48 • Photoperiodism: Attempts at Analysis 



in the middle of the dark period. This is very approximately true 

 in ordinary 24-hour cycles, but rarely so under other conditions, as 

 such work has made evident. Under the rhythm hypothesis, light- 

 breaks act not by merely breaking each long dark period into two 

 short ones, but by supplying light in the scotophile (for SDP) or 

 photophile (for LDP) phases. This has been tested extensively. 



When Claes and Lang (1947) examined the effects of 48-hour 

 cycles on Hyoscyamus (Chapter Two), they found that cycles of 7 

 hours light-41 hours darkness were noninductive. A 2-hour light- 

 break would promote flowering if given not long after the start or 

 before the end of each long dark period, but was ineffective in the 

 middle. The times of maximum effectiveness were about 16 and 

 40 hours, respectively, after the start of each main light period. 

 These results were consistent with the idea that the photophile- 

 scotophile alternation continued through the dark period with the 

 first photophile maximum (typical of LDP) 16 hours after the start 

 of the main light period and the second about 24 hours after the 

 first. Yet there was an equally reasonable alternative explanation 

 not depending on rhythms. Suppose that the light-break could act 

 together with the main light period nearest it (either before or 

 after) to constitute a long light period interrupted (without effect) 

 by darkness. On this alternative the light-break was ineffective in 

 the middle of the long dark period not because it fell in the scoto- 

 phile, as in the rhythm explanation, but because it was too far from 

 a main light period. Claes and Lang favored the second view. 



An experiment designed to avoid this ambiguity was reported 

 by Carr (1952), who used the SDP Kalanchoe grown in 72-hour 

 cycles of 12 hours light-60 hours darkness. On the Biinning theory, 

 light-breaks during the dark period should show three times of 

 maximum effectiveness in inhibiting flowering and causing the 

 correlated changes in vegetative growth, whereas on the Claes and 

 Lang alternative there should be only two, close to either end of 

 the dark period. Carr's results indeed showed three maxima, about 

 24 hours apart, although the middle one was not as well defined 

 as one might wish. Carr concluded that "the theory of Biinning 

 . . . must therefore be regarded as finally and decisively proved," 

 thereby illustrating the partisan vigor that at least enlivens if not 

 clarifies the question. 



Schwabe (1955a) repeated Carr's results but noted that the 



