TEMPORAL REGULATION IN CELLULAR PROCESSES 241 



coiitiiiiioush', and the clock continues in the proper pliase and with 

 the characteristic free-running period. 



Stopping photosyntliesis for a while is similar to a dark period, 

 insofar as products of photosynthesis are concerned. An actual dark 

 pulse causes phase shift; inhibiting photosynthesis does not. Con- 

 versely, we see that photosynthesis need not be occurring during a 

 light pulse in order for the light to cause a phase shift. 



Photosxnthesis, therefore, is an example of a clock-controlled 

 process which apparently has little or no feedback to the clock. We 

 mav thus distinguish clock-controlled chemical changes from the 

 biochemistry of the clock itself ( Hastings and Sweeney, 1958 ) . 



Finalh', we may note the lack of effect with a number of other 

 compounds of interest: gibberellin, kinetin, chlorpromazine, and 

 urethane. Azide, not shown in Fig. 9-8, is similarly ineffectiye at a 

 concentration of 4 X 10"^ M. 



Conclusions can be most readily made at this time with regard 

 to biochemical systems which are not directly involved in clock 

 function. However, we have offered unequivocal evidence for the 

 proposition that we may observe chemically induced phase shifts. 

 But, as in previous studies of this kind, the mechanism and speci- 

 ficity of the reaction is not clear. Care must be observed in such 

 experiments with respect to design and interpretation, and with par- 

 ticular attention to the question of reproductibility. It is to be ex- 

 pected that additional studies concerning various aspects of the 

 biochemistry of rhythmic systems will be of value. 



References 



Ball, N. G., and I. J. Dyke. 1954. An endogenous 24-hour rhytlim in the growth 



rate of the Avena coleoptile. /. Exp. Botany 5: 421-433. 

 Bishop, N. I. 1958. The influence of the lierbicide, DCMU, on the oxygen-e\ olving 



system of photosynthesis. Biochim. et Biophys. Acta 27: 205-206. 

 Bosch, L., E. Harbers, and C. Heidelberger. 1958. Studies on fluorinated pyrim- 



idines. V. Effects of nucleic acid metabohsm in \itro. Cancer Research 18: 



335-343. 

 Brown, F. A., Jr. 1959. Living clocks. Science i 30; 1535-1544. 

 Brown, F. A., Jr., and H. M. Webb. 1948. Temperature relations of an endogenous 



daily rhythmicity in the fiddler crab, Uca. Physiol. Zool. 21: 371-381. 

 Bruce, V. G., and C. S. Pittendrigh. 1956. Temperature independence in a uni- 

 cellular "clock." Proc. Nat. Acad. Sci. U.S. 43: 676-682. 

 BucHA, H. C., and C. W. Todd. 1951. 3-(p-Chlorophenyl)-l,l-dimetliylurea— a new 



herbicide. Science 114: 493-494. 

 Buhnemann, F. 1955a. Das endodiurnale System der Oedogoniumzellc. II. Der 



Einfiuss von Stoffwechselgiften imd anderen Wirkstoffen. Biol. Zentr. 74: 691-705. 



