ADDITIONAL REMARKS ON THE ROLE 



OF THE ENDOGENOUS DIURNAL 



PERIODICITY IN PHOTOPERIODISM 



ERWIN BUNNING 



Department of Botany, University of Tubingen, Germany 



In the discussion following my paper, I stressed several facts which 

 may be summarized as follows. 



1. Definitions. The term photophil means "light liking," not 

 "hght induced" nor "during light period." The term scotophil means 

 "dark liking," not "dark induced" nor "during dark period." 



2. Light breaks in normal dark periods. The strongest inhibition 

 by light breaks offered to short-day plants during long nights is not 

 necessarily in the middle of the night. The exact time of this maximum 

 scotophil stage depends more or less on the beginning of the dark 

 period, but also on the beginning of the preceding light period. If, 

 for instance, this maximum sensitivity in light-dark cycles of 10:14 hr 

 is reached 7 hr after the beginning of the dark period, it may be 

 reached 12 hr (or between about 7 and 12 hr) after the beginning of 

 this dark period in 5: 19-hr light-dark cycles. Thus not the splitting 

 of the dark period into its smallest possible parts is decisive; decisive 

 are processes requiring a certain time. With the beginning of the light 

 period a timing reaction starts which requires more than 12 hr (in 

 our example 17 hr) to reach a certain extreme physiological status. 

 Within this timing process a qualitative change also occurs, since light 

 breaks offered earlier than, for instance, 12 hr after the beginning of 

 the light period do not inhibit flower formation but promote it. A 

 certain time (for instance, 12 hr) after the beginning of a light period, 

 the photophil status is followed by a scotophil status, both with long- 

 day and with short-day conditions. This scotophil status reaches the 

 maximum about 6 hr after its beginning. Consequently the timing 

 reaction is a cyclic one; it is not of the "hourglass type." The cycles 

 are not only characterized by increasing and decreasing scotophil 



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