180 ILLINOIS STATE ACADEMY OF SCIENCE 



Other sandstone cliffs, of more remote location, present 

 habitats of advanced mesophytism. This consideration is 

 independent, of course, of high moisture content obtained in 

 some cases, and by a cliff of any sort of material, by pecu- 

 liarly advantageous seepage conditions related to stratifi- 

 cation. Such advantageous moisture conditions frequently 

 are found at the foot of cliffs, and occasionally on their 

 slopes. 



While the topographic location of the soil is important 

 in relation to its moisture content, the growth and develop- 

 ment of the vegetation itself is of immense importance in 

 determining what the moisture content will be. Before 

 this influence becomes operative in any marked degree the 

 other factors are controlling; the topography, the physical 

 character of the soil, what man has done or is doing to affect 

 the nature of the area. Outside of the Rock River wood- 

 land region, out in the prairie groves, a different case ob- 

 tains. In these isolated woodlots on upland prairie a num- 

 ber of ether factors besides that of soil moisture becom.e 

 prominent to differentiate them from the area which is 

 chiefly under consideration in this paper. Factors of a 

 general climatic nature put them in another class. In the 

 woodland region of the Rock it appears to the writer likely 

 that the influences of other factors are largely wiped out 

 and that the influence of soil moisture content dominates. 



Any careful examination of the plant life of the Rock 

 River woodland region will bring into notice a phenomenom 

 that appears to be a swinging of the successional trend, now 

 in the xerophytic direction, now in the mesophytic direc- 

 tion, but irregularly. All that is known of the laws of plant 

 succession goes to indicate that, under any particular 

 climatic complex, plant succession will advance to its climax 

 form through various successional stages, unless thrown 

 into retrogression by some external influence. Since the 

 local accounts repeatedly mention marked weather changes 

 in the past, since the advent of white men to the county, 

 and since the whole area is located in a traditional region, 

 one is, at first, strongly inclined to attribute the phenomenon 

 to minor climatic fluctuations, those changes in the weather 

 conditions that present marked minor departures trom the 



