

Igor | PHYSIOGRAPHIC ECOLOGY OF CHICAGO 147 
Jackson park the author has noticed accumulations of Chara 
peat amounting to one or two inches per year. 
B. The undrained swamp.—It is obvious that the processes 
outlined in the preceding paragraph must eventuate in the death 
of the lake or pond involved and its replacement by a marsh, 
entirely apart from ordinary erosive activities. Indeed, as has 
been stated, these activities are relatively unimportant here; 
this fact is shown by the absence of ordinary sediments from 
most peat beds. As the aquatics make the pond shallower 
and shallower they make it more and more unfit for themselves 
and fit for their successors, viz., those plants which grow along 
pond margins. Among the first plants of this type are various 
sedges (Carex), also the bulrush (Scirpus lacustris), though this 
latter species is more characteristic of the half-drained margins 
than of those under discussion here. Other marginal plants of 
our peat bogs are Menyanthes trifoliata and Potentilla palusinis. 
The vegetation that follows may be called typical of peat 
bogs. The dominant plants are usually shrubs, especially the 
leather leaf (Cassandra calyculata) ; this plant may be so abun- 
dant as to give tone to the landscape. /%g. 19 shows some Cas- 
sandra islands in a sedge swamp. It is clear that the islands 
represent places where in the original lake the water was shal- 
ow. The present remnant of the lake is shown at the left. Not 
only have the sedge zones advanced upon it from all sides, but 
centers of sedge growth appear also in shallow places in the 
lake itself. Just as the sedge zone encroaches upon the lake, 
when conditions become favorable, so the Cassandra zone 
advances on the sedges. Again atree zone advances on the 
shrubs, as will be seen farther on. The zonal arrangement of 
plant societies that has just been seen is a feature of most peat 
bogs, and is due to the symmetry of lake and bog conditions. 
It will be observed that along the lake margin the zones advance 
toward a common center, while on the islands the advance is 
from a center. Eventually, of course, the marginal and island 
zones will merge. o 
Besides Cassandra many other plants are commonly found 
Ser's5 
Vs, 
Tond 
