124 THE FLOWERING PROCESS 



5 (Fig. 5-6), incandescent light is a mixture of red and far-red (al- 

 though the red effect predominates in flowering) while fluorescent light 

 contains red but virtually no far-red. Thus it appears that long-day 

 plants, at least, require a mixture of the two forms of phytochrome 

 for optimal flowering. It is clear that extension of the day-length 

 depends upon red light (F-phytochrome). What is the requirement 

 for far-red (R-phytochrome) ? It is known that far-red light causes 

 the elongation of stems, and it is also known that stem elongation 

 accompanies the flowering of all the long-day plants used in the above 

 studies. As a matter of fact, it appears that in some species flowering 

 may be caused by anything which will cause the stem to elongate 

 (e.g. gibberellins). Thus it seems that flowering of long-day plants 

 requires F-phytochrome during a long period so that the flower 

 initiating processes may take place, but R-phytochrome (or perhaps 

 less F-phytochrome) is also required so that the stems will elongate 

 and the expression of flowering can reach its fullest development. 



The Action of Phytochrome in Flowering 



A complete, detailed description of phytochrome's action in the 

 flowering process is still a part of the future, but certain aspects of 

 the problem are beginning to come into focus. We know a little about 

 the form of the pigment which must be active, and we know that it 

 must be acting as an enzyme. But we do not know what its enzymatic 

 activity might be, nor do we know anything about the biochemical 

 steps in flowering which might be influenced. 



1. F-phytochrome as the Active Entity 



The data of Fig. 7-9 seem to clearly indicate that the F-phytochrome 

 produced by a red-light interruption of an inductive dark period 

 (cocklebur or soybean) some way inhibits the flowering process, and 

 that the time required for this inhibition to be complete so that it 

 cannot be reversed by reversing the pigment is about 30 min. In 

 chrysanthemum the time is longer, but the principle is the same. An 

 important diff'erence is that the pigment will decay in the dark to 

 R-phytochrome in less time than is required for the inhibitory act to 

 be complete. In both cases the decay time may be around 30 min, 

 but in cocklebur and soybean this is long enough for complete 



