342 GROWTH FACTORS AND FLOWERING 



The first observation was made in a cold-requiring plant, a biennial 

 strain of Hyoscyamus niger (Lang, 1956a,b). It soon was followed by 

 numerous similar observations in other cold-requiring and likewise 

 in long-day plants, all grown under strictly noninductive conditions. I 

 want to show just one example, Samolus parviflorus, a long-day plant 

 (Fig. 2), and then give a list of those long-day plants in which positive 



Fig. 2. Gibberellin-induced flower formation in Samolus parviflorus, a 

 long-day plant, grown on a 9-hr short-day. From left to right: controls; 

 plants treated with 1, 2, 5, 10, and 20 ftg of gibberellin daily. 



results have been reported (Table V). Of particular interest is a 

 study by Biinsow and Harder (1956a). These authors found that 

 gibberellin induces flowering in some long-short-day plants (two 

 species of Bryophyllum ) when grown on short day, that is, substitutes 

 for the long-day part of their induction. 



As in the case of auxin, after having established the effect of a 

 substance on the response in which we are interested, a number of 

 questions will promptly arise. The first one is, has this effect any 

 physiological meaning? The chemically known gibberellins are prod- 

 ucts of certain strains of a mold {Fiisarium monilijorme, sexual stage 

 Gibberella fujikuroi) and have not been found to occur in any other 

 organism.- Can we put any faith o nthe effects of such as extraneous 

 material? The most direct and conclusive way of answering this 

 question would be to see whether our plants contain some gibberellin- 



- Note added in proof: Recently, one of the Fusariuin gibbereUins, gibberellin 

 Ai, has been reported to occur also in bean seeds (J. Macmillan and P. J. Suter, 

 Naturwissenschaften, 45, 46, 1958). 



