234 CONTROL MECHANISMS IN CELLULAR PROCESSES 



recorded for 20 days. As long as the vial was exposed to a light- 

 dark cycle, the time of day at which the peak in luminescence oc- 

 curred was exactly the same. But on constant light the peak oc- 

 curred about 45 minutes later each dav, so that the free-running 

 period was about 24 hours 45 minutes. 



An important observation, which the design of these experiments 

 relies upon, is that a pulse of bright light, applied at the proper time 

 in the cycle, causes a phase shift, which phase shift persists so long 



72 84 



TIME - HOURS 



Fig. 9-4. Shifting of the phase of the rhythm of luminescence by a single 

 exposure of the cells to light. The cells were grown with alternating light and 

 dark periods of 12 hours each (LD) and were pipetted into test tubes at hours. 

 They were then replaced in LD conditions, and the typical rhythm of lumi- 

 nescence is shown. Cells remaining in the dark after 60 hours continue to show 

 the rhythm with a period of approximately 24 hours, although the amplitude 

 decreases {solid line). Cells treated similarly, except for a 3-hour exposure to 

 light between 66 and 69 hours, also show the rhythm of luminescence {broken 

 line) but with a shift in the phase or time at which the maxima occur. The num- 

 ber of hours by which the phase is shifted is dependent upon the intensity and 

 color of the light used for irradiation. 



as we are capable of assaying the system (Hastings and Sweeney, 

 1958 ) . This is illustrated in Fig. 9-4 for the flashing rhythm. It is 

 similar in the case of the glow rhythm, however. 



Cells kept on a 12 hours light-12 hours dark schedule were trans- 

 ferred to the dark at 60 hours, and the controls, left in the dark there- 

 after, showed a free-running period of about 24 hours. The experi- 

 mentals were exposed at 66 hours to a 3-hour light pulse, and the 

 large phase shift is evident. The number of hours by which the 

 phase is shifted is dependent upon both the intensity and color of 

 the light used for irradiation and upon the duration of the light pulse 



