466 



PLANT PHOTOPERIODISM 



system since cell growth can be effected by a host of diverse chemical 

 agents ranging from inorganic nutrients, water supply and sugars, to 

 the more complex and subtle agents as the growth hormones. This 

 great sensitivity does not appear to be true of the clock mechanism. 



INTEGRATED ANALYSIS 



We will now attempt to put all these facts together in a diagram- 

 matic scheme (Fig. 14) of the relationship of the three principal 



ACTIVATION BY 

 BLUE VIA YELLOW 

 PIGMENTS 



MEDIATION BY 

 GIBBERELLIN. 

 AUXIN , KINETIN 



Fig. 14. Hypothetical scheme of the relationship between: (1) the 

 photoreaction site, (2) the photomorphogenic response system, and (3) 

 the photoperiodic response system. For details, see text. 



spheres of activity: (1) site of the photochemical reaction, (2) the 

 cell growth response of the photomorphogenic system, and (3) the 

 photoperiodic system. Action spectra evidence indicates that the pig- 

 ment system for induction has a single maximum at 660 m/x, with 

 relatively little absorption in the blue. The only native pigments in 

 the plant kingdom that have been observed to have this particular 

 characteristic are the straight chain tetrapyrroles in which the four 



