1104 BIOLOGICAL EFFECTS OF RADIATION 



Willstatter and Mieg (85) found that the carotene content of leaves 

 varies with the season of the year; the greatest amoujit was found in 

 stinging nettle and horse-chestnut leaves during the flowering season of 

 both plants. The formation of carotene seems also to be influenced by 

 light. 



Parallelism between the rate of development of chlorophyll and of 

 xanthophyll in visible light was reported by Norris (49). He was able 

 to develop xanthophyll independently of carotene by controlling oxygen 

 tension, and he also found that minimum oxygen tensions are necessary 

 for the development of each pigment. 



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