Photoperiodism and Light Quality • 37 



Certain data on flowering further suggested that this dark conver- 

 sion might determine the length of the critical dark period. Borth- 

 wick et al. (1952b) reported that if Xanthium plants were given a 

 brief far-red exposure at the beginning of a dark period (end of 

 the high-intensity white light), less than 7 hours of darkness were 

 required for induction. If they received a brief red treatment 

 instead, 9 hours of darkness were required, compared with the 

 Sy 2 sufficient with no treatment after the white light. Downs (1959) 

 has also shown that the quantitative SDP millet, Setaria italica, 

 which flowers rapidly with 12-hour nights but very slowly with 

 8-hour nights, will also flower rapidly with 8-hour nights if a brief 

 far-red treatment is given at the beginning of each. This far-red 

 promotion of flowering is reversed by red, and red alone has no 

 effect at the start of the dark periods. (See Fig. 3-3.) 



At this point one may well wish for the solace of a theory 

 unifying all these data. Such a theory exists (see Borthwick, Hen- 

 dricks, and Parker, 1956) and can be briefly summarized. At the 

 end of a long white-light period, the pigment is almost completely 

 in the far-red-absorbing form; evidence for this is that red given 

 then has little or no effect, and far-red a much larger one. It is 

 this far-red-absorbing form that brings about the inhibition of 

 induction in SDP and the promotion of induction in LDP. Thus 

 SDP require a dark period long enough to allow thermal conver- 

 sion of the far-red-absorbing form and its continued absence for 

 some time, whereas LDP are inhibited by too long a dark period 

 since this conversion and absence are unfavorable. Hence red (or 

 white) light-breaks inhibit SDP induction and promote LDP 

 induction by returning the pigment to the far-red-absorbing form. 

 This theory takes into account all the data so far presented, and 

 even fits the observation (Chapter Two) that the dark period for 

 Xanthium has to be longer if the temperature is lowered, since 

 thermal conversion to the red-absorbing form will be slowed. The 

 only difficulty is that it does not fit the equally valid data to be 

 considered next. 



According to the theory, far-red given to LDP at the start of a 

 dark period barely short enough to allow induction should inhibit 

 induction. Yet in at least two LDP, Hyoscyamus and dill, it pro- 

 moted induction. Also, flowering in the SDP Chrysanthemum 

 morifolium is not promoted by far-red at the start of the dark 



