KINETIC ANALYSIS OF PHOTOPERIODISM 



463 



the reversal phenomenon. The interesting feature here is that reversal 

 or inhibition of the induction response is directly proportional to 

 energy, instead of proportional to the logarithm of the energy, and 

 saturates at about 90% of complete reversal. One never seems to get 

 complete reversal, which implies that the processes are so overlapping 

 that complete spectral separation of the two cannot be attained. 



THE CLOCK 



Before presenting any integrated hypothesis of how these various 

 phenomena are related to one another, it is important to consider 



4 6 



RED ENERGY 



8 10 12 



(seconds of exposure) 



Fig. 10. Energy-response function for a photoperiodic system. The 

 flowering response of a short-day plant, Xanthiiim, is shown when main- 

 tained on 8- and 12-hr photoperiods. A red exposure of the duration indi- 

 cated was interposed in the middle of the dark period. The incident energy 

 was varied by varying time; the irradiance was constant. The floral stage 

 was observed by microscopic examination of the terminal bud and evalua- 

 tion of its stage of floral development. Graph plotted from data of Salis- 

 bury and Bonner (1956). 



certain essential characteristics of the clock. We have shown that the 

 time precision required for the perception of seasonal time with an 

 accuracy of one month requires a clock with an accuracy of the order 

 of 10%. If the organism is exposed to a range of temperatures of 



