164 ILLINOIS STATE ACADEMY OF SCIENCE 



the water, due to the load carried, is frequently commented 

 upon, whereas before the advent of the whites its clearness 

 was equally noticed. 



The artificial planting that has been done in the county 

 is mainly of an ornamental nature and there is no evidence 

 as yet of the further establishment of these introduced 

 species by natural means. As a factor this is still negligible. 

 Unintentional weed introduction and the natural migration 

 and establishment of weed species of plants has, probably, 

 been very great since the settlement period about the middle 

 of the nineteenth century. One need refer to the case of 

 the giant ragweed (Ambrosia trifida) alone for an indica- 

 tion of how widespread this influence may be upon the 

 native vegetation. This species, locally known as "horse- 

 weed," is having a marked ill effect economically in its in- 

 vasion of farm areas. The entire matter of weed introduc- 

 tion is a practically unknown factor, however. The effect of 

 crop raising on the native vegetation is, like its reciprocal, 

 the effect of native vegetation on crop areas and the raising 

 of different farm crops, practically an unknown territory. 

 One aspect of this that seems likely to come up before many 

 years for careful investigation by ecological methods is the 

 effect of grazing by domestic stock on the permanency and 

 the yield of farm pasturage. The ecological method of 

 regulating grazing on western ranges has already been suc- 

 cessfully worked out and applied. The effects of grazing 

 animals on the Rock River woodlands will be considered 

 here, however, after the plant ecology of the areas has been 

 dealt with. It needs merely to be mentioned now as one 

 of the prime factors of the region in the ecology of the 

 native vegetation. Plant diseases among the native species 

 is a wholly unknown factor so far as the past is concerned. 

 References occur, to be sure, in various records of county af- 

 fairs, but these references are all so general in nature as to 

 be of little value, consisting mainly of such statements as 

 that after the severe winters of a certain period there was 

 much disease among the native trees. It will probably re- 

 main one of the unknown factors of the past. 



There remain, then, for later discussion the topics of 

 clearing woodlands, cutting out various species of trees 



