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TEXTBOOK OF BOTANY 



need not be very intense. The main thing is that the daily photoperiod be 

 lengthened. 



If the intensity of this supplemental light is between 1/500 and 1/2000 

 that of a bright summer day, it is sufficient for the initiation of flowers 

 in most of the plants that have been tested. Some species bloom when 

 the intensity of the supplemental light is only 2 to 5 times that of bright 

 moonlight. The longer rays of light are more effective than the shorter 

 ones. 



Second, there are the plants that bloom only when the days are rela- 

 tively short, usually under 12-14 hours. The longest daily light period 

 under which one of these species will bloom is called its critical photo- 

 period. These are the plants that bloom in early spring and autumn. 

 That is, the initiation of flowers in them occurs during short days. The 

 young flowers may grow to maturity during the season they are initiated, 

 or thev may remain dormant until the following spring, as they usually 

 do in trees and shrubs. 



Fig. 143. Effects of length of day on vegetative growth and Hower formation 

 in morning glory. C, control plant exposed to long days from time of germination. 

 Figures indicate the number of short-day periods to which the other plants were 

 exposed during the seedling stage. All plants completed their development in long 

 days. Photo by Victor A. Greulach. 



