490 



RHYTHMS IN PLANTS AND ANIMALS 



that in Drosopliila to suggest that we are confronted with a very 

 general feature of daily rhythms. 



Figure 6 shows a phase shift in Eiiglena attained by an advance 

 (short transients) following a 12-hr light signal that began in the 

 night phase; similar signals beginning in the day phase cause phase 

 shifts attained by delays. There is evidence in Fig. 6 that two transient 

 cycles precede the new steady state, but there is no doubt that in this 

 organism the reset is more nearly complete in one cycle than is the 

 case in Drosophila and the hamster (Bruce and Pittendrigh, 1958). 



to 





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o 



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ICi. 



-I — I — r— 1 — 1 — I — I I I 



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o 



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N. 7 .- 



Fig. 8. Steady-state phase shifts in {left) Drosophila and (right) the 

 hamster {Mesocricetus auratus). The abscissa represents the hour of the 

 original cycle at which the phase-shifting 12-hr light signal was begun. 

 Hour zero on the abscissa corresponds to "dawn" of the previous steady 

 state. The ordinate represents the total phase shift in hours after a new 

 steady state is attained. Positive phase shifts (points above the line) corre- 

 spond to delays of the eclosion peaks or activity onsets and negative phase 

 shifts correspond to advances. 



Figure 7 shows two phase shifts in the hamster caused by single 1 2- 

 hr hght signals. The new steady-state phase is always preceded by 

 many clear transients, more even than in Drosophila. Perhaps associ- 

 ated with this prolonged duration of transients is the failure of any 

 signal so far used to cause a "saturation" phase shift in the hamster: a 

 signal falling at hour 12 fails to shift phase a full 12 hr. Figure 8 com- 

 pares the steady-state phase shift in Drosophila and hamster following 

 12-hr light signals given at various times throughout the cycle. In 

 Drosophila the shifts (all saturated) are achieved by delays up to hour 



