Igor] PHYSIOGRAPHIC ECOLOGY OF CHICAGO gi 
conditions outlined above undergo radical changes. From this 
point it will be necessary to discuss two phases in the growing 
river, the bluff phase and the bottom phase. We have left the 
clay ravine bluffs in a state of temporary climax, clothed with 
luxuriant mesophytic forest trees and with a rich undergrowth 
of vernal herbs. More and more the erosive processes are 
conspicuous laterally, and 
widening processes prevail 
over the more primitive 
deepening. As a result, 
the exposure to wind, sun- 
light, and changes of tem- 
perature increases; thé 
moisture content of the 
slopes becomes less and 
less. The rich mesophytic 
herbs, including the liver- 
worts and mosses, dry up 
and die. The humus oxi- 
dizes_ more rapidly, and 
a xerophytic undergrowth 
comes in. In place of 
Hepatica and its associ- 
ates, we find Antennaria, 
Poa compressa, Equisetum 
hyemale, and other xero- eo ce eae ae 
phytic herbs ; Polytrichum i ce bagi soc opiate 
also replaces the mesophy- forces prominent, and vegetation slight on 
tic mosses. The first signs the dripping slopes. 
of the new xerophytic flora ae 
are seen at the top of the ravine slope; indeed the original 
x€rophytic plants may never have been displaced here by the 
ravine mesophytes. As the ravine widens, the xerophytic plants 
creep down the slope, often almost to the water’s edge. Some 
of the young ravines between Evanston and Waukegan show 
xerophytes at the summits of the slopes. Fig. 5 shows a 


