A Study on the Effect of Evaporation and Light on the 
Distribution of Lichens 
CHARLES C. PLitr AnD Lours J. PEssin 
An investigation has recently been completed of the effect 
of climatic conditions on the distribution of Polypodium poly- 
podioides in Mississippi by the junior writer. It was found 
that the evaporating power of the air plays the main réle in 
determining the distribution of this epiphytic fern. The inten- 
sity of the light, the humidity of the air, the temperature in the 
immediate vicinity of the tree and the temperature of the bark 
to which the fern was attached exerted only an indirect influence 
on the occurrence of this polypody.* 
The aim of the present paper is to give the results obtained 
from a study of the effect of certain climatic conditions on the 
distribution of the lichens on a tree on Mt. Desert Island, Maine, 
Lat.44°26’ N. The writers sought to determine what correlation 
exists between the evaporating power of the air, the intensity 
of the light, and the distribution of the lichens along the height 
of the tree and around its circumference. 
A tree (Quercus rubra) a little over thirteen meters in height 
and about one-half meter in diameter at two meters from the 
ground was selected for the investigation. The tree was located 
in a rather dense woods in the immediate vicinity of the Harps- 
well Laboratory. A small part of this oak projected above the 
tops of the neighboring trees. From the base to the top of tree, 
zones, about a meter apart, were measured off vertically along 
the tree. In each individual zone a careful study of the lichen 
flora was made. Three of the zones were selected for the study of 
the climatic conditions. These were located at 1 to 2 meters, 
5.5 to 6.5 meters, and 9 to 10 meters above ground. In each of 
three selected zones a white, spherical, porous-cup atmometer 
was set up on the northeast, northwest, and south sides. Eva- 
poration was recorded once a day, generally at nine o’clock in 
the morning. At these stations the light intensity was -also 
measured. Light readings were usually made at nine and eleven 
* This paper will appear in ECOLOGY within the year. 
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