386 



GROWTH FACTORS AND FLOWERING 



hibit the synthesis of flowering hormone, since it is effective in floral 

 inhibition only when apphed before the dark period has ended. Co- 

 baltous ion inhibits flowering when it is apphed during the critical 

 night and thus probably influences this phase of the flowering process. 

 Recently, gibberellic acid has been tested in this type of experiment. 

 Figure 2 is typical of four such experiments with this compound. Al- 



35- 



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Days before and after a single 14 hour dark period. 



Fig. 2. The promotive effect of gibberellic acid upon flowering as a 

 function of its time of application. The vertical lines through the points 

 which represent the effects of gibberellic acid upon floral stage indicate 

 standard error. All treatments applied previous to 8 hr after the beginning 

 of the dark period except one are significantly above the control line, and 

 all treatments applied after this time do not differ significantly from the 

 control line. Plants were prepared and dipped in 1.0 X IQ-^M gibberellic 

 acid (6 drops Tween 20/liter) as explained in earlier publications 

 (Salisbury, 1957b; Salisbury and Bonner, 1956). Inductive dark period 

 was begun October 17, 1957. Ten plants per treatment; weather over- 

 cast. 



though the data are quite variable, it is clear that gibberellic acid 

 applied previous to the end of the critical night promotes flowering, 

 while this substance applied just previous to and following this time 

 has no significant effect upon flowering. It is interesting to note that 

 gibbereUic acid causes cocklebur stems to elongate regardless of when 

 it is applied in relation to the inductive dark period. 



