242 ILLINOIS ACADEMY OF SCIENCE 



the beech (Fagus grandifolia) and the black gum (Nyssa 

 sylvatica) are confinecl to the eastern border of the state. 

 Their migration farther to the west, as well as that of 

 many herbaceous forest species, must be greatly hampered 

 by the generally north-south direction of the drainage 

 lines. The east-west transition is therefore more rapid 

 than the north-south transition. The resultant of the two 

 transitions is a direction from about north-northeast to 

 south-southwest for the boundaries of the range of certain 

 species, as the beech, and for the general tension line be- 

 tween forest and prairie in eastern Illinois and western 

 Indiana. 



Thus, of the western tributaries of the Wabash river 

 crossed in a given latitude, those farther to the east would 

 show the richest forest flora. The valleys of the eastern 

 creeks of the Embarras itself have several trees not pres- 

 ent, so far as the writer has been able to find, in the area 

 of the western creeks, except near their junction with the 

 Embarrass. Among these are the Kentucky coffee-tree 

 (Gymnocladus dioica), the big shell-bark hickory (Carya 

 laciniosa) and river birch (Betula nigra). 



Vegetational Changes. 



The views of Gleason* as to the development of vege- 

 tation in the middle west indicate the probable existence 

 of several historic periods after a prairie-like vegetation 

 had established itself upon the upland: (1) a period of 

 forest advance up and from the valleys; (2) a period of 

 prairie fires in which the prairie vegetation increased its 

 area, except where sloughs or streams protected the for- 

 ests from fire; and (3) the present period in which fires 

 are prevented, but forest spread has been checked by culti- 

 vation. The xerophytic oak forests of the Cumberland 

 county region are plainly in situations which are protected 

 from fire. This may be either because the forests have 

 been preserved in such areas, or because the absence of 

 fires has enabled forests to develop recently, during or 

 since the period of fires. Probably both types of xero- 

 phytic oak forests are present — the survivals of prairie 

 fires and those which have developed more recently in pro- 

 tected situations. 



4 Ann. Ass. Am. Geog. 5 :135-136, 1915 ; Bot. Gaz, 53 :38-49, 1912, 



