544 
HERBERT C. HANSON 
per square decimeter, a ratio of aboat 5:1. As the readings for the 
third set were made in the morning and those of the first set in the 
afternoon, the difference in the ratios of the two sets are most Hkely 
due to the lower humidity, higher temperature and stronger Hght, 
causing greater differences between the center and south peiiphery 
during the first experiment. 
The differences in the transpiration of Ulmus fulva leaves is about 
twice as great as the differences in Fraxinus pennsylvanica leaves of 
July 1 1 . This greater ratio between the exposed and sheltered leaves 
in Ulmus is partly due to weather conditions. On July 11 the tem- 
perature was lower, the humidity higher, and the sunlight was less 
bright than on July 22. The physical factors in the center and at the 
south periphery were therefore more alike. 
On June 23, 1916, a cloudless, warm day with a light breeze, an 
experiment was performed on a well-formed isolated Acer saccharinum. 
Two potometers of south periphery leaves were prepared, one was 
placed at the south periphery, the other at the center. A potometer 
of center leaves, also, was placed at the south periphery. At the end 
of 50 minutes readings were made and the positions of all potometers 
were changed from south periphery to center or vice versa, and allowed 
to run 50 minutes after about 5 minutes for adjustment had been 
allowed. 
The potometers, containing south periphery leaves, placed at the 
south periphery, lost 10.5 cc. and 11.95 cc. When these were moved 
to the center of the tree the losses were 3.95 cc. and 2.85 cc, respect- 
ively. The center leaves lost 5.4 cc. at the south periphery and 1.2 
cc. in the center. The temperature at the periphery of the tree was 
practically the same during the 105 minutes. The evaporation from 
Livingston's porous cups was 4.6 cc. in 50 minutes at the south peri- 
phery and 3.4 cc. in the center. 
The amount of water lost by transpiration is increased from about 
3 to over 4 times when the potometer is changed from center to the 
south periphery. The small differences in temperature and evapora- 
tion in the two positions compared to the great differences in trans- 
piration show that plants, compared with mechanical apparatus, are 
more sensitive to environmental factors. Comparison cannot be 
made between the three potometers in this experiment as the leaf area 
was not measured. Comparison can be made only between the posi- 
tions of the same potometer. 
