548 
HERBERT C. HANSON 
the same part of leaves of like age and that the measurements were 
made in typical parts of the sections so that no error would be caused 
by the thickening due to fibro-vascular bundles. 
Acer saccharum. — The study of an isolated tree showed that the 
center leaves were on an average 38 percent as thick as the south 
periphery leaves. This increase was caused mostly by the great 
palisade development, the thickness of the palisade tissue in the center 
leaves being about 25 percent that of the south periphery leaves. In 
the center leaves the palisade made up 38 percent of the total thick- 
ness, while in the south periphery leaves it made up 58 percent. The 
thickness of the sponge tissue and upper epidermis was about one half 
as great, and the lower epidermis three fourths as great in the center 
leaves as in the south periphery leaves. 
Great differences in structure were found in the leaves from the 
two positions. The south periphery leaves had two layers of palisade 
and these layers were far more compact than the single layer in the 
center leaves. The sponge tissue was more compact, the bundles 
and water storage tissue more abundant, the cells in the upper and 
lower epidermis more regular, and the number of crystals greater in 
the south periphery leaves than in the center leaves. 
The weight of the green leaves per given area and the weight of 
the water-free leaves in the center were 46 percent and 38 percent of 
the weights at the south periphery. The water content based on 
green weight of the center leaves was 75 percent, and of the vsouth peri- 
phery leaves 52 percent. 
Factor measurements showed that the amount of evaporation and 
the rate of the wind in the center of the tree were respectively 67 per- 
cent and 28 percent of the amounts at the south periphery. The 
light within the crown was 0.0086, while at the south periphery it 
was 1. 00. 
Less pronounced differences were found in the leaf-structure of 
trees growing in the forest. The thickness of the leaves growing at 
the base of the trees were 44 percent the thickness of the leaves at the 
apex. The apex leaves were 77 percent the thickness of the south 
periphery leaves of the isolated tree and the base leaves 92 percent the 
thickness of the center leaves. The weight of the green leaves and the 
weight of the water-free leaves of the base leaves were 47 percent and 
35 percent the weights of the apex leaves. The light intensity of the 
base leaves was 0.0076. Leaves were found growing in a light in- 
