62 THE FLOWERING PROCESS 



these plants will not flower on long days alone, even when treated 

 with gibberellins, the effect could not be due to residual gibberellins 

 coming from the donor plant.* Furthermore, the receptor plant 

 could be used successfully as a donor in a second graft, and leaves 

 which had grown out after the induction treatment could also be 

 successfully used as donors. Thus it seems clear that in this plant 

 gibberellins are capable of substituting for long days, but that short 

 days are also required for elaboration of the flowering hormone. 

 Once made, the hormone will induce grafted receptor plants through 

 at least two graft "generations" (see Chapters 9 and 10 for further 

 discussion of grafting experiments). 



Gibberellins seem to be closely involved with the biochemistry of 

 flowering in cold-requiring and long-day plants, and we can await 

 many interesting developments in this field. At the same time it 

 should be clear that the entire story is rather complicated. Evidence 

 from extraction and reapplication experiments turns out to be 

 somewhat less straightforward than was originally imagined. 



3. Substance E. 



H. Harada and J. P. Nitsch (29) at the Phytotron at Gif-sur-Yvette, 

 near Paris, have obtained some very interesting results with extracts 

 of a number of cold-requiring and long-day plants. On their chroma- 

 tograms a peak of growth regulator activity which they called 

 substance E appeared beginning at the time of stem elongation in 

 bolting plants. Using a whole field of hollyhocks and a year of 

 laboratory work, enough of this substance was obtained for a 

 number of physiological experiments. It caused bolting and flowering 

 of long-day Rudbeckia speciosa and Nicotiana sylvestria plants, as 

 well as a cold-requiring variety of Japanese chrysanthemum. 



As in the case of the soak water extracts and gibberellins extracted 

 from vernalized plants, substance E is of interest because of its close 

 correlation with the vernalization process. It could well be that cold 

 treatment, for example, causes formation of substance E which then 

 causes bolting (is it thus identical with vernalin ?). 



Substance E is also interesting because chemical analysis shows 

 that it does not have the molecular structure common to GA-1 to 

 GA-9 (not enough material was obtained to completely identify it 



* H. Haranda at Gif-sur-Yvette near Paris has obtained very similar results 

 with cold-requiring varieties of Chrysanthemum morifolium. 



