THE PHOTOPERIODIC PROCESS 



JAMES BONNER 



Division of Biology, California Institute of Technology, 

 Pasadena, California 



We will in this discussion consider the series of events which character- 

 ize the photoperiodic process as it occurs in plants. I wish it to be 

 understood that I approach this discussion in all humility. A great 

 many facts about photoperiodism have accumulated since the formula- 

 tion of the principle by Garner and Allard in 1 920. These facts are not 

 only numerous but complex, and it does not appear to be possible 

 today to put all of them together into one coherent model. Perhaps it is 

 impossible for one individual even to remember simultaneously all the 

 relevant facts. This discussion will be limited principally to the photo- 

 periodic lore relating to the flowering process, to reproduction. The 

 photoperiodic control of flowering is on the whole inductive, a term 

 which will be defined below. Photoperiodic control of vegetative 

 growth, on the contrary, is largely noninductive. A clear distinction 

 can therefore be made between these two important sets of photo- 

 periodic responses. 



Biologists are now aware that plants may be classified with respect 

 to their responses to relative length of day and night. Thus we have 

 short-day plants, long-day plants, and day-neutral plants. Day-neutral 

 plants are those which disregard relative length of day and night and 

 in which flowering is controlled by other factors, as by age, number of 

 nodes, and previous history of cold treatment. The flowering behavior 

 of short-day and long-day plants, on the other hand, is determined by 

 relative length of day and night. Thus, the short-day plant flowers only 

 under day lengths shorter than some critical value. The long-day plant, 

 on the contrary, flowers only under day lengths longer than a critical 

 value. Actually the two groups might better be referred to as long-night 

 and short-night plants respectively, since the length of the dark period 

 is the primary determinative factor in each case (Hamner and Bonner, 



„ 245 



