20 



THE PLANT SOCIETIES OF 



their own kind, inasmuch as the critical seedling stages cannot be 

 passed successfully. 



The life history of the rock ravines, or canons, is somewhat differ- 

 ent. When the ravine vegetation is at its height, the moisture and 

 shade are greater here than in the clay, hence the high development of 



Fig. 6.— Xerophytic bluff of St, Peters sandstone at Starved Rock on the Illinois river, show- 

 ing conifers and other plants of dry rocks. lniluen;e of erosive forces seen at the base. 



liverworts and their associates. As the ravine widens, these extreme 

 shade forms are doubtless drivA out almost immediately by xero- 

 phytes, since intermediate or mesophytic conditions are seldom seen 

 where the soil is rock. Furthermore, the xerophytic conditions become 

 much more extreme on rock bluffs than on clay bluffs. This is well 

 illustrated at Starved Rock (seey?^. 6), where the dominant tree vege- 

 tation is coniferous, consisting especially of the white pine {Pinus 

 Strobws) and the arbor vitae {Thuya occidentalis). The herbs and 

 undershrubs here are also pronouncedly xerophilous, resembling the 

 vegetation of the sand dunes, e. g., Se/agmeUa rupestris, Campanula 

 rotundifolia, Pellaa atropurpurea, Taliiium teretifolium, Opuntia Rafi- 



