INDUCTION OF FLOWERING 313 



under some of the treatments. The flowers which had been initiated 

 under 8-hr days were the most advanced in their development (Fig. 

 lA). Under 16-hr days, there were no flowers at 23 °C (Fig. IB) but 

 definite inflorescences when a 4°C temperature was given during the 

 first 8 hr of the light period (Fig. IC). A similar 8-hr treatment with a 



Fig. 1. Growing points of Xanthiiini plants of the same age photo- 

 graphed 15 days after the beginning of the following treatments. A. 8-hr 

 days at 23 °C. B. 16-hr days at 23 °C. C. 16-hr days at 4°C during the first 

 8 hr, 23 °C during the subsequent 8 hr of light. D. 24-hr days at 4°C dur- 

 ing 8 hr, 23°C during the remaining 16 hr. (In all cases, the leaves have 

 been removed in order to photograph the growing points.) 



4°C temperature produced no flowers under continuous light (Fig. 

 ID). 



In summary, cockleburs which do not flower under long days of 16 

 hr at 23 °C can be induced to flower under the same long days when 

 the temperature is dropped to 4°C during the first 8 hr of the long 

 days (Fig. 2). 



This experiment may be interpreted by saying that a labile com- 

 pound promoting flowering is formed during the first part of the dark 

 period. When the dark period is too short, this labile compound does 

 not have time to be transformed into a stable one and is destroyed 

 during the subsequent light phase. This destruction may be prevented 

 by dropping the temperature. 



The results presented here have also a more general meaning. They 



