312 



CONTROL OF REPRODUCTION 



Vigorous Xauthium plants (J. Bonner's strain) were raised under 

 smog-free air in tlie Earhart Laboratory under long days (full day of 

 July plus additional incandescent light to produce days of 18 hr). The 

 temperature was 23 °C (day) and 17°C (night). At time zero, the 

 plants were distributed among the various treatments, 8 replicates be- 

 ing used per treatment. During the following 13 days, the plants re- 

 ceived, during the light periods, artificial light of about 1,000 ft-c 

 (mixture of fluorescent and incandescent light). At the end of the 

 experiment, the plants were returned to the original, long-day condition 

 under natural light. At that time, the flower buds were macroscopically 

 visible on the plants in which flowering had been induced. Photographs 

 were taken 2 days later. 



The results are presented in Table I. At 23 °C (day and night), no 



Table I. Effect of V'arious Light and Temperature Treatments on the Flowering of 



Xauthium pcnsyhanicum 



" Eight replicates in each case. 



flowering was obtained when the dark period was 0, 4, or 8 hr, but 

 complete flowering was obtained when the length of the dark period 

 was 16 hr. Complete flowering could be achieved with a 16-hr day, 

 however, when low temperature (4°C) was given during the first 8 hr 

 of this long day. Various temperature treatments given during con- 

 tinuous illumination were unable to cause flowering. 



Figures 1 and 2 give the aspect of typical growing points produced 



