126 THE FLOWERING PROCESS 



3. F-Phytochrome in Flowering 



Two things seem clear: First, the final appearance of phytochrome 

 controlled responses such as flowering must depend on many 

 metabolic steps following the initial absorption of light by phyto- 

 chrome. This would help us to understand its ability to control so 

 many different physiological processes. It also seems quite likely 

 that many of the final manifestations of its action occur in response 

 to changes in growth regulators such as the auxins or the gibberellins. 



Second, phytochrome is a means tlirough which the plant detects 

 the presence or absence of light in the flowering process. Does 

 phytochrome act as an on-ofiT switch, so that when sufficient F-phyto- 

 chrome has disappeared the flowering timer begins to operate ? Or 

 is the dark conversion of phytochrome itself both the switch and the 

 timer ? The very success of the threshold experiment (Fig. 7-8) begins 

 to cast some doubt on how phytochrome can act as a switch at all. 

 An on-ofiF switch is an all-or-none piece of machinery, but the 

 threshold response is a quantitative, graded one. There are many 

 fascinating problems in this field, and some of them will be discussed 

 in the next chapter. 



