558 RHYTHMS IN PLANTS AND ANIMALS 



the same basic phenomenon as in flowering is indicated by the action 

 spectra of both flowering and stem elongation of peas, and even in seed 

 germination the same pigment is involved (Hendricks, 1956). 



The most disturbing new fact in photoperiodism is that temperature 

 can substitute for light. De Zeeuw (1957) and also Nitsch (p. 225) 

 found that Xanthium grown in a 16-hr photoperiod at 23 °C would not 

 flower, as is well known. But when the first 8 hr of the photoperiod were 

 spent at 4°, the plants were induced. A very similar effect was found by 

 Paton (unpublished) for peas. These are (at least the variety Green- 

 feast) long-day plants; in short days flowering is retarded for as much 

 as several weeks. When there is a 7-14° temperature drop or rise in 

 the middle of the dark period, they flower as early as the same plants 

 on a long photoperiod. We are only just starting to collect facts on the 

 effects of thermoperiod on flowering; we already know other cases 

 where temperature alters the flowering response to photoperiod (straw- 

 berries, Bidens). A good analysis of the effects of thermoperiod as 

 compared with photoperiod has been carried out only for vegetative 

 growth. 



Is there any further evidence of the reality of the Bunning cycle in 

 plant development? For this we have to return to some of the earliest 

 papers of Biinning (1932) in which he shows the existence of an 

 autonomous cycle in the nyctinastic movements of leaves of Phaseolus. 

 When he kept his plants in darkness at different temperatures, the 

 autonomous cycle was shghtly different, and he found a Qxo of 1.21- 

 1 .25 for the temperature effect on the rate of the cycle. These experi- 

 ments were not very convincing because of the relatively poor re- 

 sponses of the plants, so that the results of Leinweber (1956), claim- 

 ing that there was no temperature coefficient for the Biinning cycle, are 

 apparently generally accepted, even by Biinning himself, of whom 

 Leinweber was a student. In the meantime, however, I had set up a 

 lapse-time movie camera with which plants could be photographed in 

 extremely weak infrared light. This setup was placed in different dark- 

 rooms at different constant temperatures. Preliminary results show 

 that at 23° the nyctinastic movements of Phaseolus have a period of 

 23.1 ± 0.5 hr (see Fig. 3). At 14° the period is 30.5 hr. These points 

 lie exactly on Bunning's original curve (1932). Therefore I assume 

 that Biinning was correct and that Leinweber introduced in his experi- 



