46 • Photoperiodism: Attempts at Analysis 



clock may be used would still vary greatly from organism to organ- 

 ism. Most other workers, however, consider the clock truly endog- 

 enous. For summaries of the state of this field with particular 

 reference to animals and microorganisms, see Pittendrigh and 

 Bruce (1959) and Brown (1959); a recent symposium also covers the 

 field in great detail (Biological Clocks, 1960). Only experiments 

 directly concerned with photoperiodism and flowering will be con- 

 sidered below. 



ENDOGENOUS CIRCADIAN RHYTHMS 

 AS THE BASIS OF PHOTOPERIODISM 



In the view of Bunning and co-workers, the endogenous 

 circadian rhythm of plants passes through two phases of more or 

 less opposite sensitivity to light: a "photophile" (light-liking) phase 

 in which development is favored by light and a "scotophile" (dark- 

 liking) phase in which light is unfavorable. These phases are said 

 to be distinguishable by leaf movements as well as by differences 

 in rates of respiration, photosynthesis, cell division, and other 

 processes. As phases of a circadian rhythm they are affected but not 

 caused by light-dark alternations; they are the means by which 

 the plant can ''time" the light or dark exposures it receives. A 

 particular version of this view, now considerably modified by 

 Bunning (1948, 1959b), has provided the stimulus for much of the 

 work on the problem. It relates SDP and LDP specifically by pro- 

 posing that in both types each phase of the rhythm is about 12 

 hours long, but whereas in SDP the photophile normally starts 

 soon after illumination, in LDP it starts only some 8 to 12 hours 

 after the start of light. Thus long photoperiods give the SDP 

 excessive light in its scotophile, whereas short photoperiods give 

 LDP most of the light in the scotophile and little in the photophile. 



An example of the kind of evidence supporting this proposal 

 is from Bunning and Kemmler (1954). They found that flowering 

 in the LDP dill occurred on a daily schedule of 17% hours light- 

 6*4 hours darkness, but was more rapid if a 2-hour dark period 

 was given 3 hours after the start of each main light period (making 

 the schedule 3 hours light-2 hours dark-12% hours light-6 1 /. hours 

 dark). This observation is consistent with the idea that dill has a 

 scotophile phase that occurs shortly after the start of the main 



