PLATE 16 
A. —Bidens frondosa. Seedling planted in greenhouse; germinated December 23. 
Beginning January 30, when flowering had begun, the plant received electric illumi¬ 
nation from sunset till midnight, in addition to the natural daylight. The total 
original growth of the plant attained under natural illumination, including ripened 
fruits, is seen in front of the cardboard. As is shown, this original stem growth died, 
but new shoots appeared, and these reached giant proportions under the influence of 
the long daily illumination period. Thus, rejuvenescence was effected by use of a 
favorable light period. This plant may be regarded as embodying two different 
types of development at different periods in response to change in the duration of the 
daily light period. Photographed June 21. 
B. —Cosmos. The branch at right flowered promptly in response to the short winter 
day to which it was exposed, and the usual evidences of advancing senility followed 
in due course. The branch at left of screen remained sterile and continued to grow 
in response to the added electric illumination from sunset till midnight. The domi¬ 
nant influence of this branch over the flowering branch in its effect on basal portions 
of the plant is evident. In this instance two different types of development take 
place simultaneously on the same plant in response to difference in the light period 
(compare with A). Photographed March 25. 
