INFLUENCE OF GIBBERELLIN AND AUXIN 345 



gibberellin in flower formation, specifically, in the photoperiodic 

 control of this process? Let us approach this problem step by step and 

 first examine the effect of gibberellin on flower formation in short-day 

 plants. Not nearly as many species have been examined as in the case 

 of cold-requiring and long-day plants, but in those obhgate short-day 

 species that were examined (Kalanchoe blossfeldiana: Harder and 

 Bunsow, 1956, 1957; Xanthium and Biloxi soybean: Lang, 1957; 

 Perilla ocymoides: Lona, 1956a) gibberellin did not induce flower 

 formation under long-day conditions. Dr. Wittwer found promotion 

 of flowering in a facultative or quantitative short-day plant, Cosmos 

 (later in this part), and some authors report promotive effects of 

 gibberellin treatment on short-day induction in various short-day 

 induction in Xanthium (Lincoln and Hamner, 1958; Greulach and 

 Haesloop, 1958), but gibberellin does not seem to substitute for photo- 

 induction in this class of plants, nor, very significantly, for the short- 

 day part of induction in the long-short-day plants studied by Bunsow 

 and Harder (see Harder and Bunsow, 1957). On the other hand, all 

 physiological evidence that we have (from grafting experiments) sug- 

 gests that the floral stimuli of long- and short-day plants are identical 

 (Lang, 1952; Zeevart, 1958; Lang and Khudairi, unpublished data). 

 We must therefore conclude that gibbereUin is not the "photoperiodic" 

 floral stimulis, nor a substitute for this stimulus. 



Another conclusion can be drawn by taking a closer look at the 

 gibberellin response in long-day plants. Two things can be said in this 

 connection. The great majority of rosette type long-day plants to 

 which gibbereUin has been applied, has responded with flower forma- 

 tion. A few negative results have been reported (Lona, 1956b; Lona 

 and Bocchi, 1957; see also Lona's report in the next paper); how- 

 ever, I am not sure whether by extending and intensifying the treat- 

 ment positive responses could not be obtained at least in some of these 

 plants. However, the degree of response varies very much from one 

 long-day plant to the other. This can be seen by comparing Fig. 2, 

 Samoliis, with Fig. 4, Silene armeria. Silene needs considerably more 

 gibberellin and more time, and makes considerably more stem growth 

 before it responds with flower formation. But in their photoperiodic 

 sensitivity the two plants do not differ very much. This situation 

 indicates that gibberellin is only one of several factors in the photo- 



