468 



PLANT PHOTOPERIODISM 



of Lang (p. 329) certainly indicates that gibberellin is not a substitute 

 for the photoreaction. Auxin has little influence on these processes 

 unless extremely high levels are used. This is true of most other 

 chemical factors such as inorganic and organic nutrient substances. 



As has been mentioned previously, the clock itself is also a highly 

 refractory system. It is the close coupling of these two refractory sys- 

 tems which makes possible the phenomenon of photoperiodism. In 

 the environment, apparently day length and temperature are the only 

 two factors that have the key for setting the clock. On the basis of 

 this assumption, the diagram shows two-way arrows coupling the 

 clock to the photoperiodic system. The work of Biinning (p. 507) has 

 aptly demonstrated that photoperiod does not control the period of 

 the clock. It merely controls the phase, and it is the phase relation- 

 ships between the photoperiodic stimuli and the endogenous rhythmic 

 system which appear to be the controlling factor in photoperiodism. 

 This type of hypothesis has been presented many times by Biinning 

 and his associates. 



A hypothesis of the relationship between the photoperiodic stimulus 



PHOTOPERIODIC REGIMEN 



I DARK I LIGHT I ^l 



r~ 12 hr^T^ 12 hr-^ ( 



\/ PHOTOPHIL 



t 



r 



SCOTOPHIL 



_1_ 



ENDOGENOUS RHYTHM 



Fig. 15. Hypothetical relation of the photoperiodic stimulus to the endog- 

 enous rhythm. A. 12:12 hr dark-light cycle and a light-break (LB) are 

 diagrammatically shown in relation to a 24-hr endogenous rhythm. See 

 text for explanation. 



and the endogenous rhythm is presented in Fig. 15. There is evidence 

 that a lag period exists between the time of starting or stopping the 

 photoreaction and the response of the endogenous rhythmic system 

 (Biinning, p. 507). If the end of a light period occurs at such a point 



