DAILY RHYTHMS 501 



paper where damage is considered simply in terms of the hght having 

 fallen in the scotophil fraction of the cycle. 



It seems to the present writers that photoperiodic and thermo- 

 periodic effects on growth efficiency are indeed related, as their dis- 

 coverers suggest, to the distinct phenomenon of endogenous daily 

 rhythms. We feel, however, that this relationship cannot be properly 

 elucidated as long as the discussion is restricted to the special terms of 

 BUnning's view that the endogenous cycle comprises distinct photophil 

 and scotophil fractions. The most fruitful insights and suggestions for 

 new experiments arise, we believe, from the broader picture given by 

 the generalized oscillator model and the particular coupled-oscillator 

 scheme outlined above. A strong feature of the model is its general 

 prediction of entrainment effects (Phtendrigh and Bruce, 1957). An 

 oscillating system is entrained, or driven, when it is energetically 

 coupled to another oscillating or periodic system. Pittendrigh (1958) 

 has described three types of entrainment that might relate to biological 

 cases; two are pertinent here: ( 1 ) in unilateral entrainment one system 

 completely drives the other which is unable to feed back on the driving 

 periodicity that, accordingly, exclusively determines the phase and 

 period of the coupled system; (2) in mutual entrainment there is feed- 

 back, and the coupled oscillators share a frequency (with determinate 

 phase relations) that is some intermediate between the natural periods 

 of the separate components. All entrainment involves control of phase 

 and period in the entrained oscillation, and in general it can only be 

 effected within a restricted range of periods close to the natural period 

 of the entrained system. Tribukait (1954) has illustrated this rule in a 

 biological case: the natural period of mouse locomotory activity is 

 about llVi hr, and while it can be entrained by light cycles to other 

 periods, such entrainment fails below 19 hr and above 26 hr. 



Entrainment enters into the operation of daily rhythms in at least 

 two major ways. First, the proper functioning of the overall endoge- 

 nous oscillation depends on its strictly unilateral entrainment by the 

 environment; the periodicity of dawns entrains the A oscillation and 

 thus controls the slight innate error of its period and establishes its 

 adaptively appropriate phase. The B oscillations in the cell are them- 

 selves directly affected by the environmental temperature cycle (which. 



