ENDOGENOUS DIURNAL PERIODICITY 525 



beginning of the light or the dark period the plant was supposed to 

 start a single process with a specific duration of, for instance, 10, 12, 

 or 14 hr. If light is offered while the dark-induced process, or darkness 

 while the light-induced process is still going on, there will be a specific 

 reaction of the plant. With respect to short-day plants, for example, 

 the light-induced timing process coincides approximately with a light- 

 liking or "photophil," the dark-induced timing process with dark- 

 liking or "scotophil" status. 



But this explanation had to be improved. In experiments with 

 longer dark periods of about 40 hr, a light break will have different 

 effects, revealing that the alternation of photophil and scotophil phases 

 is going on (Figs. 20. 21). Claes and Lang (1947), Wareing (1954), 

 and others tried to explain this without the concept of endogenous 

 periodicity by assuming some interaction between the light break and 

 the preceding or ensuing main light period. Of course they had to 

 postulate in addition that this interaction requires a certain time 

 relation. The main light period and the light break are cooperating 

 only as long as they, together with the interrupting dark period, fit 

 into a duration of about 24 hr (Figs. 20, 21). It would be strange 

 indeed if this had nothing to do with the endogenous periodicity. 



Experiments with still longer dark periods in certain cases showed 

 a continual alternation of photophil and scotophil phases for about 3 

 days (Fig. 7). In other cases there was at least a continuation of 

 cycles with higher and lower photophil character. In yet other cases 

 even this was missing. To summarize: Sometimes one light period 

 induces several consecutive cycles of different photoperiodic sensi- 

 tivity; in other cases it induces only one or two of these cycles. This 

 is exactly the same situation as with the light-regulated diurnal cycles 

 in turgor pressure, growth rate, and spore discharge. 



Therefore, the best and simplest way to combine the known facts is 

 to assume that the light- and dark-induced timing processes in photo- 

 periodism are phases of the endogenous diurnal rhythm, regulated by 

 the fight-dark alternation. I cannot see any reason why these facts 

 should be complicated by postulating quite different timing processes 

 in the diurnal change of sensitivity to light and darkness on the one 

 hand, and in diurnal physiological capacities on the other hand. 



We do not yet know how the diurnal change of light sensitivity is 



