LEAF-STRUCTURE AS RELATED TO ENVIRONMENT 539 
The order of the light values in isolated trees, beginning with the 
lowest, is seen to be Acer saccharum, Acer saccharinum, Fraxinus 
pennsylvanica, Celtis occidentaUs, Tilia americana, Ulmus americana, 
Acer negundo, and Quercus macrocarpa. In the forest the order is 
Acer saccharum, Tilia americana, Quercus alba, Quercus rubra, and 
Quercus macrocarpa. Comparing the isolated and forest individuals 
of the same species it may be noted that the trees in the forest have 
the lower light intensity. 
2. Evaporating Power of the Air 
Livingston's standardized porous cups were used for measuring the 
evaporating power of the air. The cups were placed in the trees be- 
tween 8 and lo A.M. and taken down about 5 P.M. The racios of the 
amounts of water evaporated from the crown to that from the south 
periphery in isolated trees, or from the base to the apex of the crowns 
of forest trees were lowest in isolated trees of Ulmus americana and 
Acer negundo (i : 1.44, i : i 48) and greatest in isolated Acer saccharinum 
and Tilia americana (1:2.3, 1:2.2). The ratios of five other species 
ranged between these. The ratios in the isolated trees were always 
less than the ratios in forest trees of the same species, for example, in 
the isolated tree of Quercus macrocarpa the ratio was i : 1.3, while in the 
forest tree it was i : 2.2; This is due to the greater humidity at the 
base of the forest tree as compared with the humidity within the crown 
of the isolated tree. 
The temperature and humidity data indicate that the differences 
in the amounts evaporated in the sun and in the shade positions cannot 
be explained by these two factors alone. The movement of the air 
seems to be the controlling factor. As the water evaporates from the 
cups in the exposed situations it is rapidly carried away from the 
vicinity of the cups by air currents. As the wind data show, there is 
much less air movement within the crown than at the south periphery, 
so it is very probable that a vapor blanket is formed about the cups 
within the crown. This blanket would very likely be formed also 
about each leaf, Renner (28) concluded that the thickness of the 
vapor blanket varies with the leaf size. In this way the saturation 
deficit would be decreased, and evaporation and transpiration lowered. 
Radiant energy may also play an important role in causing the dif- 
ferences, because it is greater in the exposed than in the sheltered 
positions. As the readings were not taken upon all of the trees at the 
