172 



central Illinois, — variations in local conditions may produce 

 minor changes in the plant-covering, but that the resulting sub- 

 sidiary associations revert in every case to the typical bunch- 

 grass prairie. The great extent of the prairie formation, the 

 aggressiveness with which it displaces the minor associations 

 within it, and the resistance which it offers to the encroach- 

 ments of the forest, characterize it as a temporary climax type 

 of vegetation. That the prairie of Illinois is, however, being 

 rapidly displaced by the forest is no longer a matter of doubt. 

 Cowles ('01) has shown that for the Chicago district the cul- 

 minating, or climax, type of vegetation is the mesophytic forest. 

 While a similar mesophile association is the climax for the 

 central part of the state, the intermediate stages here may be 

 very different on account of differences in the physiography or 

 in the biotic environment. 



In the typical Illinois prairies the encroachment of the 

 forest upon the prairie progresses along the drainage lines. The 

 prairie soil always contains a sufficient amount of humus for 

 the growth of many species of forest plants, and the succession 

 of associations is hastened by the changes in soil and topogra- 

 phy due to stream action. The more permanent supply of soil- 

 moisture along the streams is a condition not found on the 

 prairie, and this constitutes a weakness at the most critical 

 point of its defense. In this sand region none of these favora- 

 ble conditions is found. There is no humus in the sand, and 

 there is no surface drainage, and consequently no erosion, no 

 base-leveling, and no increased water supply, — all of which in 

 other places so facilitate the extension of the forest. 



The first tree-growth that invades the sand prairies is a 

 xerophytic association composed mainly of the black-jack oak. 

 Quercus marylandica, and usually known as black-jack timber 

 (PI. XXL, Fig.l). The almost complete absence of water- 

 courses prevents its extension in long belts paralleling streams, 

 and, instead, it is found in large masses, with more or less 

 rounded outlines, on the larger sand deposits, or in narrow 

 .strips following the dunes. It is limited entirely to the sand, 

 never invading the more fertile fields of Miami loam, nor en- 



