

190r | PHYSIOGRAPHIC ECOLOGY OF CHICAGO 173 
adapted themselves to an essentially xerophytic life through 
living in undrained swamps. Again it may be true that inhabi- 
tants of undrained swamps are better able to withstand a partial 
burial than are other plants. 
Vegetation appears to be unable to capture a rapidly moving 
dune. While many plants can grow even on rapidly advancing 
slopes, they do not succeed in stopping the dune. The move- 
ment of a dune is checked chiefly by a decrease in the available 
wind energy, due to increasing distance from the lake or to barri- 
ers. A slowly advancing slope is soon captured by plants, because 
they have a power of vertical growth greater than the vertical 
component of advance. Vegetation commonly gets its first foot- 
hold at the base of lee slopes about the outer margin of the com- 
plex, because of soil moisture and protection from the wind. 
The plants tend to creep up the slopes by vegetative propaga- 
tion. Antecedent and subsequent vegetation work together 
toward the common end. Where there is no antecedent vegeta- 
tion, Ammophila and other herbs first appear, and then a dense 
shrub growth of Cornus, Salix, Vitis cordifolia, and Prunus Virgint- 
ana, Capture may also begin within the complex, especially in 
protected depressions, where Saiz longifolia is often abundant. 
D. The established dunes—No order of succession in this 
entire region is so hard to decipher as is that of the estab- 
lished dunes. There are at least three types of these dunes 
so far as the vegetation is concerned, and it is not yet possible 
to figure out their relationships. The continuation of the con- 
ditions as outlined in the preceding paragraph results in a forest 
society on the lee slope, in which is found the basswood, together 
with a most remarkable collection of mesophytic trees, shrubs, 
and climbers, which have developed xerophytic structures. These 
dunes are evidently but recently established, as is shown by the 
absence of a vegetation carpet; furthermore the slopes are 
almost always steep. 
Again, there are forest societies in which the pines dominate, 
either Pinus Banksiana or P. Strobus. These arise from a heath, 
composed in the main of Arctostaphylos and Juniperus. The 
