CHICAGO AND VICINITT. 19 



bluff in place of the witch hazel and its associates. And it is not long 

 until xerophytic or semi-xerophytic thickets prevail, in place of the 

 former mesophytic undershrubs. Among the more characteristic of 

 these shrubs are the hop tree (Ptelea trifoUatd), bittersweet (Celastrus 

 scandens), sumachs [Rhus typhina and R. glabra), chokecherry 



Fig. 5. — Open ravine at Beverly Hills, showing gentle slopes covered with a less mesophytic 

 vegetation than is shown in^^. 2. Dominance of oaks in place of maples and basswoods. 



(Prunus Virginiand), nine-bark (Physocarpus opulifolius), wild crab 

 (Pyrus coronarid). Two small trees are common on stream bluffs, the 

 service berry {Amelanchier Canadensis) and the hop hornbeam {Ostrya 

 Virginica); this last species is perhaps the chief character tree of river 

 bluffs, and is rarely absent. Perhaps the best examples of xerophytic 

 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 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 



