446 



PLANT PHOTOPERIODiSM 



Other words, response-wise, the organism sees the logarithm of the 

 intensity. This holds for the plants which we have studied at all values 

 above 0.001 % of noon sunlight. 



The dashed curve of Fig. 4 is plotted as the logarithm of the sun- 

 hght intensity for values above 1 mw/cm- and therefore is propor- 

 tional to the biological responses. When so plotted, it is evident that 

 the response to full sunlight rises with extreme rapidity in the morn- 



ft-c 



10 n 12 I 



SOLAR TIME 



-30 -20 -10 10 

 TWILIGHT PERIOD (mir> 



Fig. 4. Linear and log plots of sunlight intensity, 7^, on a horizontal 

 surface for clear days at the spring and fall equinoxes. The solid curve is 

 the same as in Fig. 3 for March and September. The dashed curve is the 

 logarithm of the irradiance, log l^, and the dotted curve is the logarithm 

 of 0.01 Iji, which corresponds to a heavily overcast sky. The horizontal 

 dashed line is the assumed upper limit of saturation of photoperiodically 

 sensitive plants. The graph on the right is the time course for twilight in- 

 tensity on an expanded scale from data of H. H. Kimball (1938). Zero is 

 the time of appearance of the edge of the solar disk. 



ing and falls quickly in the evening. The rate of change of intensity 

 during the twilight period is very great, as shown in the right-hand 

 portion of Fig. 4. The period extends for approximately 30 min beyond 

 the time of actual disappearance of the solar disk. In a period of about 

 20 min at dawn, it rises from approximately 10 to 100 ft-c. The 

 horizontal dashed hne at 100 ft-c is the intensity below which the 

 more photoperiodically sensitive plants saturate. 



