336 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
account of their isolation within the deep forest these factors as a rule do 
not affect them. The steepness of the slope would cause some washing of 
fruits and seeds and young plants; but in depressions and among the prairie 
shrubs and grass bunches there would be adequate protection from washing. 
Seedling trees and shrubs occur, as reported by Pool, Weaver, and Jean 
and by the writer, in spite of the washing. 
The data presented in this paper and in the paper by Pool, Weaver, 
and Jean appear to prove that the dryness of the air, caused by the direct 
exposure to sunlight and wind, and the low water content of the soil, caused 
by the high evaporating power of the air and the run-off due to the steep 
slope, are the determining factors in the maintenance of the prairie inclu- 
sions against forest invasion. The air near the surface of the prairie is 
usually two to four times as dry as in the surrounding shrubs and woods. 
The water content of the first foot of soil frequently falls below the non- 
available point in the prairie, while this rarely happens in the shrub and 
forest. In the prairie, conditions for the germination of tree and shrub 
seeds are usually not favorable. In case the seeds do germinate, the seed- 
lings are frequently exposed to very dry air and to non-available soil water. 
The sickly appearance of the tree seedlings observed in the prairie may be 
accounted for in this way. 
Summary 
1. Prairie inclusions have been reported as occurring in the deciduous 
forest climax in Ohio, Illinois, Iowa, and eastern Nebraska. 
2. This paper presents the results of an intensive study of a small 
prairie inclusion near Peru, in southeastern Nebraska. 
3. The evaporating power of the air was measured by Livingston stand- 
ardized atmometers. The daily average losses from May 10 to September 
7 were: high prairie (near summit of steep slope) 20.6 cc, low prairie (near 
base of steep slope) 16.9 cc, shrub zone at base of prairie slope 9.0 cc, red 
oak-hickory forest below the shrub zone 7.9 cc, bur-oak forest near summit 
of north slope 9.5 cc 
4. The average of eight total soil-water content readings, at o-io cm. 
depth, from May 18 to July 29 were: high prairie 14.5 percent (wilting 
coefficient 13.0 percent), low prairie 15.5 percent, shrubs 23.8 percent, red 
oak-hickory forest 31.2 percent (wilting coefficient 15.5 percent), bur-oak 
forest 20.8 percent (wilting coefficient 15.2 percent). For the depth of 
10-30 cm. the averages were: high prairie 15.2 percent (wilting coefficient 
1 1.9 percent), low prairie 17.0 percent, shrubs 23.6 percent, red oak-hickory 
forest 27.8 percent (wilting coefficient 12.5 percent), bur-oak forest 20.7 
percent (wilting coefficient 12.5 percent). 
5. The data presented in this paper tend to prove that the chief factors 
in the maintenance of the prairie inclusions from invasion by the surround- 
ing shrubs and trees are the great evaporating power of the air caused by 
