INFLUENCE OF GIBBERELLIN AND AUXIN 341 



vegetative growth. Some other cases have been reported in which 

 auxin seems to promote the manifestation of induction, but lack of 

 time makes it impossible to discuss them here. 



I would, however, like to mention briefly that, although relatively 

 high auxin levels are unfavorable to photoinduction — in short-day 

 plants for certain, and perhaps in long-day plants as well — effective 

 induction may possibly depend on the presence of a certain, minimum 

 auxin level in the plant. De Zeeuw and Leopold (1955) found that 

 young Brussels Sprouts plants, which were not yet capable of respond- 

 ing to floral induction, could be made to do so by auxin application. 

 It is conceivable that plants too young to flower have too low an auxin 

 level, and that at least one of the physiological meanings of the "ripe- 

 ness-to-flower" state of Klebs is the attainment of a certain auxin 

 level in the plant. Brussels Sprouts requires thermoinduction (a period 

 of low temperature), and I do not know of similar experiments with 

 long- and short-day plants; such experiments, however, might prove 

 very interesting. 



In addition to auxin effects on photoinduction, we thus have post- 

 inductive effects of auxin in flower formation, and we may have pre- 

 inductive effects. These effects may be opposite; that is. some may 

 promote the visible response, others may inhibit it. In different species, 

 these various effects may be quantitatively different, and this may 

 account for differences in the auxin responses which we may encounter 

 in different plants. We must examine very carefully with what kind of 

 auxin effect on flower formation we may be dealing, before we try to 

 classify and to generalize it. 



GIBBERELLIN 



With gibberellin there was more direct reason to check for effects 

 on flower formation than there was with auxin. Most cold-requiring 

 and long-day plants, when kept in a noninductive environment, grow 

 in the habit of rosettes, without any elongate stem. Gibberellin was 

 discovered as a promoter of stem elongation (Yabuta and Hayashi, 

 1939; see also review by Stowe and Yamaki, 1957). It was obviously 

 desirable to see whether this substance would also cause stem forma- 

 tion in a stemless plant, and whether this might result in flowering. It 

 soon was found that the one and the other did occur. 



