476 RHYTHMS IN PLANTS AND ANIMALS 



1956). Beling's (1935) review already discusses the possible rela- 

 tion of tiie bee's Zeitgedachtnis to persistent rhythms; but neither 

 her discussion nor those of Kalmus in 1935 and 1938 appear to have 

 impressed other workers on daily rhythms. In fact, frank use of the 

 word "clock" has entered the biological literature only since 1950- 

 1954 following the clear demonstration of chronometry in animal navi- 

 gation. And only since then have students of persistent rhythms explic- 

 itly recognized and treated their problem as one of time measurement. 

 Associated with this reorientation, and doubtless largely due to it, has 

 been the discovery that persistent daily rhythms in protists, plants, 

 and animals are temperature-independent in the sense that their period 

 (measuring time) is nearly invariant over a wide range of tempera- 

 tures (cf. the discussion in Pittendrigh and Bruce, 1957). 



The evolutionary and physiological relationship of daily rhythms, 

 on the one hand to the other, more elaborate cases of chronometry, 

 and on the other hand to the diverse phenomena called photoperiodism 

 is an attractive and many-sided problem. 



The present authors (Bruce and Pittendrigh, 1957b; Pittendrigh 

 and Bruce, 1957; Pittendrigh, 1958) have adopted the working 

 hypothesis that a fundamentally similar time-measuring system, which 

 they consider as a self-sustaining oscillation in cellular activity, under- 

 lies all cases of persistent daily rhythms as well as the bee's Zeitge- 

 dachtnis and the chronometry involved in animal navigation. Apart 

 from its broad empirical support, this hypothesis was adopted for the 

 unity and ultimate simplicity it implies; that living systems, having 

 once evolved the seemingly difficult function of a temperature-inde- 

 pendent time measurement, have exploited the basic oscillation in- 

 volved — doubtless with modification — wherever time measurement 

 has proved adaptively useful. 



We have not, in the papers cited, considered photoperiodism in 

 relation to this hypothesis. But it is clear that insofar as the photo- 

 periodic control of flowering amounts to a relatively temperature- 

 independent time measurement of night length (Long, 1939) we 

 would seek to relate the phenomena, anticipating that a common 

 cellular timepiece was again involved. Bunning (1937) has, of course, 

 been the pioneer in this field and has urged for many years that 



