442 



PLANT PHOTOPERIODISM 



December 21 to the summer solstice of June 21 is zero at the equator; 

 1:47 at 15°; 3:52 at 30°; 6:50 at 45°; and 13:00 at 60°. At all lati- 

 tudes, the day length is slightly over 12 hr at the spring and fall 

 equinoxes of March 2 1 and September 2 1 , respectively. It is this pre- 

 cisely repeating cycle of day length which organisms use for determin- 

 ing seasonal time. 



20 



18 



16 



-D 14 



I 12 

 ^ 10 



z 



UJ 



DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 

 MONTH. 21st DAY 



Fig. 1. Day length cycle at various latitudes. Data plotted from "Tables 

 of Sunrise, Sunset and Twilight," U. S. Naval Observatory (1945). 



The weekly rate of change in day length was calculated for each 

 week centered about the twenty-first of each month. The rate of change 

 is plotted in Fig. 2 as minutes change in day length per week and the 

 percent change in day length for that week. The week was used as the 

 basis for these estimations on the assumption that the plants and 

 animals with the most precise timing mechanism can probably per- 

 ceive yearly or seasonal time with an accuracy of one week during the 

 spring and fall seasons. This seems to be a reasonable assumption, con- 

 sidering the soybean data presented by Borthwick and Parker (1939) 

 and other observations on flowering of plants, bird migrations, and 

 similar phenomena. 



The rate of change in day length is minimal during the summer and 

 winter solstices and maximal at the spring and fall equinoxes. The rate 

 at the equinoxes for 30° latitude is 14 min/week or about 1%. For 

 45°, it is 26 min/week or 1.8%, and for 60°, 44 min/week or 3.1%, 

 and the corresponding monthly rates are 3.5, 6.3, and 11%. The best 

 opportunity for perceiving seasonal time is during the equinoxes. It is 



