ENDOGENOUS DIURNAL PERIODICITY 533 



days within 48-hr cycles, long-day plants and short-day plants con- 

 tinue to change from the photophil to scotophil stage. The results of 

 these experiments were just as predicted by assuming the role of the 

 endogenous diurnal periodicity. This was described in my paper. 

 (Further experiments like these have been published by Biinsow, 

 1953a,b.) Thus one short day is able to induce at least two full cycles 

 of 24 hr each, i.e., two photophil phases alternating with two scotophil 

 phases. 



4. Light breaks in extremely long dark periods. For more than 

 fifty years it has been known that the endogenous diurnal periodicity 

 of normal plants grown in diurnal light-dark periods may fade away 

 within 2 to 3 days after bringing the plants into continuous darkness. 

 Therefore, most research workers preferred etiolated plants for study- 

 ing the endogenous periodicity. In view of these facts, no one expected 

 a continuation of the diurnal change from the photophil to scotophil 

 phase for more than about two days in any case (BUnning, 1954). 

 But in certain species, in which the conditions controlling the rhythms 

 are favorable, the endogenous diurnal periodicity, as detected by 

 recording the leaf movements, continues for more than two days in a 

 prolonged dark period, though the plants are adapted to normal light 

 conditions. In such instances the diurnal changes from photophil to 

 scotophil stage are going on too. One of these experiments was men- 

 tioned in my paper, and others have been published (for example, 

 Clauss and Rau, 1956). Thus one hght period can sometimes induce 

 three consecutive cycles of 24 hr each. To expect the same result in 

 every experiment with extremely long dark periods means to neglect 

 the long-known facts mentioned above. 



Thus it is not significant that, as Dr. Wareing has stated, the direct 

 evidence for such endogenous rhythms in photoperiodism is rather 

 slight. The situation with respect to diurnal changes of sensitivity to 

 light and darkness is exactly the same as with other processes gov- 

 erned by the endogenous rhythm. If we record leaf movements of a 

 normal greenhouse plant in continuous light of high intensity or in 

 continuous darkness, the evidence for an endogenous rhythm will be 

 very slight too. This tenuous evidence, however, stimulated experi- 

 ments with more suitable conditions, resulting in excellent evidence. 

 Thus, both with diurnal leaf movements and with diurnal changes in 



