14 THE AMERICAN BOTANIST 



sloAvly encroaching upon the prairies, or that the encroachment 

 became measurable as soon as the prairie fires were checked. 

 The prairie is not an extraordinary thing to be explained only 

 by some strange or fanciful causes ; it owes its origin to ages of 

 humid climate in the east and southeast. These great climatic 

 types, acting upon the plant world through evolution and 

 elimination, gradually developed the two extreme types of 

 vegetation, each of which was especially adapted to its own en- 

 vironment. After the close of the glacial period, migration of 

 each of these types brought them in contact in Illinois and 

 neighboring states and a struggle for supremacy began be- 

 tween them. The outcome is decided mainly by two sets of 

 factors ; first the control of the environment by the vegetation 

 and second the climatic conditions of temperature and rainfall. 

 In the first case, the prairie vegetation, by virtue of its close 

 sod, tends to prevent the proper germination and growth of 

 the forest tree seedlings. Prairie fires, following the advent 

 of man also tend to restrict the growth of the forest. On the 

 other hand the forest has control of the light supply for the 

 herbaceous layers and the well established trees are resistant 

 to fire. Above all, the climatic conditions are favorable to 

 forest. The balance has been in general in favor of the forest 

 and it has advanced slowly upon the prairie. The greatest 

 speed of advance has been along the lines of least resistence, 

 the water courses, and has resulted in long strips of forest par- 

 alleling the streams and usually widest on the east side of 

 streams or marshes where they were better protected from 

 fire. In the sand regions the forest distribution is not regu- 

 lated in that way because of the absence of small streams, but 

 it does show a possible relation to fires. Where the sand lies 

 in disconnected ridges separated by strips of moist or swampy 

 ground acting as fire-breaks there is a good growth of forest 

 on the higher ground. — Dr. H. A. Gleason in Bulletin of the 

 Illinois State Laboratory of Natural History. 



