PAPERS ON BIOLOGY AND AGRICULTURE 179 



ment of the complex, and undue emphasis may therefore 

 easily be given it. Nevertheless, in spite of the obscurity 

 of such factors as temperature and evaporation by observa- 

 tional methods, there seems to be sufficient evidence to re- 

 gard soil moisture as the prime factor. As the soil moisture 

 content increases the succession here progresses towards 

 the oak-maple climax, and as it decreases the succession 

 retrogresses towards xerophytism, towards something very 

 like what the old prairie groves of the county now are or 

 towards a somewhat less xerophytic form of such wood- 

 land. This is without reference in this place to the par- 

 ticular immediate causes. 



There has not appeared to the writer to be any marked 

 difference in the vegetation upon soil derived from lime- 

 stone or sandstone, or, indeed, a difference in the vege- 

 tation upon any of the soils of the region that is dependent 

 upon the character of the soil material itself, ergo as 

 sand or lime. Of course on a residual sand, for example, 

 the physical characteristics of such a soil yield in the early 

 stages of plant succession a very different habitat than a 

 clay soil. But these differences are not due to any chemical 

 feature apparently. They are, in later stages of the succes- 

 sion, wiped out completely. It is true that several of the 

 plant species known commonly as those of limestone do 

 occur most frequently on such sites. Pellaea atropurpurea 

 is a notable example. But this fern occurs also on sand- 

 stone, even if less frequently. This species with Pellaea 

 gracilis rarely, and with Cystopteris fragilis, C. bulbifera. 

 Woodsia ilvensis, and of course Polypodium vulgare, are 

 the frequenters of rock wall crevices and cliffs. Much of 

 the diversity that appears between sandstone and limestone 

 cliffs seems to be explainable along the Rock and its tribu- 

 taries by certain other factors. For one thing the sand- 

 stone cliffs are mainly along the Rock River, where man's 

 activities have been greatest, and these cliffs present a 

 more xerophytic habitat largely because of such activities. 

 Castle Rock, for instance, on the east shore of the river be- 

 tween Oregon and Grand Detour, composed of nearly white, 

 non-fermuginous sandstone, a very pure silica, is to a great 

 extent without vegetation owing to trampling by visitors. 



