KINETIC ANALYSIS OF PHOTOPERIODISM 



443 



probably significant that reproductive cycles in both plants and animals 

 are usually initiated in the spring and fall, when the rate of change in 

 day length is maximal. 



These data clearly demonstrate that organisms which are capable of 

 perceiving seasons by the measurement of day length must have a 

 clock that has a precision of the order of 1 to 3% if they are to meas- 



mm 



wk 



42 

 35 

 28 



21 



14 

 7 

 



2 - 7 



o 



u. -14 



o 



21 



UJ 



-28 

 -35 

 -42 



% 

 3.0 

 25 

 2-0 

 1.5 

 1.0 

 Q5 

 0.0 

 -0.5 



- 10 



- 15 

 -^0 

 -Z5 

 -3.0 



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

 MONTH, 21st DAY 



Fig. 2. Rate of change in day length. Values were calculated from data 

 of Fig. 1 for latitudes of 30°, 45°, and 60° in minutes per week and per- 

 cent. The rate is the average change for each week centered about the 

 21st of the month. The percent change is the ratio of the average weekly 

 change to the length of a 24-hr day (1440 min). Multiply ordinate values 

 by 4 for approximate monthly rates. 



ure seasonal time to one week and 4 to 12% for an accuracy of one 

 month. These are indeed high orders of precision for the poikilother- 

 mic organisms which have no means of controlling their body tempera- 

 ture. 



There are few environments in which the temperature does not 

 fluctuate in a somewhat random manner over at least ±5°C. There- 

 fore, there should be a variation of several hundred percent in the 

 reaction rate of the usual thermochemical reactions over this range 

 in the poikilothermic organisms which would include the microorgan- 

 isms, plants, and cold-blooded animals. In the case of the homoio- 

 thermic or warm-blooded animals, there is a high degree of tempera- 



