168 ILLINOIS STATE ACADEMY OF SCIENCE 



succession was in the reverse direction. The prairie groves 

 have exhibited a movement of plant succession towards 

 xerophytism from a rather high form of mesophytism, as 

 retrogression due to the activities of man. Near Lincoln, 

 Nebraska, an area of prairie, bearing the usual sod grasses, 

 was made, by the activities of man, to exhibit a movement 

 of the plant succession towards mesophytism from some de- 

 gree of xerophytism. Some forty years ago seedling trees 

 were planted in the prairie sod grass. Now the sodded 

 condition has disappeared and the area has shown progres- 

 sion into a rather mesophytic woodland. 



Among the species given as representative of the prairie 

 groves, or their remains, the farm woodlots of today, Quer- 

 cus macrocarpa is a typical border line tree between prairie 

 and woodland. It is also, however, a species of very differ- 

 ent habitat, occurring in places of great soil moisture, as 

 for example, on the Leaf River flats. It is to be found, too, 

 in other habitats of soil moisture conditions apparently in- 

 termediate between these, where it may be a left-over from 

 another set of conditions. It is quite possible that a statis- 

 tical investigation would show it to be more plentiful in 

 that portion of the area being dealt with that is west of 

 the Rock River. 



Having now given some attention to what may be thought 

 of as the two extremes of woodland growth, the prairie 

 grove or farm woodlot association of today, just outside of 

 the Rock River woodland region and the climax form of 

 woodland within that region, some consideration of the in- 

 termediate stages and deviations from the customary forms 

 of these are in order. As might be expected the physio- 

 graphic diversity afforded by an intrenched river valley 

 in the way of differentiation of habitat conditions along 

 the line of mesophytism, that is increased soil moisture and 

 decreased evaporation, has given rise to practically all 

 variations between the two extremes. The typical prairie 

 grove or farm woodlot association can be found today with- 

 in the Rock region as well as upon the prairie. From the 

 oak-hickory association of xerophytic tendencies the relative 

 frequency of the more mesophytic oaks, Quercus alba and 

 Quercus rubra, increases as the succession goes in the meso- 



