1912] GLEASON & GATES—RATES OF EVAPORATION 487 
the trees near the atmometer gave the following results: Carya 
cordiformis, 62.5 per cent; Quercus marilandica, 19.2 per cent; Q. 
velutina, 14.3 per cent; Celtis occidentalis, Gymnocladus dioica, and 
Quercus rubra, each 1 per cent. The proportion of the young trees, 
however, is very different. Between 80 and go per cent of them are 
Carya cordiformis. They dominate in every clearing, although 
occasionally they are accompanied by a few young trees of Quercus 
velutina. Young trees of Q. marilandica are not at all plentiful and 
occur only at the edges. The luxuriance of the growth of vines 
is the most prominent feature of the change from the Quercus 
velutina association. Nearly everywhere in the mixed forest vines 
are the conspicuous ground cover. This is never true of the typical 
Quercus velutina association. Most prominent of these is Psedera 
quinquefolia, which not only carpets the ground with green, but 
also climbs to a considerable height. More noticeable as climbers, 
however, are Celastrus scandens and Vitis vulpina. Shrubs are 
prominent only near the margin of the association, although a few 
species are scattered throughout. The commoner species of the 
herbaceous vegetation are Scrophularia leporella, Anychia cana- 
densis, Parietaria pennsylvanica, Hedeoma pulegioides, Asclepias 
phytolaccoides, Galium circaezans, Aquilegia canadensis, Galium 
pilosum, Silene stellata, and Cacalia atriplicifolia. Two atmometers 
were maintained in this association. One was located near the 
margin of the grove, while the other (fig. 5) was near the center. 
From an inspection of the data given in the diagram it is seen 
that the bunch-grass, the blowout, the Quercus velutina, and the 
mixed forest associations all show noticeably different amounts of 
evaporation. The instruments in these associations were located 
within half a kilometer of each other, in a region of comparatively 
uniform topography. The variation in the physical factors of the 
environment is not sufficient to explain the wide variation in the 
amount of evaporation. Accordingly the vegetation, which is 
the most variable factor of the environment, appears to be the 
essential factor in determining the evaporation rate. This, how- 
ever, is accomplished indirectly, since vegetation influences evapora- 
tion chiefly through the control or modification of the wind, the 
temperature, and the humidity, all of which affect the evaporation 
