a 
— £r 


Igor] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 93 
bluffs and is rarely absent. Perhaps the best examples of xero- 
phytic stream bluffs near Chicago are along Thorn creek. One 
of the most interesting things about these bluff societies is the 
frequent presence of basswoods and sugar maples. Doubtless 
these trees look back to the mesophytic associations that have 
otherwise disappeared. As would be expected, the last of the 


Fic. 5.—Open ravine at Beverly ~~ gsi gentle slopes covered with a less 
mesophytic vegetation than is shown n fig. 2. Dominance of oaks in place of 
maples and basswoods. 
mesophytes to die are trees, because they are longer lived than 
herbs and shrubs, and also because their roots reach down to the 
moisture. But they cannot be succeeded by their own kind, 
inasmuch as the critical seedling stages cannot be passed success- 
full lly. 
The life history of the rock ravines or cafions is somewhat 
different. When the ravine vegetation is at its height, the mois- 
ture and shade are greater here than in the clay, hence the high 
development of liverworts and their associates. As the ravine 
