390 



GROWTH FACTORS AND FLOWERING 



,4.0 X 10"' M Co+ + 



Control 



I I I 1 1— I 1)11 



2 4 6 8 10 12 14 



SECONDS OF RED LIGHT (ofter 4 hrs dorkness) 



Fig. 6. Red light interruptions of various duration applied 4 hr after the 

 beginning of a 16-hr dark period (July 11, 1957). Eight plants per treat- 

 ment, buds examined after 9 days. 



Some insight into this "protective effect" of the cobaltous ion against 

 light inhibition may be obtained from another kind of experiment. 

 Cobaltous-treated and untreated plants are given an inductive dark 

 period, and this dark period is interrupted at various times with a 

 saturating amount of light. The results of such an experiment are 

 shown in Fig. 7. The right half of the curves are approximately the 

 same as those obtained in the critical night experiments shown in Fig. 



H — y- — I — I— (- 



2 4 6 8 10 12 14 



HOURS AFTER BEGINNING OF THE DARK PERIOD 



16 



Fig. 7. The effects of 15 sec of red light interruption given at various 

 times during a 16-hr dark period upon flowering of control plants and 

 plants treated with 4.0 X IQ-^m C0CI2 (6 drops Tween 20/liter). August 

 8, 1957; 16 plants per treatment; buds examined after 9 days. 



