PLATE i 
A .—Bidens frondosa L. Plants from seed which germinated December i. The 
individual at right was exposed to the natural daylight period of winter supplemented 
with electric illumination of low intensity from sunset till midnight each day. Indi¬ 
vidual at left received similar treatment till January 30 after which it was exposed 
to the natural daylight period only. Shortening the duration of the daily illumination 
g eriod promptly initiated flowering and open blossoms were seen March 1, the plant 
aving attained a height of 15 inches. When the photograph was made, April 17, the 
plant at left had ripened its seed and was dead. The plant at right, which remained 
under the long daily illumination, had attained a height of 6 feet without flowering. 
This species shows the typical behavior of short-day plants. 
B.—Late-flowering cosmos (C. bipinnatus Cav.), showing change from the flowering 
condition back to the vegetative stage as a result of increase in length of the daylight 
period to which the plant was exposed. This plant, which germinated May 15, was 
exposed to a io-hour day till June 13, and thereafter it was exposed to the full daylight 
period of summer. Flower buds appeared as early as June 3 and flowering had begun 
by June 19. It is apparent that when photographed, July 20, the new branches 
developing under long-day conditions were primarily of the vegetative type, though 
some flower buds were still developing. Though the indeterminate vegetative type 
of growth is thus restored by increase in length of the daily illumination period, the 
flowering condition, once it has been established, is completely destroyed only very 
slowly and with difficulty, if at all. 
