GROWTH REGULATORS AND COCKLEBUR FLOWERING 387 



3. In the third type of experiment, plants are induced with a single 

 dark period of varying lengths. Control plants in such an experiment 

 indicate the length of the critical night, and plants treated with growth 

 regulators indicate whether or not a given substance influences the 

 duration of the critical night. A number of experiments are summar- 

 ized in Fig. 3. Of the above-mentioned growth regulators, only the 



5.0-- 



4.0 -■ 





-2.0 + 



A. is a generalized curve 

 for the ouxins, 2,4-D, 

 dalapon, moleic hydroxide, 

 and 2,4-dinitrophenol. 



Control 



1.0 -■ 



/ 

 / 



/Cobaltous 

 / Ion 



4—1 \ \ 1 1- 



\^ 



I 



6 <> Q- 



10 



4 6 8 10 12 



Length of the Dork Period in Hours 



^^ — \ — I — I — h 



14 



16 



Fig. 3. The eflfect of cobaltous ion upon critical night. Points refer to 

 an experiment in which the inductive dark period was begun August 11, 

 1956 (Salisbury, 1957b). Curve A summarizes approximately 6 other 

 experiments. 



cobaltous ion increases the minimum length of the dark period required 

 for the first perceptible signs of flowering. This period may be in- 

 creased as much as 2 to 3 hr depending upon cobaltous ion concentra- 

 tion. All other compounds except gibberellic acid inhibit the degree of 

 flowering as measured 9 days after induction, but do not influence the 

 critical dark period. Experiments in progress indicate that gibberellic 

 acid promotes flowering somewhat with slight or no shortening effect 

 upon the critical night. 



FURTHER EXPERIMENTS WITH GROWTH REGULATORS 



Each of the growth regulators mentioned above might be further 

 investigated in relation to flowering. The general growth inhibitors 



