Interactions of Light and Dark Period Lengths • 23 



niger would initiate flowers. As long as the light-dark cycles totaled 

 24 hours, flowering occurred with at least 1 1 hours of light per 

 cycle, and was most rapid with 15-16 hours. When cycles totaling 

 48 hours were used, however, flowering occurred with as few as 

 9 hours light per cycle, and reached its maximum rapidity with 

 13 hours per cycle. Thus longer total cycle lengths actually reduced 

 the "critical daylength" by at least two hours, in spite of the fact 

 that the shorter daylength was active with a much longer dark 

 period. 



Differing but equally complex results were obtained by 

 Takimoto (1955) in experiments in which he exposed the LDP 

 Silene armeria to 10-day treatments of cycles composed of various 

 durations of light and darkness. Flower initiation was most rapid 

 in continuous light. In cycles with light periods of 12 hours or 

 shorter, initiation occurred only when the associated dark periods 

 were shorter than 13 hours; in cycles with light periods of 14 or 16 

 hours, however, even dark periods of 24 or 32 hours duration failed 

 to prevent initiation. Some of the interactions between light and 

 dark periods in the SDP Biloxi soybeans were studied by Blaney 

 and Hamner (1957). Only a few of the results will be mentioned 

 here, but this paper provides one of the best examples of the com- 

 plexity of such interactions and resultant difficulty of reaching any 

 general conclusions on the problem at present. The Biloxi soybean, 

 like most SDP, requires several cycles of appropriate photoperiodic 

 treatment to initiate flowers. When plants were given 7 cycles of 

 8 hours fluorescent light and 16 hours darkness, then placed on 

 long-day greenhouse conditions, high flowering values were ob- 

 tained. Hence 8-hour light periods and 16-hour dark periods 

 together constitute an inductive cycle. However, when each portion 

 of such an inductive cycle was examined separately, the following 

 results were obtained. Seven cycles of 8 hours light alternating with 

 24-hour or 26-hour dark periods resulted in no induction at all. 

 Seven cycles of 16-hour dark periods alternating with light periods 

 either 4 hours or shorter, or longer than 12 hours, also resulted in no 

 induction. For further results and tentative conclusions the original 

 paper should be consulted. The concept of a minimum critical 

 dark period requirement was still supported since induction was 

 never brought about by any cycle with less than a 10-hour dark 

 period, no matter what the associated light period; however, it 



