576 RADIATION BIOLOGY 



between light and darkness accelerated streaming. Using Vallisneria as 

 the test object, Fitting demonstrated that the acceleration in proto- 

 plasmic streaming was caused only by a change from weak to stronger 

 light and not by a change from strong to weak illumination. He also 

 found that the change in illumination must take place relatively rapidly 

 in order for such an effect to take place, since no change could be noted 

 in protoplasmic streaming if the increase took place slowly. Because only 

 the illuminated parts reacted, he concluded that no impulse was con- 

 ducted in this process. 



Since protoplasmic streaming thus reacts only to a relatively rapid 

 change in illumination, Seifriz (1943) expressed the view that the effect 

 is due more to a shock effect than to the actual influence of light. He 

 stated that any shock, whether due to hght, heat, electricity, or mechani- 

 cal factors, would presumably accomplish the same result. He was 

 unable to show any stimulating effect of light on the streaming in 

 Myxomycetes. He therefore found it possible that tissues, which like 

 the Myxomycetes are commonly exposed to light, are not activated by it 

 and that tissues not naturally exposed to light, such as the inner cells of 

 stems, are more sensitive and respond by an increase in streaming. 



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