480 



Minnesota Plant Life. 



exposure to wind, a slight modification of the chemical or phys- 

 ical condition of the soil, or any one of innumerable other varia- 

 tions in the surroundings may suffice to alter the vegetation 

 which has become established. Thus, in a well-kept lawn, 

 where conditions are normally mesophytic, plantains and dande- 

 lions both of them mesophytic in structure will prob- 

 ably be among the most troublesome weeds. But let the 

 lawn be established on a soil -too porous and easily dried, or 

 let the season be below the average in rainfall, and very promptly 

 xerophytic mat plants, such as spurges and prostrate verbenas, 

 and xerophytic grasses, such as the sand-burs and other similar 



FIG. 239. Modern hardwood forest of the St. Croix valley, near Osceola. After photograph 

 by Professor W. R. Appleby. 



vegetation, will begin to encroach upon the turf. Vegetation 

 coverings, wherever they are formed, are quite as sensitive to 

 changes in surrounding conditions as is a city lawn, and for 

 this reason the infinite variety of woods, fields, marshes, swamps 

 and prairies has come to exist. 



A very broad classification of the influences which affect plant 

 adaptations is that made by the German authority, Schimper. 

 He distinguishes essentially i, climate; 2, substratum, and 3, 

 neighbors. By climate he means the complex of atmospheric 

 and cosmic influences, including, for example, all the influences 

 of solar light and heat and those of atmospheric vapor and wind. 



