KINETIC ANALYSIS OF PHOTOPERIODISM 



445 



(Taylor and Kerr, 1941; Moon, 1940). At all times, however, the 

 peak of energy of combined sun and sky radiation is in the visible. 



Much more significant are the very great changes in intensity. The 

 daily variation in light intensity with clear skies during the summer 

 and winter solstices and spring and fall equinoxes are given in Fig. 3. 

 It is seen that clear sky intensities do not vary by more than a 2:1 

 ratio between the summer and winter solstices. However, during the 



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Fig. 3. Sunlight intensity on a horizontal surface for clear days at lati- 

 tude 42° N. for the 21st of December, March, June, and September. Data 

 taken from that of I. F. Hand (1950) for the Blue Hill Observatory near 

 Boston, Massachusetts. Multiply ordinate by 100 for approximate value in 

 foot-candles. 



winter, the average intensity is very much less than that indicated here 

 because of the preponderance of cloudy weather in much of the 

 Temperate Zone. The peak value of light intensity at the summer 

 solstice is approximately 10,000 ft-c which is about 100 mw, cm-. 

 During periods of heavy overcast, the intensity may fall to a few 

 percent of the maximal clear sky value. It is these extreme variations 

 in intensity that must be smoothed out in the plant's response if they 

 are not to introduce serious random variations in the biological 

 measurement of day length. The photomorphogenic red-light responses 

 such as anthocyanin synthesis, opening of the hypocotyl hook, inhibi- 

 tion of the hypocotyl and leaf expansion in bean, and inhibition of the 

 first internode in Avena, all follow a response rate which is approxi- 

 mately proportional to the logarithm of the incident intensity. In 



