366 BOTANICAL GAZETTE [DECEMBER 
lower stations, holding the lower stratum of air to a certain extent 
against displacement by air drainage. 
One object in obtaining these temperature data was to ascer- 
tain whether temperature differences might be correlated with the 
invasion of lower latitude plants, like Asimina triloba, into the 
lower levels. The data obtained, however, are contradictory. The 
mean temperatures are undoubtedly higher at the higher lying sta- 
tions, the last points of invasion, and also the last points of cession 
by relict northern species. On the other hand, the consistent 
higher minima at the lower levels might well permit a slightly longer 
growing period, and perhaps even milder winter conditions. Other- 
wise stated, the temperature optimum for southern species might 
not be so nearly approximated on the lower stations as at the higher, 
but a point above the minimum might be maintained throughout a 
longer period each year. The main reason for the confinement of 
southern invaders to the bottom lands, however, must probably be 
sought in the more favorable moisture conditions at these levels. 
The writer does not feel that attempts at close correlations between 
temperature and vegetation over such small differences in altitude 
would be very profitable at the present stage of development in 
ecological science. It is interesting to find, however, that fairly 
consistent small differences in temperature do coexist with small 
topographical differences. 
Summary 
1. This paper is a study of the ecological factors at seven repre- 
sentative topographical points in the Illinois State Park at Starved 
Rock, during the growing season of 1921. The factors studied were 
(a) soil moisture, (6) evaporating power of the air, (c) evaporating 
power of solar radiation, and (d) temperature of air and soil. 
2. Observations were taken with special reference to their 
influence on seedling growth, because of the importance of the latter 
as a factor in succession, 
3. Soil moisture was found to vary (a) seasonally, falling off 
after the close of the spring rains and reaching a point below the 
minimum necessary for plant growth during a considerable portion 
