Dec., 1909.] 
Evaporation in a Bog Habitat. 
*9 
Livingston’s experiments (3) with the atmometer at Tucson 
have shown that the evaporating power of the air, aside from its 
indirect effect upon soil moisture, is an important factor in plant 
development. Several species of plants were grown in soil which 
was kept as nearly as possible at its optimum moisture content, 
and their behavior in relation to the rate of evaporation was 
studied. Two species which were able to transmit water to the 
leaves faster than it was lost by transpiration, grew vigorously, 
even during a period of drought. Several other varieties were 
unable to provide the excess water for growth during the period 
of drought, but remained quiescent, and resumed their growth 
upon the return of the season of lower evaporation. Other 
plants not only failed to provide the excess water for growth 
during the drought, but did net respond even on the coming of 
the season of lower evaporation and soon died. It is concluded, 
from these experiments, that the evaporating power of the air 
controls desert vegetation to a great extent, for it inhibits the 
growth of plants which are not able to adjust themselves to the 
low evaporation rate, and thus plays an important part in the 
determination of centers of plant distribution. 
Further work (4, 5,) has brought out the value of the atmom- 
eter in the differentiation of habitats. While the amount of 
rainfall, through its effect upon soil moisture, is effective upon 
vegetation over large areas, the evaporating power of the air 
may vary greatly within these areas, and within neighboring 
habitats. Data taken in the Missouri Botanical Garden showed 
that the average ratio of the evaporating power of the air in the 
open field, and in the shade of a coppice was approximately as 
2.5 to 1 . About the same ratio was apparent in an open straw- 
berry patch, and beneath a shade tent. 
Atmometer readings taken at Tucson, and at different alti- 
tudes in the Santa Catalina Mountains indicate a gradual decrease 
in the rate of evaporation with altitude. Considering as unity 
the standard Tucson atmometer at 2412 feet, the relative loss of 
the instruments at 6000, 7500, and 8000 feet, was .8, .5 and .4 
respectively. These conclusions with reference to the decreasing 
rate of evaporation at higher altitudes have been corroborated by 
similar experiments conducted by Shaw in the Selkirks (6). 
Transeau (7) continued the study of the relation of plant 
societies to evaporation. He placed instruments in different 
plant habitats about Cold Springs Harbor, Long Island, com- 
paring all readings with that of a standard instrument in the 
Carnegie Garden. He reported an evaporation of 100% on an 
open gravel slide, and showed that the partial invasion of the 
slide by vegetation produced a decrease of 40% in evaporation. 
The rate in a forest habitat varied from 50% in the open wood to 
10% in the swamp forest. In the light of these data, it is easy to 
see why plants, accustomed to the swamp environment, cannot 
succeed in an open woods with a rate of evaporation five times as 
