360 GROWTH FACTORS AND FLOWERING 



the effects of gibberellin on this photoperiodic class, Xanthium pen- 

 sylvanicum being selected as the experimental species. The first experi- 

 ment, which will be reported on elsewhere, included an analysis of the 

 effects of gibberellin on the vegetative as well as the reproductive de- 

 velopment of Xanthium under long and short days. Only the data re- 

 lating to reproductive development will be considered here. 



The seeds were planted in a greenhouse on April 1, 1957, and the 

 plants were kept under long days (16.5 hr minimum) until 28 days 

 after planting, when 12 plants were treated with a \0~^M solution of 

 gibberellic acid in 0.25% Dreft, each plant receiving 1 ml of solution 

 in the form of drops applied to the leaves with a pipette. Twelve con- 

 trols were similarly treated with the 0.25% Dreft solution alone. Six 

 plants of each group were retained under long days, the other six be- 

 ing placed under short days of 8 hr in the greenhouse by means of an 

 automatic short-day device. The final data were taken 71 days after 

 treatment. 



In the second experiment the plants were similarly treated with 

 gibberellin after being retained under long days (18 hr minimum) for 

 28 days after planting (July 15). Five control plants and five treated 

 with gibberellin were retained under long days, while 25 of each were 

 placed under 8-hr days. After two long dark periods, 5 of these plants 

 from each group were returned to long days, the process being re- 

 peated on alternate days, providing groups of plants which had re- 

 ceived 2, 4, 6, 8, and 10 induction periods. The final data were taken 

 21 days after the beginning of the treatments. 



In the first experiment all plants under long days remained strictly 

 vegetative, while all plants under short days bloomed profusely at 

 about the same time. Although the gibberellin-treated plants averaged 

 54.8 burs per plant as compared with 50.7 for the controls, the differ- 

 ence was not statistically significant. The burs of the control plants 

 were slightly heavier than those of the treated plants, on both a fresh 

 and dry weight basis (Table I). 



In the second experiment, growth of the stems of the treated plants 

 was much greater than that of the controls regardless of photoperiodic 

 treatment (Table II). While the stems of the control plants given four 

 or more long nights were somewhat inhibited in growth, gibberellin ap- 

 parently overcame such photoperiodic inhibition. Contrary to the 



