440 PLANT PHOTOPERIODISM 



Other seasons were suitable for such processes. This problem is typi- 

 cally characteristic of the Temperate Zone. For survival, it is necessary 

 for Temperate Zone organisms to be able to predict the onset of un- 

 favorable periods far enough ahead so as to get the year's growth and 

 reproductive phases over with before the onset of cold or hot weather 

 or drought. 



The host of physical and chemical problems posed by biological 

 photoperiodism in plants and animals might be resolved into four 

 general questions: 



1 . What are the physical and chemical mechanisms by which light 

 mediates growth and reproduction? 



2. What is the nature of the internal time-measuring system or 

 clock which is coupled to the photochemical stimulus? 



3. How are the photochemical and thermal stimuli and their conse- 

 quent growth-mediating systems coupled to the clock? 



4. In the case of plant photoperiodism, how is it that the same 

 photochemical stimulus is capable of invoking such a great variety of 

 responses, often of opposite character in different species, at all stages 

 of development from seed germination to maturity? Is there any 

 evident pattern by which one might organize these apparently diverse 

 responses? 



The following discussion attempts to analyze the extent to which 

 present kinetic information contributes toward answers to these ques- 

 tions. The discussion will be limited largely to plants, since it is 

 here that we have the most complete information on the photochemical 

 aspects, but many of the considerations may be applicable to animal 

 photoperiodism as well. 



THE RADIATION ENVIRONMENT 



Before proceeding with an analysis of the biological responses, it is 

 desirable to examine the natural environment for clues as to the pre- 

 cision required in the measurement of day length and the magnitudes 

 of the spectral and intensity variables of sunlight. Throughout evolu- 

 tionary development, living organisms have been exposed to three types 

 of superposed celestial oscillations. The first is the diurnal cycle of 365 

 days per year caused by the rotation of the earth about its axis in the 



