GROWTH REGULATORS AND COCKLEBUR FLOWERING 389 



red light SVi hr (or 4 to SVi hr) after the beginning of a 16-hr indue- 

 tive dark period. If plants are treated instead with saturating or even 

 subsaturating amounts of light near the middle of a 16-hr inductive 

 dark period, they remain vegetative, and thus the saturating amount of 

 light cannot be determined. After SVi hr plants treated with a saturat- 

 ing amount of light still flower to some extent. Such an experiment is 

 shown in Fig. 5. Approximately 7 sec of light was sufficient to cause 

 the maximum degree of floral inhibition of control plants (curve A). 

 Plants treated with cobaltous ion at the beginning of the inductive 

 dark period and illuminated with the control plants are inhibited in 



3 5 7 10 15 



Seconds of Light Interruplion (5 1/2 hrs) 



Fig. 5. The effects of various durations of red light applied SVi hr after 

 the beginning of a 16-hr dark period (Oct. 3, 1957). Treated plants were 

 dipped" in 5.0 X IQ-'M CoClo (6 drops Tween 20 liter) just previous to 

 induction. Twelve plants per treatment, buds examined after 9 days. 



their flowering by the cobalt and also by the light interruption, but it 

 can be seen from curve B of Fig. 5, that light is less effective in in- 

 hibition of cobaltous-treated plants than untreated control plants. The 

 amount of light required to bring about the maximum inhibitory effect 

 has been quite difficult to determine, but two out of six experiments 

 seem to indicate that the amount of light required to saturate the 

 process is approximately the same whether the plants have been treated 

 with cobaltous ion or not (Fig. 6, for example) . The other four experi- 

 ments are inconclusive or perhaps suggest that more light is required 

 for cobaltous treated plants than untreated controls. 



