110 THE FLOWERING PROCESS 



come to my attention. Any degree of certainty that the process is 

 really controlled by phytochrome should rest at least upon evidence 

 that red light strongly influences the process, while far-red light 

 reverses this influence (or in some cases vice versa). All of the time- 

 independent responses in the table have been studied in this way, 

 although some of them have been studied much more intensively 

 than others. In the case of photoperiodism, however, some responses 

 are included simply because photoperiodism itself has been clearly 

 shown in a few instances to be a phytochrome-controlled response 

 (e.g. flowering, germination of seeds, and inhibition of axillary buds 

 in cocklebur). In other cases evidence with controlled light qualities 

 is either not available or conflicting. Thus certain aspects of floral 

 development, as well as development of reproductive structures in 

 bryophytes, have been shown to be controlled by photoperiod, but 

 no work has been done to show that red or far-red light is effective 

 in promoting or inhibiting the process. In the case of dormancy, 

 J. P. Nitsch (29) has preliminary results indicating that phytochrome 

 takes part in the response, but P. F. Wareing obtained negative 

 results with woody species in a prehminary experiment which was to 

 be reported in this book. We have also obtained negative results with 

 some alpine species. Thus much work remains to be done with plant 

 responses to photoperiod. 



It is interesting to note that the table contains some good examples 

 of phytochrome control in the green algae, the bryophytes, the ferns, 

 and in the higher plants including both gymnosperms and angio- 

 sperms. No evidence has been found for phytochrome in the fungi 

 nor in the algae other than the green algae, however. Thus phyto- 

 chrome appears in the evolutionary line in which chlorophylls A 

 and B are the only light receptors in photosynthesis. Phytochrome 

 may be absent from the other lines, but of course future work could 

 change this viewpoint. 



Is it not rather impressive that so many natural plant processes 

 are under the control of a single pigment system? It is an evidence 

 for parsimony and uniformity in natural processes. At the same time 

 it poses an intriguing question: How can a single biochemical 

 mechanism control so many different kinds of processes? The 

 workers at Beltsville have suggested that some very central compound 

 in metabolism might be involved, such as coenzyme-A. Reactions 

 of coenzyme-A are certainly general; it takes place in carbohydrate, 



