KINETIC ANALYSIS OF PHOTOPERIODISM 441 



radiant energy field of the sun; the second is an annual cycle arising 

 from the inclination of the earth's axis in its plane of rotation about 

 the sun; the third is the lunar cycle. 



The gravitational forces of the moon produce monthly tidal cycles 

 which impose considerable environmental changes on coastal marine 

 organisms. The moon also reflects some sunlight onto the earth during 

 the night at certain parts of the lunar cycle. Although this intensity at 

 the earth's surface is low, it cannot be ignored as a factor in growth. 

 As will be shown later, the intensities of moonlight are well within the 

 significant range of plant photomorphogenic responses. 



The diurnal and annual cycles produce precisely changing sequences 

 of day length and daily supply of energy which are largely responsible 

 for the seasonal changes in weather. In most of the Temperate Zone, 

 temperature and precipitation patterns approximately coincide with 

 the day-length cycle. The coincidence is never precise, and neither 

 temperature nor rainfall is a sufficiently reliable index of seasonal time 

 for those organisms which must make drastic physiological adjustments 

 in preparation for a period of adversity. It is the length of the natural 

 photoperiod for which many plants and animals have evolved a sea- 

 sonal time-measuring mechanism. However, it is not enough that the 

 organism be able to elicit different responses to different day lengths; it 

 must, in addition, have some means of comparing its day-length signals 

 with an internal scale of time which is both precise and independent 

 of wide variations in light intensity, temperature, water supply, and 

 other environmental factors. 



Cycle of Day Length 



The time-sensing precision required of the biological clock can be 

 estimated from standard tables of sunrise and sunset. The well-known 

 monthly variation in day length is graphed in Fig. 1 for the latitudes 

 from 30° to 60', which include the major crop-producing areas of the 

 Temperate Zone. It is probably in this general range that most of the 

 photoperiodically sensitive plants have evolved. The refraction of the 

 sun's rays by the earth's atmosphere and the finite diameter of the solar 

 disk makes the day length constant at approximately 1 2 hr and 7 min 

 at the equator. In the Northern Hemisphere, at the solstices, the total 

 day-length variation in hours and niinutes from the winter solstice of 



