94 BOTANICAL GAZETTE [ FEBRUARY 
widens these extreme shade forms are doubtless driven out 
almost immediately by xerophytes, since intermediate or meso- 
phytic conditions are seldom seen where the soil is rock. Fur- 
thermore, the xerophytic conditions become much more extreme 
on rock bluffs than on clay bluffs. This is well illustrated at 
Starved rock (fig. 6), where the dominant tree vegetation is 



Fic. 6.—Xerophytic bluff of St. Peters sandstone at Starved rock, on the Illinois 
river, Showing conifers and other plants of dry rocks. Influence of erosive forces 
seen at the base. 
coniferous, consisting especially of the white pine (Pinus Stro- 
bus) and the arbor vitae ( Thuya occidentalis). The herbs and 
undershrubs here are also pronouncedly xerophilous, resembling 
the vegetation of the sand dunes, e. g., Selaginella rupestris, Cam- 
panula rotundifolia, Pellaea atropurpurea, Talinum teretifolium, 
Opuntia Rafinesquit, etc. The entire bluff flora down to the 
river's edge is xerophytic, except in shaded situations. 
When a stream in its meanderings ceases to erode at the base 
of a bluff, increased opportunity is given for plant life. Through 
