LIGHT AND VEGETATION 141 



General Characteristics of Photoperiodism 



This division into three categories — long-day plants, short- 

 day plants, and those indifferent to daylength — has the 

 advantage of simpHcity and is useful for classifying plants 

 raised within a specified range of temperature. 



Some interesting facts relating to photoperiodism have 

 been learnt from the recent work of Professor P. Chouard 

 and others in France and from that of investigators in America 

 and Russia. The experiments were made on plants cultivated 

 under normal conditions, i.e., in rich soil and receiving, at 

 least for the greater part of the day, the illumination without 

 which plant life is impossible. 



The necessity of long nights for the flowering of the 

 chrysanthemum is most probably due to the slowness of 

 the nocturnal chemical reactions which are indispensable for 

 the transformation of the terminal buds into flower buds. 

 These reactions obviously start with the substances elaborated 

 through photosynthesis, from which most of the organic 

 compounds constituting nearly the whole of the plant are 

 derived. They are completed only if their chain remains 

 unbroken by light. It is not surprising that they, too, like all 

 chemical reactions, are dependent on temperature; if the 

 temperature is too high (38° C.) or too low (10° C), they do 

 not achieve the expected result with long nights and the buds 

 are not formed. 



On the other hand, the action of light in preventing 

 flowering remains effective whatever the temperature may be, 

 which seems to indicate that light acts directly on one of the 

 substances whose chain of transformations ends in the 

 production of buds. 



These substances have not yet been isolated and their 

 nature and composition is still uncertain. They can, however, 

 be made to pass from one plant to another, which shows that 

 they are similar in character in spite of the difference in the 

 behaviour of species. 



If a short-day plant. A, is cultivated in the summer, it 

 does not flower because the nights are short, but when it is 



