38 



Photoperiodism: Attempts at Analysis 



period, as it is in Xanthium and millet (see Borthwick, 1959). Still 

 more complicated, yet confirmed now by the Beltsville group whose 

 theory it confounds, is the response of the Japanese morning glory, 

 Pharbitis nil. 



Fig. 3-3. Effect of far-red supplement at the end of the light period on the SDP 

 millet (Setaria italica). All plants were grown with 16 hours of light; at the end 

 of each light period the following treatments were given, represented by the 

 plants from left to right: no further radiation; five minutes of far-red; five 

 minutes of far-red followed by five minutes of red. (Photograph from Downs 

 [1959], by permission of the American Association for the Advancement of 

 Science, and courtesy of Drs. R. J. Downs and H. A. Borthwick, U. S. Depart- 

 ment of Agriculture.) 



Pharbitis seedlings grown at about 26° C can be induced to 

 flower by one or more 16-hour dark periods, and red light-breaks 

 (perceived by the cotyledons) 8 or 10 hours after the start of the 

 dark period completely inhibit induction. This is a typical SDP 

 response. But the effects of red light are not reversed by far-red; 

 far-red itself inhibits flowering when given during the dark period. 

 Far-red even inhibits when given at the start of the dark period, 

 and this effect is reversed by red. Thus the red, far-red reversible 

 system is present and active, but in a way unlike that suggested by 



