[Chap. XXXI INITIATION OF FLOWERS 343 



Herbaceous short-day plants that have been tested bloom during the 

 long days of summer if for a few successive davs thev are placed in a 

 dark room from about the middle of the afternoon until the next morning 

 (Fig. 143). The number of these short-day exposures that are eflFective 

 depends upon the species of plant and also upon the temperature. One 

 or two exposures are sufficient to initiate flower primordia in some plants. 

 Short days are necessary for the further growth of the flower primordia 

 after they are initiated in some species, whereas in other species the 

 flowers may continue to develop in either long or short photoperiods. 



These short-dav plants will bloom even when exposed to the long 

 days of summer if the intervening dark period is made longer, for exam- 

 ple, from the evening of one day to the morning of the second day there- 

 after. The dark period must be continuous. If it is interrupted, even bv 

 very short light periods, flower primordia do not form. Such facts as these 

 have led some investigators to suggest that the short-day plants should 

 be called long-night plants. An idea of the importance of the long dark 

 period may be obtained by comparing the behavior of the plants under 

 difl^erent photoperiods as summarized in Table 12. 



Table 12. Initiation of Flowers in Cocklebur in Relation to the Length of 

 Alternating Periods of Light and Darkness 



If short-day plants are exposed only to long daily light periods, they 

 remain vegetative for an indefinite time and grow much larger than 

 when thev are exposed to short days. Advantage is sometimes taken 

 of this fact. After the plants have grown to some desired size in long 

 days, they may be transferred to short days. In this way larger plants 

 with a greater number of flowers and fruits may be obtained. If the 

 seeds germinate in spring the plants are exposed to long days in earlv 



