PHOTOPERIODIC CONTROL OF FLOWERING 279 



experiment are shown on an action-spectrum curve for the inhibition 

 of the germination of seed of lettuce {Lactuca sativa L. var. Grand 

 Rapids) (Hendricks and Borthwick, 1955). The various points ob- 

 tained for repromotion of flowering in cocklebur agree very well with 

 the curve for inhibition of seed germination (Fig. 2) . 



Red, Far-Red Reaction in Chrysanthemum 



The discovery of reversibility in the controlling reaction of flowering 

 immediately opened the way to experiments that deal with other 

 features of this photoreaction and of the responses it controls. Such 

 studies have been made with a number of different kinds of plants, but 

 illustrations here are drawn largely from recent work with chrysan- 

 themum {Chrysanthemum morijoUum Ramat) (Cathey and Borth- 

 wick, 1957). 



Young chrysanthemum plants grown from cuttings recently rooted 

 and under photoperiodic conditions inhibitory to flowering were given 

 an inductive treatment consisting of 8 cycles of 9 hr of high-intensity 

 light alternating with 15 hr of darkness. Irradiation treatments were 

 given in the form of brief dark-period interruptions during these 8 

 cycles. A post-inductive period of 6 more daily cycles of 9 hr of light 

 gave time for development of flower primordia initiated by the induc- 

 tion treatment. During this 6-day period 4 hr of incandescent-filament 

 light was given in the middle of each dark period to make sure that 

 further induction did not occur. The plants were dissected on the 

 fourteenth day after the start of induction. The results were recorded 

 as coded numbers identifying the stage of floral development of each 

 plant; zero represented complete vegetativeness and 10 indicated that 

 perianth primordia were present on all florets. 



Experiments were performed to test the effectiveness of different 

 energy levels of red light on the inhibition of flowering of chrysan- 

 themums, the light source being a group of 8-foot slimline fluorescent 

 lamps equipped with a red filter consisting of two layers of red cello- 

 phane. In a typical experiment (Table II) the terminals of each of the 

 unirradiated control plants, which received 8 daily photoperiods of 

 9 hr, enlarged to form a globular structure, the receptacle of the 

 inflorescence, that had either no evidence of floret primordia (stage 

 4) or 1 to 3 rov/s of them around its lower part (stage 5). In both of 



