INDUCTION OF PHASE SHIFT 549 



they would have been without Hght, although possibly more syn- 

 chronous. It is conceivable that the UV-induced phase delay was 

 photoreversed and a photoinduced phase delay substituted for it, but 

 this experiment cannot decide that issue. 



DISCUSSION 



This demonstration that phase shifts in cellular rhythmicity can be 

 induced by ultraviolet irradiation should prove encouraging to the 

 investigator who hopes to find clock escapements for the rhythm- 

 controlling timepieces at the cellular level. It is of further interest that 

 the UV-induced damage is photoreversible. This brings the phe- 

 nomenon into line with a large number of diverse biological mecha- 

 nisms, none of which in the past has been directly associated with 

 rhythms. The high efficiency of ultraviolet both in absolute energy and 

 in time for induction must ultimately be compared with induction 

 efficiencies of the longer wavelengths that are generally associated 

 with phase shift, but are not usually associated with either nucleic 

 acid absorption nor with photoreactivation. Separate avenues of phase- 

 shift induction may thus be employed by visible and by far ultra- 

 violet radiant energy within the cell. In the preliminary speculations, 

 the conspicuous intranuclear and nucleolar (Kimball and Gaither, 

 1951; Ehret, 1955b) effects of far ultraviolet should not go unnoticed. 



While there is little known about the chemistry of mating substance 

 or its biosynthesis, the present results can be described within the 

 framework of the heuristic scheme previously adopted for Paramecium 

 (Ehret, 1953). In that terminology, the times at which cells are most 

 sensitive to phase shift are those at the middle and end of the scoto- 

 philic phase, when mating-substance precursor is being synthesized 

 and during the time just prior to the conversion of such precursors to 



mating substance. 



SUMMARY 



1. Phase shifts in cellular rhythmicity in Paramecium can be in- 

 duced by low doses of far ultraviolet irradiation. 



2. Stages most sensitive to induction are the middle and late por- 

 tions of the scotophilic phase. 



