48o 



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 v^egetation 

 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 



r 



l''iG. -'.jy. — Model II hardwood forest of the^St. Croix valley, near Osceola. After iihotoKraph 

 by Professor W. R. Appleby. 



vegetation, will begin to encroach upon the turf. \'egetation 

 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. cli)iiafc: 2, substratum, and 3. 

 neighbors. By cliuiaic he mcnis the complex of atmospheric 

 and cosmic inllucnccs, including, for example, all the intluences 

 of solar light and heat and those of almospheric vapor ami wind. 



