PAPERS ON BIOLOGY AND AGRICULTURE 165 



from them, fires in such areas, grazing within them, with 

 such consideration of the principles of plant succession as 

 appear pertinent, and the matter of possible climatic fluc- 

 tuation. It may be well to emphasize that what is de- 

 sired even in a qualitative investigation of these factors is 

 some knowledge of the attendant alterations in such things, 

 for example, as soil moisture content, physical character of 

 the substratum, effects on the plant assemblages, as well 

 as more general information and certain outstanding prin- 

 ciples or isolated facts. 



THE PLANT ECOLOGY 



Just what the Rock River woodlands contained in plants 

 besides tree species before white settlements got under 

 way is somewhat speculative. The most striking thing 

 about the early accounts appears to be the great similarity 

 between the species of trees then present in the woods along 

 the Rock and in the woods well back from the river, that is 

 in many of the prairie groves. Mention of "large oaks" 

 is frequent, and sometimes a diameter of three or four feet 

 is given for them as well as for elms. Large walnut and 

 butternut are also spoken of and the hard or sugar maple. 

 The WTiter believes that there was more mesophytism than 

 now, that if not a very much higher degi'ee of it, it was at 

 least greater in areal extent. 



It is believed, too, that the climax form today of the Rock 

 River woodland region is one of very considerable mesophy- 

 tism. The highest expression of this is to be seen only in 

 spots of woodland, while certain larger areas exhibit it 

 also but expressed in lower terms. It must be remembered 

 that the region presents other woodland associations of 

 high mesoph\i:ism besides this climax form. These are the 

 bottomland association and the streamside association, al- 

 most identical in floral composition, the former of which is 

 notable on the islands while the latter is common on the 

 banks of the Rock and other streams of continuous flow. 

 Here the ruling species of trees are Acer saccharinum, Ulmus 

 americana, U. pubescens, Salix nigra, S. fluviatilis, Acer ne- 

 gundo, Fraxinus americana. When ungrazed, Ambrosia tri- 

 fida and Urtica dioica are characteristic also. The climax 

 form when at its highest expression contains such trees 



