LEAF-STRUCTURE AS RELATED TO ENVIRONMENT 
545 
2. Surface Area and Lobing 
In nearly all cases the leaves from the center or from the base of 
the crown were larger than those from the south periphery or apex. 
The greatest differences were found in the second crop of leaves during 
the season on forest forms of Tilia americana, in the first crop leaves of 
isolated Fraxinus pennsylvanica and in the first and second crops of 
leaves of isolated and forest individuals of Quercus macrocarpa. The 
production of a second crop of leaves was due to the warm weather in 
June following a cold spring. The least differences in surface extent 
were found in the isolated Tilia americana and Acer saccharum. Us- 
ually the leaves from the exposed positions were more deeply and 
narrowly lobed, and more prominently toothed than the leaves from 
the protected positions. The lobing of the south periphery and the 
apex leaves of Acer saccharum was less deep than in the center or base 
leaves. 
3. Green and Dry Weights: Water Content 
Two methods were employed in determining the green and dry 
w^eights and the areas of the leaves weighed; one by means of the 
Ganong leaf area cutter, the other by weighing entire leaves and then 
determining their area by means of the proportional weights, using 
solio paper for the leaf prints. Sufficient material was used in each 
method so as to render the error negligible. The leaves were always 
collected late in the afternoon. The green and dry weights per square 
decimetei- of leaf surface and the water content are given in the fol- 
lowing table : 
In every case the green and dry weights were lower, and the water 
content higher, in the shaded than in the sunny positions. The ratio 
between the weights is greater in the dry weights than in the green 
weights, showing that more solid material was laid down in the leaves 
where the light and other factors were more intense. The highest 
water content was found in the shade leaves of Acer saccharum, A. 
saccharinum and Fraxinus, while the lowest was in Quercus alba and 
Q. rubra. The greatest differences in water content between the sun 
and shade leaves were found in Fraxinus and Acer saccharum, while 
the smallest occurred in Quercus rubra and Q. alba. According to the 
amount of dry material in the leaves at the south periphery or apex 
the trees fall into the following order: Fraxinus 1.639 g., Q. macro- 
carpa 1.272 g., Q. rubra 1.190 g., Q. alba 1.173 g., Tilia americana 
