PLATE 17 
A. —Stems of Cosmos, grown in winter in greenhouse. Electric illumination was 
used from sunset till midnight as a supplement to the natural daylight. Sections of 
stems in center and at right show the effects of cutting back m causmg local swelling 
of the nodes, followed by development of new branches. This behavior indicates a 
strong upward flow of sap from below, resulting from exposure to a length of day fav¬ 
orable to elongation of the vegetative stem. This behavior is not observed when toe 
plant is exposed to short-day conditions. The section of stem at left illustrates toe 
condition of the nodes at the time cutting back was done. . . 
B. —Hibiscus moscheutos. Seedlings of previous summer s growth which remained, 
out of doors during toe winter. Beginning March 27, toe two plants at left were 
given a io-hour day while toe control plants at right continued to receive toe full 
seasonal length of day. Under the io-hour day small, weak basal shoots finally ap¬ 
peared in May, as seen in front of the cardboard. These shoots were unable to grow 
and soon perished. New shoots appeared on toe controls early in April, and these 
continued to develop normally. Photographed June 21. 
