far-reaching discharge of the pore structure. 

 This in its turn would be liable to affect the passage of water and solutes 

 in two ways, differing in principle: 



1. Since a discharge of the micelles building up the boundary layers 

 must lead to a reduction in their mutual electric repulsion, their cohe- 

 sion forces will prevail and cause a contraction of the illuminated struc- 

 ture and a narrowing of the pores and intermicellar spaces; or this con- 

 striction of the intermicellar spaces will follow as a result of a reduced 

 degree of swelling in the membraj\es caused by the discharge of the 

 micelles and the ensuing loss of hydration. 



2. The behavior of water cannot, however, be explained by this con- 

 ception. For the permeability of water three different possibilities must 

 be taken into account: (a) an increase in the osmotic capacity of the 

 tissue or a decrease in the wall pressure; (b) a change in the friction to 

 which the water threads are subject when moving through the pores of 

 the membrane. In an illuminated system the friction could be expected 

 to increase as a result of the reasons given under paragraph 1, but this 

 effect would possibly be compensated by the decrease in the electrostatic 

 braking forces of the pore walls after their partial discharge; (c) the par- 

 ticipation of an electroosmotic process in the meaning of Loeb (1920a,b. 

 1921). Such an additional force would interfere with the normal osmotic 

 suction force of the tissue and, according to its direction, accelerate or 

 retard the velocity of the water flow. Since, according to the afore- 

 mentioned experiments on Elodea and model systems, preexisting mem- 

 brane potentials are affected by illumination, if the diaphragm is light- 

 sensitive, light may influence the water intake indirectly by modifying 

 the electroosmotic component of the tissue. 



In order to permit a decision between the different possibiHties dis- 

 cussed in the foregoing, Brauner and Brauner (1940) made special experi- 

 ments on root tissue of Daucus carota. Using a gravimetric method, they 

 investigated the water intake and output in light and darkness, in dis- 

 tilled water, in hypertonic and hypotonic solutions, and during different 

 time intervals. 



In distilled water the photoreaction appeared as an increase in the rate 

 of water intake, its extent varying with the exposure time. The magni- 

 tude of the reaction reached its maximum after about 30 min (Hght inten- 

 sity 36 X 10""^ cal/cmVsec; initial temperature 25°C). 



In hypertonic sugar solutions (0.5 and 0.7 AI glucose) the water loss of 



