510 RADIATION BIOLOGY 



extreme forms by the counteracting effect of light. If the counteraction 

 of light is incomplete or absent, the dark reaction might pass unrecog- 

 nized in the plant or the animal. This might also be the result if the 

 control system, once adequately established, modifies the organ of per- 

 ception so as to ensure the continuance of the induced chain of reactions. 

 Indeterminate, or day-neutral, forms, which might be considered as 

 developing equally well in continuous light or ordinary natural cycles of 

 light and darkness, could in these or related ways still be controlled by 

 the same initial reaction as responsive forms, but pass unnoticed. Indeed 

 the great variation in presence and degree of physiological modification 

 in response to darkness is evidence of the universality of the response. 



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