188 JOURNAI, OF FORESTRY 



2. The evaporation, measured by the Livingston porous cup atniom- 

 eters, was found to be from i^ to 2^ 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 per cent to 6 per cent higher within the crown. 



5. A wind of low velocity caused greater differences in the air move- 

 ment between the center and the periphery of the crown than a strong 

 wind. The wind, was found to be from i^^ 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 Fra.vinits 

 pennsylvanica, the south periphery leaves lost from 3 to 6 times as much 

 as the center leaves; in Uhnus americana, about 12 times as much. 

 Even w4ien the potometer containing south periphery leaves is placed 

 under similar conditions with the potometer containing 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 tolerant 

 trees is less per unit area than the dry weight of the leaves of the least 

 tolerant trees, as leaves from Acer saccharmn contain 1.029 grain of 

 dry matter per unit area, while leaves from Quercus macrocarpa con- 

 tain 1.272 grains. 



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 epider- 

 mis and cuticle than the leaves from within the crown. 



E. R. H. 



American Journal of Botany, November, 1917, Vol. 4, pp. 533-560. 



