ECOLOGY AND THE FLOWERING PROCESS 33 



instantly, the rate of decrease of F-phytochrome will be rapid; in a 

 typical twilight it might be less rapid. When the level is low enough 

 the dark processes will begin in the leaf, and thus the plant will 

 "know" it is in the dark. Even this could be gradual: the processes 

 beginning gradually as F-phytochrome gradually decreases. 

 Obviously two factors will determine the time when the dark phase 

 of flowering will begin : first, the quantity of F-phytochrome as a 

 function of light and metabolism; and second the sensitivity of the 

 flowering process to F-phytochrome. Plants might well differ in this 

 sensitivity. Some could initiate the dark phases of the flowering 

 process when the level of F-phytochrome has decreased by a relatively 

 small amount, and others might require almost complete removal of 

 F-phytochrome for initiation of the dark processes. As mentioned 

 above (Fig. 3-3) these responses to phytochrome often exhibit a 

 logarithmic relationship to light intensity. 



In the morning the situation must be exactly reversed. As light 

 gradually builds up in intensity, production of F-phytochrome will 

 gradually increase until light formation is greater than metabolic 

 removal, and a net amount of F-phytochrome begins to accumulate. 

 When the level is high enough, the dark phases of the flowering 

 process will be stopped. Of course, it is quite possible that the 

 biochemistry of stopping is quite diff'erent from that of starting. 



It appears from the above discussion that there are two times that 

 could be noted during twilight : the time at which the net amount of 

 F-phytochrome begins to change significantly in response to change 

 in light intensity, and the time at which the quantities of F-phyto- 

 chrome become critical in the flowering process. In theory we could 

 state the first time exactly, but the second time could extend over the 

 period during which the processes of flowering gradually stop or 

 start in response to gradual changes in quantity of F-phytochrome. 

 So far the state of our science has not advanced far enough to study 

 either of these times according to the theoretical treatment given above. 



A more practical experimental approach has been to transfer 

 plants to complete darkness at various times during evening twilight, 

 and thus to learn how dim the light intensity must be before plants 

 left in the twilight flower as much as plants placed in total darkness. 

 Morning twilight may be studied by reversing the procedure ; plants 

 which were all placed in darkness at the same time the previous 

 evening may be removed from the darkness and placed in morning 



