HERBERT C. HANSON 
549 
tensity of 0.0024, fifteen centimeters above the ground. The thickness 
of these leaves was 30 percent the thickness of the south periphery 
leaves of the isolated tree. The palisade and sponge tissue were very 
loose. 
Tilia americana. — The total thickness of the center leaves of 
isolated Tilia americana in Minneapolis was 52 percent the thickness 
of the south periphery leaves. This difference was caused chiefly by 
Figs. 5-8. Tilia americana. Fig. 5, Isolated tree. Leaves from south peri- 
phery. Fig. 6, Isolated tree. Leaves from center of crown (Lincoln). Fig. 7, Iso- 
lated tree. Leaves from center of crown. Fig. 8, Forest tree. Leaves from base. 
the great increase of palisade tissue, the center leaves having only 
22 percent as much palisade as the south periphery leaves. The 
palisade tissue composed 34 percent of the thickness of the center 
leaves, and 81 percent of the other. The sponge tissue is changed to 
palisade in the south periphery leaves. There are four layers, usually, 
of palisade in the latter leaves and only one in the former. The cells 
in the center leaves are larger, more irregular, more often funnel- 
shaped, the air-spaces are larger and more numerous, the bundles and 
