121 



gration even of a few miles would require thousands of years. The 

 animals which feed upon acorns do not usually carry them a long 

 distance, and those which are carried away are generally eaten. 

 The acorns are produced in large numbers and lie thickly on the 

 ground beneath the trees, but, as reported also by Britton (ipo_^: 

 578), most of them are not viable. Reid (i8pp: 29) reports that in 

 England rooks carry acorns to some distance and that isolated young 

 plants may be found at a considerable distance from fruit-bearing 

 trees. It is his idea (i8pp: 31) that the "accumulated accidents 

 of some thousands of years" are sufficient to explain the distribution 

 of oaks in England. In the Illinois sand region seedling oaks are 

 always few in number and are never found on the prairie. Acci- 

 dents can not be invoked here to explain a migration so regular, so 

 continuous, and apparently so rapid, and the whole question must 

 be left unanswered. 



When once established the forest is permanently dominant, un- 

 less destroyed by man or by some exceptional physiographic 

 changes. In the former case there may be a temporary reversion to 

 the bunch-grass association. One such case was observed near 

 Forest City, in the Havana area. The ground was occupied by a 

 good growth of bunch-grasses, including Panicmn pseudopuhesccns, 

 Lcptoloina cognatnui, Sorghastrnm niitans, Andropogon scoparms, 

 Tridens Haviis, Bouteloua curtipcndida, Paspalmn sctaccuni, and 

 Care.v Miihlenhergii. There were thickets of Rhus canadensis, var. 

 illinocnsis, and mats of Opuntia Rafinesquii, and numerous intersti- 

 tial plants of Cassia Chamacchrista, Ambrosia psilostachya, and 

 Monarda punctata. None of the perennial group was present. Nu- 

 merous young plants of Qucrcus rnarilandica and Oucrcns velutina 

 were appearing, indicating the approaching" end of the bunch-grass 

 association. 



Within the forest may be distinguished two well-marked asso- 

 ciations, related to each other by a clearly defined order of succes- 

 sion, and differing in their habitat and component species. Of these 

 the pioneer is the black oak association. 



The Forest Formation 



the black oak association 



The associations of the forest formation are fewer in number 

 than those of the prairies. The first of them in order of succession 

 and the most typical of the sand region is the black oak association. 

 (PI. XIV, Figs. I, 2; XV, Figs. I, 2; XVII, Fig. i). It is found 



