Seasonal Periodicity 227 



able to complete their development under photoperiods of about 12 

 hours' duration. Since the vegetation of intermediate latitudes is 

 composed of a mixture of long-day, short-day, and indifferent plants, 

 the seasons of vegetative growth, flowering, and seed dissemination 

 vary among the species of the community, and competition is reduced 

 accordingly. In some instances genetic strains of the same species 

 growing under different climatic conditions respond differently to 

 given photoperiods, apparently, as an adaptation to differences in the 

 time of onset of adverse conditions. The possibility of intraspecific 

 variation of this sort among both plants and animals in their response 

 to all types of environmental factors must constantly be kept in mind. 



The control of the reproductive cycle by the length of day is a re- 

 sult of a complicated balance between life processes that go on in the 

 light and in the dark (Leopold, 1951). Since the intensity of light 

 which influences the photoperiodic response is far below that needed 

 for photosynthesis, we know that the reactions responsible are quite 

 different. The delicate nature of the processes involved are indicated 

 by the fact that only a few minutes, or even a few seconds, of light 

 during the middle of a long night may reverse the flowering reaction 

 of the plant. The tasseling of sugar cane can be inhibited by a short 

 light period during the night, and the possibility of retarding the 

 ripening of the crop until the most favorable time for harvesting by 

 sweeping the cane fields with search lights has been investigated by 

 Hawaiian planters. In greenhouse plants and other commercial 

 species the timing of the production of flowers, seeds, fruits, or storage 

 organs is controlled artificially by means of the photoperiodic re- 

 sponse. 



Photoperiodism similarly plays an important role in the life cycles 

 of many kinds of animals and, interestingly enough, also most fre- 

 quently controls the reproductive cycle. However, wing production 

 in aphids, metamorphosis in mosquitos, pelage changes in fur bearers 

 (Lyman, 1942), and other life processes are known to be influenced 

 by daylength. The most extensive studies have been carried out on 

 mammals and birds, and these have been reviewed by Bissonette 

 ( 1936 ) and Burger ( 1949 ) . Trout that ordinarily spawn in Decem- 

 ber can be induced to lay their eggs in August by artificially chang- 

 ing the daylength. The fresh-water pulmonate snail, Lijmnaea 

 palustris, will not lay eggs on an 11-hour day but lays abundantly 

 when the days are 13% hours long or longer. The control of repro- 

 duction by daylength in this species rather than by temperature was 

 neatly demonstrated under natural conditions by the fact that snails 

 living in a spring with practically no seasonal change in temperature 



