DUAL DAY LENGTH REQUIREMENTS 319 



challenge to our present concepts. With regard to the synthesis of the 

 floral stimulus they demonstrate that there may be more than one 

 photosensitized reaction involved in the same plant (though the photo- 

 receptor may be concerned in the different reactions). Perhaps even 

 more important, they show that our criteria for classifying plants as 

 LDP or SDP do not permit us to conclude that LD or SD induction 

 controls the same step in the synthesis of the floral stimulus in all cases. 



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