50 • Photoperiodism: Attempts at Analysis 



period," these results were consistent with the explanation pro- 

 posed. The inhibition of Xanthium induction by light-breaks given 

 early in long dark periods was explained as due to a direct nul- 

 lification of dark processes leading to flowering plus the fact that, 

 after the light-break, the high-intensity light process (Chapter Two) 

 is left unsatisfied. The induction of Xanthium by a critical dark 

 period, regardless of length of the preceding photoperiod, was 

 also cited by Wareing against Bunning's theory, since the latter 

 appeared to hold that the phase of the rhythm was regulated by 

 the start of each main light period. Thus the effect of a dark period 

 should depend on how long the light continued. 



Biinning responded to all this in considerable detail. As to the 

 Xanthium results, leaf-movement studies (Biinning, 1955) indicated 

 that in this plant the phase of the circadian rhythm is indeed regu- 

 lated by the light-to-dark rather than the dark-to-light transition, 

 thus refuting Wareing's evidence based on the opposite assumption. 

 A light-break given early in the dark period falls in the scotophile 

 induced by the transition to darkness and thus inhibits, but a light- 

 break late in a long dark period falls in the photophile that endog- 

 enously follows and thus does not inhibit. The results with soy- 

 beans may also be clarified, according to Biinning (1954), by 

 attention to the actual course of the circadian rhythm as shown by 

 leaf movements. These indicate that the rhythm continues for 

 about 30 hours in darkness, after which a period of "dark rigor" 

 (Dunkelstarr) sets in. A light-break during dark rigor brings about 

 a new photophile phase which is then followed endogenously by a 

 scotophile. Wareing's observation that the effect of a light-break 

 toward the end of a long dark period depended not on the length 

 of the dark period but on the light-break's relation to the following 

 main light period is then due to the fact that the main light period 

 and the scotophile phase of the newly reinitiated rhythm now over- 

 lap, with resultant inhibition. In addition, Biinning pointed out 

 that his observations on leaf movements would also predict the 

 existence and optimum times for the light-break promotions of 

 flowering observed by Wareing. To Wareing's position that light- 

 break effects are due to interaction with nearby light periods, 

 Biinning thus retorted: "Yes, that is so— because of the endogenous 

 daily rhythm." 1 



i "Ja, das ist so, und es beruht auf der endogenen Tagesrhythmik." 



