CHAPTER 10 



Photoperiodism 



H. A. BoRTHwicK, S. B. Hendricks, and M. W, Parker 



U.S. Department of Agriculture, Bureau of Plant Industry, Soils, 

 and Agricultural Engineering, Beltsville, Maryland 



Introduction. Discovery. Photoperiodic responses of animals. Organ of percep- 

 tion. The period involved. Action spectra: Equivalence of phenomena — The time- 

 measuring reaction. The pigment and the photoreaction. Reciprocity and energy 

 response. Correlation of some responses to light. Photoperiodic aftereffects. Flower- 

 ing dn relation to auxins. Inheritance of responsiveness to photoperiod. Influence of 

 environmental factors on photoperiodic response. Reviews on photoperiodism. Cog- 

 nizance. References. 



INTRODUCTION! 



Biological responses indicative of reactions that serve in some way to 

 measure duration of time are known for both plants and animals. These 

 are in part associated with either the light or the dark period of the daily 

 cycle and are subject to the seasonal variation of the night and the day 

 lengths. Species adaptations of both plants and animals to change in 

 season are often effected through this dependence. The term "photo- 

 periodism" has been applied to one type of these responses associated 

 with initiation of reproduction of plants and animals. Other responses 

 are known to arise from the same initial reaction, and some of these are 

 not necessarily periodic. Some periodic phenomena are of other classes, 

 an example being those possibly determined by endogenous rhythms. 



An attempt is made to treat a number of diversified biological responses 

 that might in all species be associated with the same photoreaction or 

 possibly with two different photoreactions, one in plants and the other in 

 animals. There is danger that unrelated reactions have been brought 

 into juxtaposition, but the risk is justified, if only as a challenge for more 

 intensive study of the phenomena involved. The extensive literature, 

 which has appeared in a thousand or more articles, is mentioned in the 

 section Reviews on Photoperiodism. In general, attention in this chap- 



! The names of organisms referred to in this chapter are those used by the authors 

 of the literature cited. At the first reference to an organism its presently accepted 

 name is included in parentheses. 



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