218 THE FLOWERING PROCESS 



this requirement for 8 hr or more of light, something in addition to 

 photosynthesis must be involved. Furthermore, although the effect 

 is compHcated, the 8 hr requirement seems to be independent of 

 intensity. (Subsequent level of flowering is intensity dependent, 

 indicating a photosynthesis component.) One experiment has been 

 performed to determine the action spectrum for clock restarting. 

 Using Japanese morning glory, plants were given 10 hr darkness, 

 1 hr white hght (to inhibit flowering), 10 hr under various light 

 quaUties at equal intensities using the monochrometer installation at 

 Tubingen), and then 18 hr darkness. The results of this experiment 

 provide a typical phytochrome action spectrum for restarting the 

 clock! Red hght was most effective, orange nearly as eff'ective, 

 blue much less, and green and far-red ineff'ective. 



As in Chapter 6, we may conclude that photosynthesis is required 

 to provide substrates for the dark period reactions, but then photo- 

 synthesis is uhimately necessary for growth of the plant! In addition 

 an essential part of the High Intensity Light Process must be a 

 "winding" of the clock which will measure the length of the dark 

 period. This process requires presence of F-phytochrome for a time 

 interval of about 8 hr. This also agrees with the observations of 

 Konitz (p. 123), and it may be interpreted as being in agreement with 

 the theory of Biinning (pp. 133-6), which says that hght (F-phyto- 

 chrome) promotes during one part of the 24 hr cycle and inhibits 

 during the other. It may even provide some understanding of this 

 theory. 



Tubingen 

 May 25, 1963 



