318 



CONTROL OF REPRODUCTION 



theory of the photoperiodic control of floral initiation based on a sin- 

 gle photosensitized reaction controlling the synthesis or destruction of 

 the floral stimulus, or precursor (as proposed in Borthwick et al., 

 1948a; Bakhuyzen, 1951) is not applicable to the LSDP and SLDP. 

 The sequence of photoperiodic requirements in the dual day length- 

 requiring plants is obligatory, that is, the order of LD and SD induc- 

 tion cannot be reversed, and the following argument can be made. 



Let the assumption be made that SD induction in the LSDP is identical 

 with that in the SLDP, i.e., SDi = SD2. Then, since LDi precedes and 

 LD2 follows SD induction, LDi cannot be identical with LD2. There- 

 fore, LD induction in the LSDP does not control the same step in the 

 synthesis of the floral stimulus as it does in the SLDP. If it is assumed 

 that LDi = LD2, SDi cannot be identical with SD2, and one can show 

 that SD induction in the LSDP does not affect the same reaction as that 

 in the SLDP. On the basis of our present criteria for classifying photo- 

 periodic reactions, LD induction in the LSDP is indistinguishable from 

 that in the SLDP, and the same is true for SD induction in these two 

 groups. It can be concluded then that our criteria must be augmented, 

 and we should be wary of the assumption that LD induction affects 

 the same stage of synthesis of the floral stimulus in every LDP (the 

 same doubt exists with regard to SD induction in all SDP). 



When the chemical identity of the floral stimulus is known, an in- 

 vestigation of the intermediary metabolism involved in its synthesis will 

 be feasible, and the two assumptions made at the beginning of the dis- 

 cussion, namely, that the floral stimulus and the pathway to its synthe- 

 sis are common to all plants, can be tested. Until that time comes there 

 is a need for more information about the characteristics of photo- 

 periodic induction among plants of the same category. Although the 

 dual day length-requiring plants have not contributed to a better un- 

 derstanding of photoperiodism and floral initiation, they present a 



