46 BOTANICAL GAZETTE [JANUARY 
immigration of plants along a river highway. In the isolated 
grove it must be explained by a connection, no longer existent, with 
an older forest source, or by the sporadic development of the 
forest following a discontinuous migration across the prairie. 
Considering the second alternative, it might be possible for the 
various successional stages to develop centrifugally about a small 
forest center, the first stage occupying an outer ring, while the 
following ones appeared toward the middle. This does not seem 
possible here, because the arrangement is so obviously unilateral, 
with the later stages in the succession progressively farther toward 
the northeast, while there is no obvious difference in the environ- 
ment between the two ends of the grove, which might lead to the 
readier development of the red oak stage at one end. Also, in 
every other forest examined in central Illinois, in which oaks are 
the dominant trees, it has been possible to show a definite connec- 
tion with some other body of forest, from which continuous 
migration might have taken place. In other words, oaks, wit 
their heavy immobile seeds, do not seem able to cross tracts of 
prairie to a more favorable habitat, but must migrate in an unin- 
terrupted path. There are isolated groves in Champaign County, 
“ whose structure suggests that they are the result of a discon- 
tinuous migration, but no oaks occur in them. 
Considering now the first alternative, the development of Bur 
Oak Grove through immigration along the small streams of the 
vicinity is precluded for several reasons. First, their valleys are 
too shallow to afford the necessary physiographic diversity which 
always accompanies a mesophytic type of forest in central Illinois. 
Secondly, they all flow to the south, while in Bur Oak Grove the 
more mature forest type is at the north. Thirdly, they would have 
served as well or better for the immigration of hydrophytes than 
for upland species, while, as has been shown, the hydrophytic 
vegetation of the grove consists entirely of prairie species. The 
arrangement of species in the grove is exactly similar to the unilat- 
eral arrangement paralleling water courses in central Illinois. 
The whole grove has the appearance, and conveys the impression, 
of being the margin, now the only part remaining, of some exten- 
sive body of forest immigrating from the northeast, the location 
