PLATE a 
A. —Rudbeck a bicolor Nutt. Plants from seed sowed December 19. The indi¬ 
vidual at left was exposed to the natural daylight Period of winter supplemented by 
electric illumination from sunset till midnight. The individual at right received 
only seven horn's of illumination daily. Photographed May 14. The behavior of 
Rudbeckia is typical of the group of plants which quickly flower in response to rela¬ 
tively long days, and therefore are designated as long-day plants. Ordinarily in this 
group the height attained is more or less directly proportional to the length of day. 
B. —Helianthus annum L. Seedlings which germinated January 23. The larger 
plants at right were exposed to the natural daylight period of winter and spring, while 
the smaller plants at left received only seven hours of illumination daily. Photo¬ 
graphed April 12. The time of flowering was not affected by the reduced illumination 
period, but the height attained was markedly decreased. On the other hand, if the 
illumination period is increased to 18 hours, the time of flowering is delayed. This 
sunflower shows some of the characteristics of both short-day and long-day plants. 
