20 
The Ohio Naturalist. 
[Vol. X, No. 2, 
great. The importance of pioneer shade plants as reducers of 
transpiration is also pointed out. 
The instruments used at Buckeye Lake were a slight mod- 
ification of those used by the writers just mentioned. The 
evaporation takes place from the surface of an exposed porous 
clay cup, about thirteen centimeters in length, two and one-half 
cm. in diameter, and with a wall of four millimeters thickness. 
The upper end is closed, and rounded, the lower end is closed 
tightly by a perforated rubber stopper, through which passes a 
glass tube. This tube extends down to the bottle below, which 
serves as the reservoir of water. vSince the instruments were to 
be left for rather long periods of time, a larger and more stable 
form of reservoir was required. In place of the “ Mason” jar and 
cork stopper, a bottle of 5000 cc. capacity was used, with a neck 
of sufficient slope so that the water level could readily be seen 
from above. At the mouth of the bottle, the glass tube passed 
through two rubber stoppers, the one a two-holed stopper 
inserted in the bottle, and the other with its large end down, cov- 
ering the hole, and preventing the entrance of water, but allow- 
ing free access of air. A file mark near the top of the bottle 
indicated the point to which the water level was raised on refilling. 
Throughout the experiment only distilled water, containing a 
small per cent of formaldehyde, was used. 
The interior of the cup remains free from air because of the 
surface tension of the water films closing the pores. The cup 
thus remains filled with water, and as evaporation takes place at 
the surface, more water is forced up from below into the vacuum 
by the air pressure upon the water surface in the reservoir. 
The porous cups used in this work were obtained through 
Dr. Dachnowski from the Carnegie Institute and were standard- 
ized at the Desert Laboratory at Tucson. 
When used during the growing period of plants, the principal 
defect of this instrument is that rain may enter the reservoir 
through the porous cup, and thus cause an error in the results. 
If daily readings are taken, the length of time of precipitation 
can be recorded, and corrections made for the error. But in 
taking readings at intervals longer than a day, this error must be 
neglected. 
One instrument was placed in a station of the Maple-Alder 
zone near the border of the island and was shaded by Acer 
rubrum, Alnus rugosa, and Rhus vernix. Osmunda cinna- 
momia, O. regalis, and Dryopteris cristata were growing nearby. 
The other station was in the central zone, where the principal 
plants were Sphagnum, Oxvcoccus oxycoccus, Drosera rotundi- 
folia, Eriophorum virginieum, and Dulichium arundinaceum. 
Readings were begun May 14, and taken weekly until June 11. 
No data were taken then until July 17, when the evaporation for 
five weeks was recorded. Weekly readings were then resumed 
and taken until August 21, when after another break of three 
