534 RHYTHMS IN PLANTS AND ANIMALS 



the sensitivity to light and darkness, the endogenous cycles may con- 

 tinue for one or two periods only in certain cases, for a longer time in 

 others. 



5. Long-day and short-day plants. I agree with Dr. Wareing that 

 I have changed my original opinion on several points. The most 

 essential change is the following. The regulation of the endogenous 

 cycles by the light-dark periods, as is well known, makes the short-day 

 plants scotophil in the second half of the day, the long-day plants 

 photophil. But now we understand that this is not due to a 12-hr 

 phase difference of the basic cycle in the two types. All our experi- 

 ments with different species of both types show that in long-day and 

 short-day plants the same biochemical features prevail in parts of the 

 cycle which are comparable with respect to their time position within 

 the light-dark periods. For instance, in the first half-cycle, i.e., during 

 about the 12 hr following the beginning of a light period, synthetic 

 capacities are predominant. Even the maximum sensitivity to red light 

 in the second half-cycle, about 1 6 to 1 8 hr after the beginning of the 

 light period, is common to many species of both types. Thus the 

 question now is: Why do short-day and long-day plants show antago- 

 nistic reactions to red light offered in the same phase of the basic 

 cycle, i.e., why do long-day plants show a photophil character about 

 16 to 18 hr after the onset of the light period while short-day plants 

 have a scotophil character during the same phase of the basic cycle? 

 This is not as strange as it seemed previously. Sachs has reported on 

 dual day length requirements. Thus the light, offered during that 

 highly red-sensitive phase, may be involved in two different reactions, 

 both of which are necessary for flower formation. But the light sensi- 

 tivity of these reactions is different in long-day and in short-day plants. 

 In case each of these reactions shows a light sensitivity, the plant needs 

 at first long days, afterward short days (long-short-day plants). In 

 other cases, one or the other (even both in day-neutral plants) of these 

 two reactions are sufficiently independent of light. 



REFERENCES 



Bunning, E. 1950. Uber die photophile und skotophile Phase der endogenen 

 Tagesrhythmik. Planta, 38, 521-40. 



