444 PLANT PHOTOPERIODISM 



ture compensation and the problem of a temperature-compensated 

 clock imposes a less severe requirement. 



These data force us to conclude that the timing of photoperiodic 

 responses cannot be of the chemical hourglass type which depends 

 upon the accumulation of a photoproduct or a thermochemical reac- 

 tion product that is initiated by the photoreaction. This type of postu- 

 late has been suggested by many workers in the field of plant photo- 

 periodism. It seems to us that we are forced to consider two other 

 possibilities; either the time-comparison system involves several chemi- 

 cal reactions having just the right differences in temperature coeffi- 

 cients so that they balance out to an overall coefficient of about one, 

 or some physical process is involved, having an inherently low tem- 

 perature coefficient. 



The endogenous rhythm systems having periods of approximately 

 24 hr as reported by Biinning (1948, 1954) and Hamner (1940) in 

 plants and Pittendrigh (1956) in animals have temperature coefficients 

 of about 1.2. Since the seasonal temperature follows a fairly consistent 

 pattern of average change in relation to the day-length cycle, a 

 coefficient of 1.2 would appear to be adequate for perceiving time 

 with a precision of a few weeks during most of the year. It seems that 

 these considerations of the required precision of the biological clock 

 are the basis for one of the most compelling arguments for accepting 

 the endogenous 24-hr rhythm system as the actual clock which the 

 organism uses for photoperiodically perceiving the seasons. When to 

 these facts are added the observations of Biinnino- that the red is the 

 most effective spectral region for phasing or "setting" the clock and 

 that the phasing induction is reversible by the far red, the assumption 

 appears to be a sound one. 



Spectral Energy and Intensity Variables 



The spectral energy distribution of sunlight is not greatly altered by 

 either season or weather condition. At high solar angles on a clear 

 day, the peak energy is in the green, but as the sun approaches the 

 horizon and the mass of air through which the rays must pass in- 

 creases, the blue is attenuated more rapidly than the red, and the 

 peak shifts from the green toward the orange, and finally to the red 



