104 



THE FLOWERING PROCESS 



400 



500 



600 



700 



800 



U.V. 



Violet I Blue | Green |Yellow| Orange | Red Far-red llnfro-re d 



WAVE LENGTH IN MILLIMICRONS 



Figure 7-3 



Generalized action spectra expressed on a scale of relative effectiveness, 

 and absorption spectra of the two forms of phytochrome as measured 

 primarily in vitro. The broad peaks for the action spectra are meant to 

 imply that various plant systems might have sharper peaks within the 

 indicated broad range. Difference in such sharp peaks may be ascribed 

 to side effects such as chlorophyll screening. Absorption data from the 

 Australian symposium paper of Hendricks and Borthwick (19). 



3. Control by the Pigment of Responses Other Than Flowering 



(7, 8, 10, 18) 

 A striking thing about the Beltsville experiments is that the action 

 spectra (Fig. 7-2) for short-day plants (an inhibition of flowering) 

 is very similar to the action spectra for long-day plants (a promotion). 

 Orange-red light is most eflfective in either case. Actually, by the 

 time the curve was obtained, it was already possible to recognize its 

 familiar shape. It is evident in Fig. 7-2 that the flowering curves are 

 very similar to curves showing the promotion by light of lettuce seed 

 germination, and of dark grown pea leaf elongation, which were 

 obtained in much the same way with the same instrument. The curves 

 also closely resemble those for the inhibition of stem elongation in 

 the dark (etiolation), the straightening of a hook at the end of a bean 



