DAILY RHYTHMS 477 



endogenous daily rhythms are involved in the photoperiodic control 

 of flowering. He was, however, surely led to his hypothesis for very 

 different reasons: the temperature independence of the photoperiodic 

 phenomena had not yet been discovered, and in spite of Grossen- 

 bacher's (1939) paper, it was almost twenty years before the period 

 of endogenous plant rhythms was commonly taken to be tempera- 

 ture-independent (Ball and Dyke, 1954; BUnning and Leinweber, 

 1956; Leinweber, 1956); indeed BUnning himself has long held the 

 explicit view that the period of plant rhythms was temperature- 

 dependent (BUnning, 1935). Hamner demonstrated that a 24-hr 

 rhythm is indeed involved in the flowering response of beans; but 

 whether or not the relation of rhythms to photoperiodic effects is the 

 one we would envisage — involvement of a common cellular clock — 

 is a question still far from answered. 



Use of the term photoperiodism has now been extended to cover 

 a very different type of phenomenon from that discovered by Garner 

 and Allard. Papers by Highkin and Hanson (1954) and Hillman 

 (1956) show that the growth of tomatoes and other plants is sensitive 

 to the light regime; damage ensues from continuous light or from light 

 cycles in which the period differs markedly from 24 hr. The concept 

 of photoperiodism is, in a sense, weakened by its extension to these 

 cases. The original phenomenon, both in principle and in fact in the 

 clear case of Xanthiiim, did not involve a periodicity as such: a single 

 experience of long night is an adequate releaser of the flowering re- 

 sponse in the cocklebur. But in the injurious effects of light on growth, 

 just cited, significance does attach to the period (or cycle length) as 

 such, vis-a-vis the length of a fraction (night) in one full cycle of 

 fixed length (or period); 24 hr. Thus although it technically weakens 

 the term, the new denotation of photoperiodism has more logical 

 justification, and were it strictly followed the word would be properly 

 juxtaposed with Went's (1944) thermoperiodism where again perio- 

 dicity, as such, is a crucial issue. 



The new model for daily rhythms developed in this paper has no 

 special or new implications concerning the postulated bearing of 

 rhythms to the night-length measurement involved in the flowering 

 response. But it has, we believe, a substantial bearing on the relation- 



