376 GROWTH FACTORS AND FLOWERING 



obligatory long-day requirement for flowering was significantly hast- 

 ened under long days, but did not occur in short days even though 

 gibberellin was applied repeatedly. Stem elongation was promoted 

 after each treatment of gibberellin followed by the formation of an 

 aerial rosette between each application of gibberellin. Spinacia oleracea 

 Linn. (Fig. 1), Chrysanthemum coccineum Willd., and Cichorium 

 endivia Linn., obligatory long-day plants, flowered under short days 

 when treated with gibberelhn (Table I) but only after a greater num- 

 ber of days than was required in a long photoperiod. 



Flowering was also induced with gibberelhn in Rudbeckia hirta 

 Linn, grown under short days. In this instance, however, stem elonga- 

 tion did not precede flower formation; and only single sessile flowers 

 were formed (Fig. 1), reminiscent of Murneek's observations (1937) 

 of flowering in Rudbeckia when grown near the critical photoperiod. 

 By contrast, gibberellin applied to Rudbeckia grown under long days 

 resulted in a "burning-out" of the growing tips, elongated leaves, and 

 greatly retarded flowering. 



Thus flowering was accelerated by gibberellin in facultative long- 

 day plants grown under both long and short photoperiods. In plants 

 having an obligatory long-day requirement, and grown under short 

 days, flowering was induced in some but not all species. Acceleration 

 of flowering by gibberellin of obligate long-day plants in long days was 

 generally less pronounced than with facultative long-day plants. 



Short-Day Plants 



Of the short-day plants, only with Cosmos bipinnatus Cav. has gib- 

 berellin accelerated flowering in long days. While at least two papers in 

 this symposium (Greulach, p. 359; Salisbury, p. 381) have reported 

 an acceleration of flowering in Xanthium pensylvanicum Wallr. grown 

 under an inductive photoperiod (short days), there was no response in 

 long days. 



Details of the flowering of Cosmos under short and long days as 

 modified by gibberellin are given in Tables II and III. Under a short 

 photoperiod all plants flowered simultaneously, and gibberellin had no 

 significant effect (Table II). At an 18-hr day length, however, gibber- 

 ellin treatment hastened the appearance of visible flower buds — all 

 plants flowered as early as those under a short photoperiod. By con- 



