PLANT SOCIETIES. 475 



exercising a drying effect on vegetation, and the vegetation type 

 then is largely xerophytic. 



913. Edaphic * formations. Edaphic formations may occur 

 in any of the climatic-formation areas. They are controlled by 

 the condition of soil or ground. The condition of the soil is 

 unfavorable for the growth of the general vegetation type of 

 that region, or is more favorable for another vegetation type, so 

 that soil conditions overcome the climatic conditions. These 

 areas include swamps, moors, the strand or beach, rocky areas, 

 etc., as well as oases in the desert, warm oases in the arctic zone, 

 river bottoms in the prairie and plains region, alkaline areas, etc. 

 The edaphic formations may be close or open according to the 

 nature of the soil. The edaphic formations then are infiltrated 

 in the climatic formations, the different vegetation types fitting 

 together like pieces of mosaic, which can be seen in some places 

 from a mountain top, or if one could take a bird's-eye view of 

 the landscape or from a balloon. 



914. Aquatic formations. These are made up of 'water 

 plants and are of two general kinds: fresh-water plant forma- 

 tions in ponds, lakes, streams; and salt-water plant formations 

 in the ocean and inland salt seas. 



915. Culture formations. Culture formations are largely 

 controlled by man, who destroys the climatic or edaphic forma- 

 tion and by cultivation protects cultivated types, or by allowing 

 land to go to "waste" permits the growth of weeds, though 

 weeds are often abundant in the culture areas. In general the 

 culture formations may be grouped into two subdivisions: ist, the 

 vegetation of cultivated places; and 2d, the vegetation of waste 

 places, as abandoned fields, roadsides, etc. 



IV. Plant Societies. 



916. Plant societies are somewhat definite associations of 

 the vegetation of an area marked by physiographic conditions. 

 A single plant society is nearly if not altogether identical with a 



ground. 



