LEAF-STRUCTURE AS RELATED TO ENVIRONMENT 
543 
The losses of the south periphery leaves were from 3 to 6 times as 
great as those from the center leaves when placed in their native 
situations. It is interesting to note that the south periphery branch 
in the center of the crown lost more water than the center branch at 
the south periphery. This may be due to several causes; probably 
the scomata in the leaves of the center branch closed when exposed to 
the sun (but other experiments indicate that this was not the case), 
or the center leaf may have been transpiring up to its full capacity. 
The south periphery leaf had greater capacity for transpiration than 
the center leaf, because of its greater amount of solid matter and 
chloroplasts. Graphs for the two potometers of center leaves give 
evidence by their parallelism for this view, indicating that the center 
leaves placed in the center are transpiring at almost full capacity. 
Sampson and Allen (29) account for the greater transpiration of the 
sun leaves, because they have from 20 to 60 percent more stomata. 
Hesselman (23) accounts for the increase in transpiration in his state- 
ment that the leaf surface being equal, plants transpire more as they 
have greater development of palisade. Bergen (4) explains the 
greater transpiration in the sun leaves by their greater activity, by 
their greater thickness affording a larger interior evaporating surface, 
and by their larger bundles and stems which would transfer the water 
more quickly. These graphs also show by their abrupt changes 
that the center leaves are more responsive to the environmental 
factors than the south periphery leaves. This is explained by 
the greater amount of protective material, as thicker cuticle, greater 
thickness of the leaf and more solid material in the south periphery 
leaves. 
On July 22, 1916, a clear, hot day, w^ith a hght south breeze, three 
sets of branches were used in an experiment on an isolated Ulmiis fiilva 
Mich. Each set contained one potometer of a south periphery branch 
placed at the south periphery, and one potometer of a center branch 
placed at the center. The sets were ran from i hours to 2}/^ hours, 
readings being taken every 15 minutes. The greatest differences in 
the water loss between the south periphery and center leaves occurred 
in the first set where in ij^ hours the south periphery leaves lost 1.956 
cc. per square decimeter, while the center leaves lost 0.16 14 cc. per 
square decimeter, a ratio of about 12: i. The least differences were 
in the third set, where the south periphery leaves in 2j^ hours lost 
3.10 cc. per square decimeter, while the center leaves lost 0.638 cc. 
