LIGHT AND THE PIGMENT 



117 



5-0 



4-0 



(o 3-0-- 



2-0 



l-O- 



2 4 ■ 6 8 10 12 14 16 



HOURS AFTER BEGINNING OF DARKNESS 



Figure 7-7 



The effects upon subsequent flowering of a 60-sec red light interruption 



given at various times during a 16-hr dark period to control plants and to 



plants previously treated with 3.0 x 10~^ m CoCU. Dark period initiated 



on January 16, 1960. Data previously unpublished. 



of a light flash must depend not so much upon the length of the dark 

 period preceding or following as upon the time when it is given in 

 relation to the beginning of the dark period. This would seem to 

 imply that some time-measuring mechanism is set into action when 

 the plants are placed in the dark, and that the effect of a light 

 interruption is dependent upon this time-measuring mechanism. If 

 this idea is correct, cobaltous ion must slow down this reaction. 

 Furthermore, a brief interruption of light must not reset the time- 

 measuring mechanism, otherwise we would be back where we were 

 to begin with. 



Actually, it is not even necessary to use cobaltous ion to obtain 

 results which are difficult to understand by the simple mechanism 

 proposed in the above section. For example, if a light interruption is 

 given after 8 hr in a 20-hr night, flowering is completely inhibited 

 even though the remaining 12 hr would normally cause maximum 

 flowering if they had not been preceded by the 8 hr and the light 

 flash (Fig. 8-1 in the next chapter illustrates this phenomenon with a 

 64-hr night). Is this just another demonstration of the requirement 

 for high intensity light preceding an inductive dark period (Chapter 6) ? 



