366 GROWTH FACTORS AND FLOWERING 



periods, obtained by interrupting the dark period with at least 10 ft-c 

 of light from incandescent filament bulbs from 10 p.m. to 2 a.m. 

 nightly, to keep them vegetative. They were then subjected to short 

 (8-hr) photoperiods until anthesis. Five plants were used for each 

 treatment. 



The plants were sprayed with aqueous mixtures of gibberellin and 

 Amo-1618. The mixtures were prepared by dissolving the required 

 amount of the chemical in as little ethanol as would dissolve it, adding 

 sufficient Tween 20 to produce a 0. 1 % solution, and then diluting the 

 mixture to the desired volume with distilled water. The aqueous solu- 

 tions were held at 10°. The immature part of the plant was sprayed 

 with a hand atomizer until the excess solution began to run off. Ap- 

 proximately 0.5 ml of solution was required per plant. 



The data collected at the time of anthesis included length of stem 

 and number of lateral inflorescences. 



REACTION OF PLANTS ON SHORT PHOTOPERIODS 



Sensitivity to Gibberellin 



Indianapolis Yellow chrysanthemum, which requires 10 weeks of 

 short photoperiods to induce flowering, was tested for reaction to gib- 

 berelHn. Short photoperiods were started on December 3, 1956. Cer- 

 tain plants were sprayed with aqueous solutions containing 100 ppm 

 of gibberellin on 5 consecutive days after beginning of the short photo- 

 periods as follows: 



As shown, the stems of Indianapolis Yellow plants elongated most 

 when treated during the third week of short photoperiods, which was 

 after initiation of the inflorescences but before extensive floret develop- 



