Response of Plants Relative to Length of Day and Night 
Plate 2 
A—Plants from seed which germinated September 25. Seedlings were grown in a greenhouse arti¬ 
ficially lighted from sunset till midnight each day until January 7 when test was begun. From that 
date control plant at right was exposed to the natural daylight period of winter, and began flowering 
February 9. Beginning January 7 lower portion of plant at left received the natural daylight only, 
while the upper portion received electric illumination from sunset till midnight in addition to daylight. 
In response to the short-day exposure, the lower portion showed flower buds by February 14, and the 
first open blossom appeared March 8. Responding to the long-day exposure, the upper portion re¬ 
mained in the vegetative condition and continued to increase in height. Photographed March 9 
B— Later stage of development of the Cosmos plant shown at left in 2, A. On May 7, when the 
natural length of day had increased sufficiently to inhibit flowering, the electric illumination was dis¬ 
continued and the plant was transplanted outdoors. Vegetative development of the upper portion 
continued, and when photographed August 18 the plant had attained a height of 11 feet. The lower 
portion of plant, however, continued to flower through the summer, as shown here 
