VISCOSITY, PERMEABILITY, PROTOPLASMIC STREAMING 



555 



some time before the experiment. An intensity of light of 0.5 m-c suf- 

 ficed to cause variations. 



Virgin (1947, 1948) also found the same reactions in Spirogyra and 

 Mnium. In Spirogyra, however, the time of centrifugation increased 

 when the objects were kept in the dark. It rose from 2 to 15 min in the 

 course of 4 days. There was a rapid decrease when the leaves were 

 exposed to light, and after about 3 hr the time dropped to 2 min. 



All the test objects studied, i.e., Elodea, Spirogyra, and Mnium, showed 

 that visible light causes changes in the viscosity of the protoplasm. It is 



Fig. 12-1. Ceutrifuged leaf cells of Elodea densa. One-half the area has Ween illumi- 

 nated, and the other half kept in darkness (covered with tin foil). In the illuminated 

 cells, and even in parts of the cells, the chloroplasts are moving and accumulating in 

 the distal end. (Courtesy of H. Virgin, 1950.) 



therefore possible to assume, as did Virgin (1948), that the phenomenon 

 is a general one and that it occurs presumably in all green plants. Diffi- 

 culties have nevertheless been encountered in confirming the accuracy of 

 this hypothesis. This is because the centrifugation method can be used 

 only when the test object fulfills certain criteria, so that only a few species 

 are suitable. Other methods, such as the plasmolytic, the gravity, and 

 the Brownian movement, either have too slow an action or record move- 

 ments of the particles that can be simultaneously affected by the micro- 

 scope light, since this has an effect on the state of the protoplasm. More- 

 over, the light emanated by the microscope is so strong that the viscosity 

 of the protoplasm rapidly reaches the paralytic stage. Owing to its con- 

 stancy and lack of sensitivity, this can easily give the impression that 

 the viscosity is unaffected by light. 



