PREFACE 



An international symposium was held in Gatlinburg, Tennessee, 

 October 29 to November 2, 1957, to discuss and correlate theories, 

 observations, and hypotheses on photoperiodism and related phe- 

 nomena in both plants and animals. The present volume is a record 

 of the proceedings of this meeting, which was held under the auspices 

 of the Committee on Photobiology of the National Academy of Sci- 

 ences-National Research Council and was supported financially by 

 the National Science Foundation. 



The principal objective in organizing the symposium was to present 

 a discussion of both plant and animal facets of photoperiodism in 

 order that questions, problems, and observations unique and common 

 to both areas be considered in a unified manner. It was felt that a 

 great mutual advantage could be derived from such an exchange of 

 views. The diversity of approaches in the study of photoperiodic 

 phenomena in such a wide range of organisms and the scope of the 

 field to be covered necessitated the scheduling of a large number of 

 papers. Both current and previously unpublished research, as well as 

 more inclusive review papers, were considered important in giving a 

 panoramic view of the field of photoperiodism. After deliberation, it 

 was decided not to record all extemporaneous remarks, but to en- 

 courage those who wished to have their ideas published to submit them 

 in written form. Several short articles are included as a result of this 

 procedure. 



The first section of the volume deals with a photochemical analysis 

 of the problems involved and discussion concerning the initial photo- 

 reaction. The second group of discussions is concerned with photo- 

 periodic and related phenomena in plants, including seed germi- 

 nation, photomorphogenesis of seedlings and changes in vegetative 

 structures, flowering, kinetics, and possible biochemical pathways re- 

 sulting in the observed growth. The third division deals chiefly with 



