PHOTOTHERMAL AND CHEMICAL CONTROL 355 



Stage. Such behavior on the part of a SDP toward far-red extension 

 seems very remarkable and may correspond, in an inverse sense, to 

 that of some LDP. 



We cannot consider now the mechanism of such an effect of a far- 

 red overdosage. The datum may help us, however, to understand better 

 several aspects of GA action on flowering as discussed below. 



In the meantime we shall comment on some results concerning shoot 

 lengthening. From these last experiments, we could clearly observe 

 that plants generally grow tall under both prolonged and very short 

 periods of far-red supplementary irradiation, whereas they are kept in a 

 dwarfed condition, as in SD ( ! ) , even by a short flash of red light given 

 in the middle of the nictophase. This is a characteristic response, 

 especially of the caulescent type. However, in some rosette plants the 

 rLD seems to act somewhat positively in shoot lengthening, even 

 before the eventual formation of flowering structures. Very frequently 

 after flowering, a new condition favorable to shoot lengthening ap- 

 pears. This latter may then be rather independent from photoperiod, as 

 lengthening may continue also in SD. This argument is developed 

 somewhat elsewhere (Lona, 1957b). 



The effect of red and far-red irradiations on growth phenomena 

 appears therefore to be, in several cases, distinct from their effect on 

 flowering processes. It clearly appears that growth response of plants 

 to light conditions (quality and dosage) is more uniform than flower- 

 ing response. It is interesting to note that Xanthium flowers in condi- 

 tions of growth inhibition (SD) and under conditions very favorable 

 to shoot lengthening (f-rLD). 



In view of afl the foregoing results we may now consider the action 

 of GA observed in parallel or distinct experiments. 



The trials with GA showed that our annual strain of Brassica napus 

 characteristically sensitive to far-red extension (a typical f-rLD) was 

 also sensitive to GA both for growth (shoot lengthening) and flower- 

 ing manifestations. GA had a general far-red-like effect on this crucifer. 

 In Centaurea cyanus, a typical rLDP, only shoot lengthening was 

 stimulated by GA, its effect being similar to that of far-red extension; 

 flowering was not qualitatively promoted. So, this rLDP it not GA- 

 positive for flowering, at least at the young stage of the individuals 

 considered in our trials. Also in this case, however, GA had a general 



