540 
HERBERT C. HANSON 
same time, comparisons between species cannot be made except in a 
very general way. 
3. Temperature 
Numerous simultaneous readings of temperature were made in the 
shade within the crown and in the sun at the south periphery upon 
several isolated trees of Acer saccharum, A. saccharinum, Tilia ameri- 
cana and Fraxinus pennsylvanica. The greatest differences between 
the two positions were 2.8° C. at 2:50 P.M. on Acer saccharum and 
1.8° C. at 1:00 P.M. on Tilia americana. Usually the difference was 
about 1° C. This difference was caused by the stronger light at the 
south periphery. 
4. Humidity 
Humidity readings were made with cog psychrometers simulta- 
neously within the crown and at the south periphery of the same trees 
that were used for temperature readings. The greatest differences 
were found on Acer saccharum, 16 percent, at noon. The greatest 
difference in the case of Tilia americana was 9 percent at 2 P.M. The 
usual differences in the humidity of the two positions was from i per- 
cent to 2 percent. The greatest factor in causing these differences 
was the movement of the air. Dense crowns impeded the free move- 
ment of the air far more than open crowns, so that trees with dense 
crowns, as Acer saccharum, showed greater differences than trees of 
open crowns. The greatest saturation deficits were always found on 
the exposed parts of trees from i P.M. to 3 P.M. 
5. Wi7id 
Numerous readings of wind velocity were taken by hand anemom- 
eters, operated simultaneously in the two positions. The ratios of the 
velocity within the crown to that at the periphery have been deter- 
mined from several readings on each tree. In Acei saccharum the 
ratio was 1:2.2; in ^. saccharinum, 1:2.1; in Quercus macrocarpa, 
1 : 2.0 ; in Ulmus americana, i : i .6 ; in Tilia americana, i : i .4 ; in Fraxinus 
pennsylvanica, i : 1.4; in Acer negundo, i : 1.3. 
The amount of air movement within the crown depends upon the 
openness of the crown. The two extremes in crown density in the 
above series were Acer saccharum and Acer negundo. As seen from the 
figures the least air movement within the crown in comparison to that 
at the south periphery was found in the former, and the greatest in the 
