KINETIC ANALYSIS OF PHOTOPERIODISM 469 



as to coincide with some suitable phase in the endogenous rhythm, 

 and the beginning of the next photocycle coincides with the opposite 

 phase of the rhythm after its suitable lag period, the stimulus and 

 clock systems are in phase and will proceed with a given type of 

 response; either the plant is vegetative, or it is thrown into the flower- 

 ing stage. A light flash in the middle of the dark period, however, or 

 an extension of the photoperiod through this phase so that the effective 

 end of the photoperiod occurs in some other phase of the endogenous 

 rhythm, introduces either the perturbations which have been observed 

 by Banning and Pittendrigh, or shifts the phase of the endogenous 

 rhythm completely. As yet we do not have sufficient information to 

 make any postulates as to the exact relationship between long- and 

 short-day plants. The important point here is that such a hypothesis 

 introduces the necessary extra parameters of lag-phase duration and 

 photophil phases of the rhythmic system to make it possible to explain 

 the existence of various classes of photoperiodic flowering responses 

 in plants. 



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