488 RHYTHMS IN PLANTS AND ANIMALS 



context. It was, however, noted ( 1 ) that a transient motion was to be 

 expected of perturbed oscillations achieving a new steady state and 

 (2) that transients were a promising tool for further formal analysis of 

 daily rhythms generally. We have now extended their study in Droso- 

 phila and have detected them also in the hamster (Burchard, 1957), 

 Euglena, Mus, Sceloporus, and the cockroach Leiicophaea (Roberts, 

 1957). In all these organisms there is evidence — clearest in the ham- 

 ster and Euglena — of a phase-shifting behavior sufficiently similar to 



DAY 



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A 



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DAY 2 



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24 

 HOURS 



48 



Fig. 6. A typical reset of the rhythm of phototactic response in Euglena 

 is illustrated. Seven successive days are shown, one under another, and for 

 clarity days 2 through 8 are repeated at the right so that the reset can be 

 more easily visualized. As described elsewhere (Bruce and Pittendrigh, 

 1956) each of the 12 lines per 24-hr interval represents the phototactic 

 response to a "test-light" measured turbidimetrically every 2 hr. The first 

 day shown is the last of several in LD 12:72, where the shaded portion 

 represents 12 hr of light. Starting at hr 18 in the cycle (middle of night 

 phase) a 12-hr light signal is given on the fourth day resulting in an im- 

 mediate advance of the typically daytime responses. 



