PLATE i9 
Poinsettia: 
A —Beginning July 9, the plant at left was exposed to a 10-hour day. Before the 
dole ofAurn “tkls plmt had "flowered and was rlchlyaflored The control plant at 
right, exposed to the P seasonal length of day, of course, did not 
This experiment is of interest in showing that the green chlorophyll pigments of the 
leaf may give way to other pigments with appropriate change in the light period. 
Photograph mad^Septemb ? ^ ^ ^ ^ feju ^ effected in 
the weak, pendent shoot which appeared at the tip of the stem bearing an inflores 
cence as shown in Plate 1. This rejuvenescence was accomplished by exposure to 
a daily illumination of about 18 hours, after flowering had been induced byexpiMure 
toa io-hour day as shown in Plate 19, A. The U-shaped portion of the woody stem 
resulted from the pendent position of the new shoot as it first appeared. The short 
horizontal section of stem is that on which the inflorescence was borne. Plant at 
left illustrates the tall, spirelike development of the stem, absence of branching, an 
progressive loss of lower leaves which yield their plastic nutrient materials to tiieupper 
pl^t p^, all of which are typical features of the response to a long day. Photo- 
graphed July 13. 
