46 



THE PLANT SOCIETIES OF 



most of the rock, to the mesophytic plants of the deeper soil which lies 

 over the crevices. 



Through rock decay and the accumulation of organic matter a con- 

 siderable soil comes to be developed where there was at first an outcrop 

 of bare rock. The opportunity for a shrubby vegetation eventually 



Fig. 27.— Limestone ledge at Stony Island, showing vegetation farther advanced than in 

 ^ig. 2b. The crevice shrubs here are chokecherries (Prunus Virginiana) . 



arrives, especially in the crevices. J^ig. 27 shows such a vegetation 

 getting a foothold. Among the shrubs in such places are the choke- 

 cherry {Prunus Virginiana), ninebark {Physocarpus opulifolius), poison 

 ivy {Rhus Toxicodendron), Rosa humilis, sumach (Rhus typhina), hop 

 tree {Ptelea trifoliata), wild crab {Pyrus coronarid). Still later the way 

 is open for a tree vegetation, at first xerophytic, but ultimately meso- 

 phytic, as the author has frequently observed in the Alleghanies. There 

 can be no doubt but that a temporary mesophytic climax can be 



