LEAF-STRUCTURE AS RELATED TO ENVIRONMENT 555 
bundles and water storage cells. The weight of the green center leaves 
was 60 percent the weight of the south periphery leaves, the weight of 
the water- free center leaves 42 percent. The water content of the 
center leaves was 68 percent, of the south periphery leaves 55 percent. 
Figs. 19-20. Platanus occidentalis. Fig, 19, Isolated tree. Leaves from south 
periphery (Lincoln). Fig. 20, Isolated tree. Leaves from center of crown (Lincoln). 
Fig. 2 1 , Quercus rubra. Forest tree. Leaves from apex of crown. 
The evaporation at the center was 65 percent that at the south 
periphery, and the light intensity .038. 
The studies of several trees at Lincoln showed that the south 
periphery leaves of these trees were from 13 percent to 42 percent 
thicker than the south periphery leaves of Minneapolis trees. The 
increase was due to the increase of palisade chiefly. The structure of 
the south periphery leaves from Lincoln was more xerophytic. The 
center leaves w^ere about the same in both places. 
Acer negundo. — The thickness of the center leaves of an isolated 
Acer negundo tree at Minneapolis were 79 percent that of the south 
