THE PERIODIC ASPECT OF PHOTOPERIODISM 

 AND THERMOPERIODICITY 



FRITS W. WENT 



Missouri Botanical Garden, St. Louis, Missouri 



This symposium has heralded the twiUght of the chemical approach 

 and marked the coming of age of the physical approach to the problem 

 of photoperiodism, just half a century after Klebs started his work 

 with Sempervivum, which led him in 1913 to suggest that its flowering 

 was apparently controlled by the day length. In a clear and frank 

 appraisal Bonner has shown earlier in this symposium that evidence 

 for the chemical nature of the intermediary steps between perception 

 of the photoperiodic stimulus and its manifestation is almost com- 

 pletely lacking in spite of diligent and prolonged research. The most 

 definite evidence he saw was in the grafting experiments in which a 

 photoperiodically induced scion can make the stock plant flower, 

 indicating transfer of something material from scion to stock. Also 

 the first high-energy light process seemed of a chemical nature, al- 

 though Professor Biinning (p. 507) pointed out that even this process 

 might be largely connected with the internal clock mechanism basic 

 for flowering. 



Fortunately, the discarding of so many of the seemingly fundamental 

 concepts of the chemical control of flowering need not bother the 

 theoretically inclined biologists so much, since the recognition of 

 the periodic aspects of the problem can at least partly substitute for 

 the discarded ideas, and it is most encouraging that at this symposium 

 so much new material has been presented relating to this periodic 

 aspect. 



The light reactions discussed during this symposium can be divided 

 into three groups. First are the morphogenetic processes in general, 

 without special reference to a red-sensitive process with a peak in its 

 action spectrum at 667 mix, usually reversible by far-red radiation of 

 710-730 nift wavelength. It was clear that these processes, non- 

 551 



