378 GROWTH FACTORS AND FLOWERING 



visibly vegetative were included in subsequent treatments. Flowering 

 in Cosmos which were treated with gibberellin after the formation of 

 5 to 6 nodes, was accelerated as compared to no gibberellin, to a much 

 greater extent than when sprayed at 2 to 3 nodes. It remains to be 

 seen if flowering can be induced under long photoperiods in varieties 

 of Cosmos bipinnatus, or in C. sulphureus Cav., which may have oblig- 

 atory short-day requirements. 



Two observations suggest that much work yet lies ahead before an 

 accurate picture of the effects of gibberellin on the photoperiodic re- 

 sponses of short-day plants may be framed. These are the modifying 

 effects of gibberellin on the flowering of Cosmos bipinnatus under a 

 noninductive photoperiod, and the fact that only with a few classical 

 short-day plants (Lang, 1957; Harder and BUnsow, 1957) has the 

 response to gibberellin been subject to critical evaluation. 



Stem Elongation and Flowering 



In many long-day plants and in short-day Cosmos, stem elongation 

 and flowering are induced and proceed simultaneously after photo- 

 periodic induction. Similarly, both occur when gibberellin is used to 

 replace the light requirement. With gibberellin, however, extensive 

 stem elongation usually, but not always (see Rudbeckia, Fig. 1), pre- 

 cedes flower bud formation. Thus gibberellin provides a convenient 

 means of separating the normally associated phenomena of stem elon- 

 gation and flowering. 



Pronounced stem elongation precedes flower initiation in gibbereflin- 

 treated plants, and accompanies flower bud development following 

 photoperiodic induction. This suggests that the developing flower buds 

 may form "gibberellin-like" substances capable of inducing both stem 

 elongation and flowering. That this may be true has now been con- 

 firmed by reports in this symposium, wherein extracts from developing 

 flower buds of Brassica rapa Linn. (Lona, p. 351) and immature seeds 

 of Echinocystis macrocarpa Greene (Lang, p. 329) have broken the 

 rosette habit of growth, and induced both stem elongation and flower- 

 ing in certain biennials and long-day annuals. These reports recall 

 those published by Wittwer (1943) and Mitchefl et al. (1951 ) where 

 crude extracts from developing plant reproductive organs induced stem 



