The Central Role of the Dark Period • 19 



8 hours darkness. To determine whether it was actually the day- 

 length or nightlength that was critical in this schedule, Hamner 

 and Bonner performed several kinds of experiments. 



Using artificial light when necessary, they exposed some plants 

 to schedules of 4 hours light-8 hours darkness. None of these 

 flowered, although each light period was far shorter than the critical 

 daylength of 15 y> hours. On the other hand, all plants flowered 

 rapidly under cycles of 16 hours light-32 hours darkness, even 

 though each light period was longer than the critical daylength. 

 Two conclusions come from such data. First, it seems to be the 

 length of the dark period, not that of the light period, that is 

 important for Xanthium. Second, the relative length of day and 

 night is clearly not the critical factor since the ratio of light to 

 darkness was the same in both schedules used. 



Perhaps the best evidence concerning the role of the dark 

 period in both LDP and SDP can be obtained by interrupting 

 these dark periods with brief light exposures. Hamner and Bonner, 

 for example, showed that the inductive effects of 9-hour dark periods 

 could be completely annulled by interrupting each one in the 

 middle with a minute of relatively dim (150 foot candles) incandes- 

 cent light. This "light-break" effect is widespread among both 

 response classes, and the general situation can be summarized as 

 follows (see, for example, Borthwick, Hendricks, and Parker, 1956). 



In order to be photoperiodically effective in either SDP or 

 LDP, a dark period of sufficient length has to be uninterrupted. 

 Total light energies (100-1000 kiloergs/cm 2 ) that are very low 

 compared to those of daylight, even given in a few minutes, are 

 sufficient to constitute an effective interruption. In SDP such as 

 Xanthium or Biloxi soybeans, light-breaks in otherwise inductive 

 dark periods will completely inhibit flowering. In LDP such as 

 Hyoscyamus or the Wintex variety of barley, Hordeum vulgare, 

 light-breaks in otherwise noninductive periods (that is, in schedules 

 with daylengths less than the critical) bring about flowering as 

 though the plants had been on an adequate long-day schedule. 

 As will become evident later on, light-break experiments have 

 proved very useful for further studies on the mechanism of photo- 

 periodism. At this juncture, however, they are simply presented as 

 evidence for the role of the dark periods as the single most im- 

 portant controlling factor in photoperiodism. Similarly, brief "dark- 



