132 ILLINOIS ACADEMY OF SCIENCE 
exposed to wind from all directions, and to the sun throughout 
the day. This represented the most xerophytic type of habitat 
available upon the dunes. 
Transpiration readings were recorded by means of the cobalt 
chloride hygrometric paper. Correlative readings were taken 
ot atmospheric temperature, relative humidity, direct evaporat- 
ing power of the air, soil moisture, soil temperature, and wilt- 
ing coefficient. These were measured that the true relation of 
the relative mesophytism of the stations might be better under- 
stood, and that their relation to the foliar transpiring power 
might be expressed. 
It was found that the transpiring power at station A. was 
less than that of any other station, and the water loss at sta- 
tion E. was the greatest. Stations B, C, and D, showed an in- 
crease over A. which was directly proportional to the increase 
in xerophytism. 
Transpiration was most rapid at the stations where the low- 
est average soil water and the most xerophytic leaf structures 
were found. 
It was discovered that the amount of soil moisture had no 
appreciable effect upon the transpiration stream except when 
the wilting coefficient of the soil was approached. — 
Light, wind, and relative humidity were the factors which 
most strongly influenced the fluctuation of the current. The 
transpiration index was found to rise very rapidly just after 
dawn, and more apace upon the open sands than in the shaded 
forest. 
A “saturation deficit” was developed in most instances and 
was much more in evidence upon the open dunes than in the 
forest. In fact in the forest the deficit did not appear in many 
of the readings, especially when the day was cloudy and the 
relative humidity high. 
Although the saturation deficit was very strongly developed 
upon the open sand, there was never any visible evidence of 
wilting there. In the early summer the soil water content in 
*A more extended report of the work on Tilia appeared i in Bot. Gaz. 68: 262-286. 1919. 
“‘Comparative Studies of the folia Transpiring Power.” 
