532 RHYTHMS IN PLANTS AND ANIMALS 



character in their second half. Experiments with interrupted hght 

 periods reveal the increase and decrease of photophil character in the 

 first half-cycle. In Kalanchoe, in soybeans, and in Chenopodlum, for 

 example, the maximum of the photophil character is reached 5 to 6 

 hr after the beginning of the hght period (Biinning, 1950; Biinning 

 andKonitz, 1957). 



The reactions of long-day plants lead to the same conclusion. 

 Owing to the concept of induced cycles (instead of hourglass proc- 

 esses), it was postulated that flower formation in this type should be 

 possible even under short-day conditions, that is, if after starting the 

 cycle by a short hght period, the plants were offered some additional 

 light at the right moment (the photophil stage) of the induced cycle. 

 The assumption was that only a certain length of time (for example, 

 12 hr) after the beginning of a hght period, the long-day plants 

 become photophil, both with long-day and with short-day conditions. 

 This has been confirmed, as is well known. Again, the maximum 

 sensitivity is not necessarily in the middle of the dark period, but at a 

 time on which the beginning of the light period can have a decisive 

 influence. This point may be reached, for instance, 16 hr after the 

 beginning of the light period. 



Moreover, by determining the action spectra, it became evident 

 that the fight-induced process in the first part of the cycle differs 

 from that prevailing in the second half of the cycle. This again shows 

 the endogenous change from one status to another. These facts once 

 more show the cyclic nature of the timing process. 



Also, experiments of Takimoto (1955) on the long-day plant 

 Silene armeria show that it is not the splitting of the dark period into 

 its smallest possible parts that is decisive: With long light periods of 

 14 hr or more, flowering is possible even if the dark periods are 

 extended up to 24 hr or more. This again shows that the light period 

 initiates a timing reaction, which after some 12 to 14 hr brings the 

 plant to a photophil status. 



It is known that in certain species the onset of the light period is 

 more effective in setting the phases of these cycles, while in other 

 species it is the onset of the dark period. 



3. Light breaks in 48-hr cycles. If light breaks are offered in a 

 prolonged dark period of, for example, 40 hr, alternating with short 



