644 BIOLOGICAL EFFECTS OF RADIATION 



The measurement of protoplasmic viscosity has been discussed at 

 considerable length by Heilbrunn (136). Two types of measurement are 

 most in favor at the present time. With the aid of a centrifuge, proto- 

 plasmic granules are moved through the cell, and from the speed of this 

 movement it is possible by Stokes's law to estimate the protoplasmic 

 viscosity. It is a simple matter to compare the viscosity under various 

 conditions. Absolute viscosity determinations are less easy and less 

 certain. The rate of Brownian movement is also an index of protoplasmic 

 viscosity. Both methods of viscosity determination have their serious 

 limitations. In special cases, it may be possible that the viscosity 

 measured by the two methods may be different. Thus, following 

 vacuolization of a cell, granules within the vacuoles might show rapid 

 Brownian movement, whereas centrifuge tests might indicate high 

 viscosity. Finally, one may distinguish between the viscosity of the 

 clear hyaline protoplasm and the viscosity of the protoplasm as a whole. 

 The latter may be vastly greater, owing to the presence of a high con- 

 centration of granular material. 



VISIBLE LIGHT 



It is noteworthy that so few authors have studied the effect of visible 

 light on the colloidal properties of protoplasm ; especially is it surprising 

 that no more work has been done on plant material. Using Ranunculus 

 ficaria, Weber (387) compared the form and time of plasmolysis in cells 

 of leaves exposed to darkness and light, respectively. The "light" 

 leaves were in direct sunlight. From leaves in darkness plasmolysis is 

 "convex" and rapid, whereas from those in the light it is "concave" 

 and slow. Huber (166) also noted differences in the form of plasmolysis. 

 Weber has shown that convex plasmolysis is associated with relatively 

 low, and concave with high viscosity (Heilbrunn, 136) ; also that rapid 

 plasmolysis is an indication of low viscosity. Hence he concludes that 

 the viscosity is higher in the light than in darkness, but he is careful to 

 note that the effect may be an indirect one and may be the secondary 

 result of an action of light on other vital processes (e.g., transpiration or 

 photosynthesis). 



Gassul (114, 115) first showed that leucocytes and lymphocytes in 

 spleen explants cytolyse more rapidly in the light than in the dark and 

 accumulate lithium carmine stain at a greater rate. Similar studies by 

 Earle (86) on leucocytes and fibroblasts in tissue culture are of consider- 

 able interest. He used tungsten filament lamps (15 to 200 w.), and 

 studied the effect of these on the blood cells of cats, guinea pigs, and 

 rabbits, as well as on fibroblasts obtained from chick embryos. When 

 blood is illuminated, the leucocytes go through an interesting series of 

 changes. At first there is a great decrease in viscosity, which is indicated 

 by the "tremendously increased amplitude of Brownian movement of the 



