284 CONTROL OF REPRODUCTION 



Repromotion of flowering occurred in response to the fourth applica- 

 tion of far red in such a series of alternating red and far-red treatments, 

 but it was not so strong in the final cycle as in earher ones. This is 

 attributed to the fact that performance of the experiment required 

 more than 45 min each day, and thus the plants were probably in the 

 red-irradiated condition long enough so that they began to escape 

 from control by the photoreaction. 



Effect of Prolonged Treatments with Far Red 



Another feature of the reaction shown by chrysanthemum is that 

 about 90 min of far red causes the same response as 2 or 3 min of red, 

 namely, failure of flowering. The far red does this, moreover, when 

 used alone or when immediately preceded by a brief inhibitory irradia- 

 tion with red. In the latter case one knows from results such as those 

 of Table III that during the first few minutes of the 90-min period of 

 irradiation with far red the flowering stimulus is reestablished only to 

 be redestroyed during the remainder of the period. 



The brief treatment with far red apparently changes most of the 

 far-red-absorbing form of the pigment to the red-absorbing form. If 

 the plant is then placed in darkness, the conversion of the remainder of 

 the far-red-absorbing form presumably occurs thermally. If the plant 

 is maintained in far red for as long as 90 min, however, the fractional 

 distribution of the pigment in the two forms probably remains the 

 same as at the end of 3 min and inhibition of flowering resuks from 

 the slow action throughout that time of the small amount of the far- 

 red form of the pigment. 



Of course, the actions of far red first to repromote and then to 

 reinhibit flowering might depend on the simultaneous functioning of 

 two photoreactions, one the reversible, red, far-red one and the other a 

 reaction such as that controlling anthocyanin production in red cab- 

 bage {Brassica oleracea) (Siegelman and Hendricks, 1957) and white 

 mustard (Sinapis alba) (Mohr, 1957). Experimental resuks thus far 

 do not indicate which of these two possible explanations is correct. 



Time Measurement in Photoperiodism 



Experimental results of the type described give much information 

 about the photoperiodic mechanism but leave many questions un- 



