224 Light 



that the sense organs of any type of animal conld respond to the dif- 

 ferences in the moon's gravitational effect. 



For marine animals of the littoral zone the moon's influence often 

 appears to be mediated at least in part through the change in the 

 tides, but certain lunar periodicities occur in regions where tides are 

 slight or lacking. One great difficulty in studying this problem is 

 that often the behavior pattern persists for a while under changed 

 conditions, as when the moon is obscured, apparently because of a 

 correlation with an internal rhythm. Much more investigation will 

 be required before these intriguing periodicities are completely un- 

 derstood. 



Seasonal Periodicity. The seasonal activities of plants and animals 

 are sometimes due to changes in temperature and are sometimes con- 

 trolled by the cycle of dry and rainy seasons; seasonal periodicity may 

 also be influenced by the light factor. The seasonal effect of light 

 is not so much due to differences in intensity as it is to differences in 

 total amount of light and in the relative lengths of day and night. 

 The response of organisms to daylength is known as pJiotoperiodistn. 

 The most striking manifestation of photoperiodism is the control of 

 the reproductive cycle in certain plants and animals, although other 

 life processes are involved in some instances. In these organisms the 

 reproductive phase of the life cycle is initiated by days that are shorter 

 or longer than certain critical lengths. Under natural conditions short 

 days are accompanied by long nights and vice versa. Experimental 

 manipulation of the photoperiod has revealed facts indicating that the 

 length of the night may be more influential than the length of the day 

 in controlling the photoperiodic responses of at least some of the 

 plants and animals, but the precise mode of action of the light factor 

 has not yet been determined (Farner et al., 1953). 



Photoperiodism was first discovered in relation to plants. Species 

 that flower only when the days are longer than a certain number of 

 hours and the nights are correspondingly short are known as long- 

 day plants. Short-day plants, on the other hand, flower naturally only 

 under conditions of short days and long nights. However, the re- 

 productive cycle of certain other species is not affected by daylength; 

 these are referred to as indeterminate or indifferent plants. This 

 aspect of periodicity in the light factor controls the season at which 

 long-day and short-day plants flower in any locality, and it also in- 

 fluences the geographical distribution of the species. 



Long-day plants flower naturally only in the middle or high lati- 

 tudes during the late spring or early summer. Familiar examples 

 are the radish, iris, red clover, evening primrose, spinach, the smaller 



