24 • Photoperiodism: An Outline 



also did not occur on cycles containing 16-hour light periods, no 

 matter what the dark period. 



The generalization that crucial events in photoperiodism take 

 place during the dark period is evidently not annulled by results 

 such as those presented in this section. The precise values of 

 "critical nightlengths," however, arc markedly dependent upon the 

 lengths of the associated light periods, and in a manner which 

 conforms to no simple pattern. 



INTERACTION OF DIFFERENT PHOTOPERIODIC CYCLES: 



FRACTIONAL INDUCTION IN LDP AND 



LONG-DAY INHIBITION IN SDP 



In all the experiments so far considered, not more than one 

 particular kind of light-dark cycle was used for each experimental 

 treatment, although such cycles might be repeated several times. 

 It is desirable to examine some results of using more than one kind 

 of cycle in a given treatment. Most such experiments have been 

 concerned with the effects of intercalating noninductive between 

 inductive cycles, and have naturally been conducted largely with 

 plants requiring more than one cycle for induction. The responses 

 of LDP and SDP to such treatments differ fairly consistently from 

 each other, but show considerable regularity within each class. 



Most LDP studied are susceptible to "fractional induction." 

 This is best illustrated by an example reported by Snyder (1948). 

 Plants of the plantain Plantago lanceolata showed 100 percent 

 inflorescences after exposure to 25 long-day cycles (18 hours light- 

 6 hours darkness). Exposure to only 10 such cycles resulted in no 

 flowering when followed by exposure to short-day cycles (8 hours 

 light- 16 hours darkness). However, if 10 long-day cycles were given 

 and followed by 20 short-day cycles, only 15 more long-day cycles 

 were required for 100 percent inflorescence formation. Thus the 

 effect of the first 10 inductive cycles, though insufficient to cause 

 flowering by itself, persisted throughout the short-day treatment 

 so that only 15 more long-day cycles gave the effective total of 25. 

 This remarkably accurate "memory" is apparently not unusual in 

 fractional induction. It implies that, in such LDP at least, non- 

 inductive cycles play a merely passive role and do not oppose the 

 effects of inductive cycles. 



