558 
HERBERT C. HANSON 
2. The evaporation, measured by the Livingston porous cup at- 
mometers, was found to be from ij^ to 23/3 times as great at the south 
periphery as within the crown. 
3. The temperature at the south periphery was usually but one or 
two degrees higher than within the crown. 
4. The humidity, measured by cog-psychrometers, was usually 
from I percent to 6 percent higher within the crown. 
5. A wind of low velocity caused greater differences in the air 
movement between the center and the periphery of the crown than a 
strong wind. The wind was found to be from 13/3 to 8 times as strong 
at the periphery as within the crown. 
6. Transpiration experiments showed that the south periphery 
leaves lose more water per unit area than the center leaves. In 
Fraxinus pennsylvanica the south periphery leaves lost from 3 to 6 
times as much as the center leaves; in Ulmus americana about 12 
times as much. Even when the potometer containing south periphery 
leaves is placed under similar conditions with the potometer contain- 
ing center leaves it will lose more water per unit area. 
7. The leaves from the periphery of the tree were usually more 
deeply lobed, more prominently toothed, and smaller than the leaves 
from the center of the same tree. 
8. The water content of the leaves from the center of the tree was 
always higher than that of the leaves from the south periphery. The 
amount of dry material per unit area in the exposed leaves bears a 
relation to tolerance. The dry weight of the leaves of the most tol- 
erant trees is less per unit area than the dry weight of the leaves of the 
least tolerant trees, as, leaves from Acer saccharum contain 1.029 gr. 
of dry matter per unit area, while leaves from Quercus macrocarpa 
contain 1.272 gr. 
9. The differences in the total thickness between the south periphery 
and the center leaves on the same tree are usually greater than the dif- 
ferences heretofore reported from leaves of mesophytic and xerophytic 
forms of the same species. The leaves from the south periphery have 
more palisade tissue, greater compactness of structure, thicker epi- 
dermis and cuticle than the leaves from within the crown. 
This subject, the structural response of leaves of the same plant to 
measured environmental factors, is so large that this paper can only 
be considered as an opening wedge into further investigation. De- 
tailed studies are needed on specific aspects, as transpiration, water 
content, etc. 
