TEMPORAL REGULATION IN CELLULAR PROCESSES 231 



lias been measured bv- botli nianometric techniques and bv C^^02 in- 

 corporation measurements. Sweene\ , in experiments utilizing diver 

 techniques, has demonstrated this rhythm in single isolated cells 

 (Sweeney, 1961). This, it is believed, is the first instance in which 

 a diurnal rhythm has beeii observed from measurements of single 

 cells. 



There is, in addition, a rhvthm of cell division (Sweeney and 

 Hastings, 1958). Cells divide predominantly at the time of day in- 

 dicated by the solid line, at approximately the end of the dark period. 

 We do not find that all cells divide every dav, but those cells which 

 do divide in any given 24-hour period do so at this time. Under 

 optimum conditions we have been able to obtain 65 per cent or 

 more of the cells di\iding during one day. 



Finally, there exists also a rhythm of a steady glow, which is 

 maximum at about the same time as is cell division (Sweenev and 

 Hastings, 1958). In contrast to the light emitted by flashing, this 

 steady glow is spontaneous and does not require stimulation. Its 

 intensity is very dim and not readily visible to the dark-adapted eye. 



All these rhythms continue to occur in constant conditions and 



ml 



all are similarly temperature-independent, showing Qio values in the 

 range of 0.85. 



The glow rhythm has been used in the experiments to be de- 

 scribed, and it may be assayed by placing a culture in front of a 

 phototube, as shown in Fig. 9-2 ( top ) . The increase in the baseline 

 represents the glow; the irregular vertical lines are from the spon- 

 taneous flashing of single cells. Note that the phase of the spon- 

 taneous flashing rhythm becomes displaced from the glow rh)'thm. 



Measuring the glow in this wav requires that the culture be al- 

 ways kept in front of the phototube, therefore in the dark. This 

 results in the starvation of the cells because of lack of light for 

 photosynthesis (Sweeney et al., 1959). Only three or four peaks 

 may be measured in the dark, as is suggested by the decline in the 

 magnitude of the second peak (see also Sweeney and Hastings, 

 1957). 



To overcome this difficult\' and to be able to assay manv samples 

 at a time, we assembled an automatic sampling apparatus, con- 

 structed from a Packard Liquid Scintillation counter. It has a capac- 

 ity for 100 vials on a turntable, with light sources overhead whose 

 location, intensity, and programing can be controlled. A vial is 

 moved from the turntable down to a dark chamber in front of the 



