580 PHOTOPERIODISM IN INVERTEBRATES 



lower ones. When both the time at which the exposure is made and its 

 duration are held constant, the resetting is an approximately linear 

 function of intensity, up to a saturation value of about 800 ft-c. 



The last-noted feature has permitted a determination of the action 

 spectrum for resetting. Measurements were made of the effectiveness 

 of monochromatic light (from 325 m/* to 750 m/x, in 25-m/i steps) in 

 shifting the phase. The results obtained show relatively sharp maxima 

 in effectiveness at 475 m//- and at 650 m^t. Although these peaks do not 

 correspond exactly with the absorption spectrum for the cellular pig- 

 ments, it is probable that the chlorophylls are the primary photorecep- 

 tive pigments involved. 



Several aspects of phase shifting have also been studied with the 

 glow rhythm, yielding results essentially identical with those obtained 

 with the rhythm of luminescence. A detailed analysis of the data 

 should be of use in constructing and evaluating models such as the 

 one presented to us at this symposium by Drs. Pittendrigh and Bruce. 



DISCUSSION 



The fact that the several overt rhythms in Gonyaulax have similar 

 properties supports the idea of a single "master clock." The similarities 

 include a loss of overt rhythmicity when the organism is subjected to 

 constant bright light, but a retention of rhythmicity in constant dim 

 light; the manifestation of natural periods in constant dim light or dark- 

 ness which are close to but not exactly 24 hr; an entrainment to light 

 and dark cycles which differ from 24 hr; and a phase shift of the 

 endogenous rhythm upon changes in illumination, the light perturba- 

 tions yielding no information concerning the period. In addition, the 

 period of each rhythm is essentially temperature-independent, but in 

 each case the temperature sensitivity that does exist has a Qm of less 

 than 1.0. An unusual temperature relationship of this type has also 

 been reported by Buhnemann (1955) for the clock system in Oedogo- 

 nium, suggesting that a common mechanism is involved. The idea of a 

 temperature compensation mechanism carries with it the distinct possi- 

 bility of imprecision, within the limits of adaptive selection pressure. 

 The cases with a Qio of slightly less than 1 .0, therefore, need not fall 



