LEAF-STRUCTURE AS RELATED TO ENVIRONMENT 557 
the Upper part consisting of three or four layers of very compact cells, 
the lower part of four or five layers of irregular cells; while in the center 
leaves there was usually but one layer of palisade. The bundles and 
water-storage cells, and the crystals were more numerous in the former 
also. 
The weight of the center green leaves was 58 percent that of the 
south periphery leaves, the weight of the water-free leaves 28 percent. 
The water content of the former leaves was 67 percent; of the latter, 
33 percent. 
The amount of evaporation at the center was 53 percent that at 
the south periphery; the wind, 65 percent. The light at the center 
was 0.015. 
Celtis occidentalis L. — The thickness of the center leaves of an 
isolated Celtis occidentalis at Lincoln was 63 percent the thickness of 
the south periphery leaves. All of the sponge tissue in the center 
leaves was palisaded in the south periphery leaves, so the palisade in 
the former is only 31 percent that in the latter. The differences in 
structure were again found in the compactness of the cells, the shape 
of the cells, and the cystolithic cells were more abundant in the south 
periphery leaves. The light in the center was 0.059. 
Platanus occidentalis L. — The thickness of the center leaves of an 
isolated Platanus occidentalis at Lincoln was 61 percent the thickness 
of the south periphery leaves. The palisade tissue in the center leaves 
was 46 percent that of the latter. The palisade made up 32 percent 
the thickness of the center leaves, the sponge 38 percent, the upper 
epidermis 17 percent, the lower epidermis 13 percent. The palisade 
made up 42 percent the thickness of the south periphery leaves; the 
sponge 38 percent, the upper epidermis 12 percent, the lower epider- 
mis 8 percent. The south periphery leaves had more compact tissue, 
the cells were more prolate, although there was but one layer as in the 
center leaves. The scalloped appearance of the cross section of the 
south periphery leaves was caused by the greater bundle and water 
storage tissue development as compared with the center leaves. 
Summary 
I. The light intensity, as measured by the Clements photometer, 
within the crown of 10 common broad-leaved trees was found in 
August to vary from .0076 of full sunlight in Acer saccharuni to .1132 
in Quercus macrocarpa. 
