208 
Ohio Biological Survey 
plant associations which are related to one another. Although the vegeta- 
tional features of young blufifs in gravel, rock, and clay differ from one 
another in many respects, they are more nearly related to each other, than 
are the associations of a gravel bluff and a gravel shore. Numerous 
examples point to the conclusion that topographic influences are usually 
more powerful than soil influences. 
In most of the region, there is some relation between vegetational 
and geological areas due to the influence of soil. Fresh exposures of 
gravel, rock, and clay are more different from one another, than are weath¬ 
ered ones. For this reason, the differences between the vegetation of dif¬ 
ferent soil areas are more pronounced early than late in the succession. 
Soil has an indirect influence on vegetation through its effects on 
topographic forms. Resistance or ease of erosion of rock material re¬ 
tards or accelerates the progress of the erosion cycle, and with that, the 
vegetative cycle dependent upon it. The effects of difference in resist¬ 
ance are best disclosed where two materials, in contact, are acted upon 
by the same agent. These effects may be seen along a stream which 
cuts through two or more unlike materials. The differences in the asso¬ 
ciations, along such a stream are partly due to differences in topography 
arising as a result of differences in the ease of erosion of the soil. The 
associations, as existing along that stream, do not illustrate exactly the 
succession of associations at any one place, for this is modified by the 
soil of the area in which the succession occurs. 
In some instances, topography and the physical character of the soil, 
together produce the plant association. An undissected flat is usually 
a poorly drained area. If, added to the absence of drainage lines, the 
soil is of such a nature that it drains slowly the features of bad drainage 
will be more pronounced, and the plant associations which depend upon 
poor drainage for their existence, will be better developed. 
Roth xerarch and hydrarch successions are illustrated in the Cin¬ 
cinnati region. 
The xerarch succession embraces those of dry flats, whether on up¬ 
land or terrace, and of slopes, whether of rock, gravel or clay. The suc¬ 
cessions on dry flats are relatively rapid. They are dependent for their 
advance upon biotic agencies. Topography is as yet little changing. 
Erosional features are almost absent. Biotic cycles are not complicated 
by the progress of a topographic cycle. 
The successions on slopes are slow and varied. They are produced 
by a combination of biotic and topographic agencies. Of these succes- 
