278 



CONTROL OF REPRODUCTION 



lengths. At 4000 A the effectiveness of radiant energy for soybean 

 was about 15% of that at 6500 A and for cocklebur 1 or 2%. 



Repromotion of Flowering by Far Red 



Ahhough the action of red on flowering has been reversed in all 

 four plants mentioned as well as in others (Borthwick et al, 1952; 

 Cathey and Borthwick, 1957; Downs, 1956), no detailed action 

 spectrum for the repromotion of flowering by far red is available. The 

 best information is from experiments with cocklebur (Borthwick et al., 

 1952). The cocklebur plants grew in long dark periods and short 

 photoperiods during the period of treatment. The dark periods except 

 for those of the unirradiated control plants were interrupted near the 

 middle with a brief red irradiance to inhibit flowering. These inhibited 

 plants were then given various brief irradiances with the spectrograph 

 at different wavelengths longer than about 6000 A. Results of such an 



35 



5600 6000 6400 6800 7200 7600 



WAVE LENGTH IN ANGSTROM UNITS 



8000 



Fig. 2. Action spectra for promotion (left) and inhibition (right) of 

 lettuce seed {Lactiica sativa var. Grand Rapids) germination and for 

 inhibition (left) and promotion (right) of floral initiation of cocklebur 

 {Xanthium pensylvanicum) . (From Hendricks and Borthwick, 1955.) 



