
Ig0I } PHYSIOGRAPHIC ECOLOGY OF CHICAGO 155 
individuals. Here is to be found a great wealth of alga vegeta- 
tion, including such forms as Cladophora, Spirogyra, Oedogon- 
ium, Hydrodictyon. Among the floating plants are Riccia, 
Ricciocarpus, Spirodela, Lemna, and Wolffia. There are also a 
large number of attached plants, including many species of Pota- 
mogeton, Ranunculus aquatilis, Brasenia, Nelumbo, Myriophyllum, 
Ceratophyllum, Elodea, Vallisneria, and Naias. This rank 
growth of vegetation fills the lake up rapidly, since the currents 
are not sufficient to carry off the plant remains. There isa rapid 
advance of marginal plants upon the lake, a phenomenon that is 
shown in fig. 22, where the scattered bulrushes (Scirpus lacustris) 
are seen to be soon followed by a dense bulrush society. With 
or soon after the bulrushes are a number of marginal plants, 
especially 7ypha latifolia, Pontederia cordata, Sparganium eurycar- 
pum, Sagittaria variabilis and S. heterophylla, Zizania aquatica, 
Phragmites communis, Acorus Calamus, and Eriophorum cyperinum. 
fig. 23 shows a stage in which a lake has been all but destroyed 
by a rank bulrush vegetation. 
C. The prairie—Sedges encroach rapidly upon the bulrushes 
as the new soil becomes raised more and more above the lake, 
and grasses in turn encroach upon the sedges, forming a prairie. 
fig. 24 shows an expanse of grassy prairie which has developed 
through these successive stages from Calumet lake. Skokie 
marsh and Hog marsh are undergoing transformations of this 
character also. Sometimes with the prairie grasses are a number 
of coarse xerophytic herbs, largely composites (Si/phium lacini- 
atum, S. terebinthinaceum, S: integrifolium, Lepachys, Solidago 
rigida, Aster, Liatris), with some legumes (Amorpha canescens, 
Petalostemon, Melilotus, Baptisia), Eryngium, Dodecatheon, 
Phlox, Allium cernuum. A Silphium (compass plant) prairie is 
shown in fig. 25. The prairies of our area are in the basin of 
the glacial Lake Chicago, and hence all may be referred to a 
lake or swamp origin, exactly as prairies are developing from 
Calumet lake today. This explanation of the prairie, an 
undoubted explanation for the cases in hand, must not be 
applied to the great climatic prairies farther west. Whether 
