574 



ANNUAL REPORT SMITHSONIAN INSTITUTION, L92Q. 



at 4 o'clock in the afternoon each day and again withdrawn at 9 

 o'clock the following morning. The plants thus received only seven 

 hours of light daily, while, for comparison, other similar cultures 

 Avere left exposed to the light throughout the day. In the first tests 

 a variety of soy bean known as Peking, which flowers earlier than 

 the Biloxi, was used and the plants were placed in the dark chamber 

 for the first time on July 10, at which date they had just begun to 

 blossom. The Maryland Mammoth tobacco plants, which had been 

 planted April 14, also were first placed in the dark chamber on July 





S 





/ /s / 



/s 



0CF 



FlG _ i. — Graph showing changes in length of day during the growing season in the lati- 

 tude of Washington, D. C. Ordinates indicate 2-hour intervals of the day and abscissae 

 indicate 16-day periods of the growing season. 



10. In a short time it became evident that both soy bean and tobacco 

 were responding to the artificially shortened length of day. Before 

 the end of July the seed pods of the soy bean were full grown, while 

 on the control plants remaining out of doors none of the pods were 

 more than half grown. By September 1 the leaves of the test plants 

 had yellowed and were falling and many seed pods were fully 

 matured, while both leaves and seed pods on the control plants were 

 still green. The appearance of the two sets of soy bean are well 

 shown in plate 4. The tobacco plants exposed to the shortened day 

 length began to flower August 26, while the first blossoms ap- 

 peared on the control plants remaining out of doors on October 7. 

 These results were so striking that arrangements were made for much 

 more extensive tests in 1919 with soy bean and tobacco, as well as 

 with many other species. 



