PLATE 4 
A. —Apple seedlings. Plants at right were exposed to io hours of light daily during 
the summer months; those on left received the full daylight exposure of summer. It is 
apparent that growth is more rapid and vigorous with only io hours of light daily than 
with the 14 to 15 hours of daylight of summer in the latitude of Washington. This 
would indicate a tendency toward an increased rate of growth with decrease in latitude. 
B. —Seedlings of Acer negundo. Beginning May 2, the plants at left were exposed 
to a io-hour day while the plants at right were exposed to the full day length of summer. 
It is obvious that in contrast with the apple this species grows vigorously under long- 
day conditions and could not flourish under the length of day prevailing at Washington 
during the winter months. Photographed September 22. 
27975 — 23—7 
