Igor | PHVSIOGRAPHIC ECOLOGY OF CHICAGO 159 
first an outcrop of bare rock. The opportunity for a shrubby 
vegetation eventually arrives, especially in the crevices. Fig. 
27 shows such a vegetation getting a foothold. Among the 
shrubs in such places are the chokecherry (Prunus Virginiana), 



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27. ~~ Limestone ledge at Stony island, showing vegetation farther advanced 
than in a 26. The crevice shrubs here are chokecherries (Prunus Virginiana). 
ninebark (Physocarpus opulifolius), poison ivy (Rhus Ti oxicoden- 
dron), Rosa humilis, sumach (Rhus typhina), hop tree (Ptelea tri- 
foliata), wild crab (Pyrus coronaria). Still later the way is open 
for a tree vegetation, at first xerophytic, but ultimately meee: 
phytic, as the author has frequently observed in the Alleghanies. 
There can be no doubt but that a temporary mesophytic climax 
can be reached even on rock hills, though the probability of this 
