296 



CONTROL OF REPRODUCTION 



Fig. 6. The COo metabolism of Kalancho'e during a prolonged light 

 period of 24 hr. 



INDUCTION OF CO., FIXATION 



The flowering response to photoperiodism may be induced; that is, 

 after a certain number of cycles of a favorable day length have been 

 given, flower formation will proceed even though the plant is main- 

 tained under unfavorable day lengths. In Xanthium only one photoin- 

 ductive cycle is required, but with Kalancho'e, under the conditions 

 employed in these experiments, 13 to 14 short-day cycles are required 

 for a minimum flowering response and 20 to 25 short-day cycles for 

 a maximum response. The result of an experiment to determine 

 whether the CO2 metabolism associated with short days could also be 

 induced is shown in Fig. 7. The broken line represents the metabolism 

 of this plant under long-day conditions; the solid line the CO2 metabo- 

 lism during the twentieth short-day cycle; and the dotted hne the CO- 

 metabolism after the plant had received 21 short days followed by 15 

 long days. By comparing the dotted and broken curves, it is evident 



