354 GROWTH FACTORS AND FLOWERING 



data, one may suspect that a short stimulus of red light may be favor- 

 able to flowering as well as a long irradiation period with far red, while 

 a long treatment with red may produce negative results, and flowering 

 may also fail by treatment with a brief flash of far red. 



In short, we may assume that the same irradiation may have a differ- 

 ent effect, or even an antithetic one, depending upon the length of 

 action or perhaps also on the photocyclic period in which it is applied. 

 Apart from the legitimacy of such an assumption, and neglecting for 

 the present the case of Hyoscyamiis, with which we have not yet ex- 

 perimented, we wished to learn whether there is an equal effect from 

 applying a short irradiation {V2 hr) in the middle of the night phase 

 and that of giving a prolonged irradiation of several hours (8 hr 

 natural light + 8 hr extension of colored light + 8 hr dark, or also 9 

 hr natural light + 15 hr colored light). The resuhs of the experiments 

 in this regard (including SDP) have not yet reached an advanced 

 stage; they may, however, facilitate our further comments with relation 

 to the action of GA. They have been carried out only on a few species, 

 namely, Centaiirea cyanus, Brassica napus, and Xanthium italicum. 



The results revealed that Centaurea cyanus and Brassica napus 

 behaved in practically the same manner toward both a long and a short 

 extension made up respectively with red and far-red light. Centaurea 

 flowered, say under a long red extension, and also on a short one, and 

 so did Brassica under f-rLD. As far as our LD tests are concerned, we 

 may consequently assume that three groups are to be distinguished, 

 namely: (a) typical red positive (Centaurea); (b) typical far-red 

 positive (Brassica); (c) red and far-red-positive (Mimulus). A fur- 

 ther category probably may be individuated in those plants, such as 

 Hyoscyamus, that flower under a long far-red extension and also on a 

 brief night breakage with red light, but not under a long red extension 

 or a brief far red. Experiments are to be pursued to demonstrate 

 whether these characteristics are really distinct or only relatively so. 



In Xanthium italicum it appeared that, while a flash of far red acts 

 positively on flowering, a prolonged far-red irradiation ( 8 hr natural 

 day + 8 hr fr + 8 hr dark) tends to act negatively. Indeed, the flower- 

 ing of individuals cultivated under f-rLD was visibly delayed in 

 respect to those cultivated under SD. If the treatment was limited to 

 one photocycle, only some of the SD individuals reached the flowering 



