ASSOCIATIONS DUE TO CONDITIONS OF SOIL 483 



462. Temperate plant associations due to special conditions 

 of soil. Even where the climate is a moderate one as regards 

 temperature and rainfall, peculiar soils may cause the assemblage 

 of exceptional plant associations. Some of the most notable of 

 such associations in temperate North America are those of the 

 salt marshes, the sand dunes, and the peat bogs. 



In salt, marshes the water supply is abundant, but plants do 

 not readily absorb salt water by their roots, so that the plants 

 which grow in salt marshes usually 

 have something of the structure and 

 appearance of xerophytes. Some of 

 them are fleshy (Fig. 368), and some 

 species are practically leafless. 



Sand dunes, whether along the 

 seacoast or near the Great Lakes, 

 offer a scanty water supply to the 

 roots during much of the year, and 

 the soil water contains less of the 

 raw materials for plant food than is 

 offered by that of ordinary soils. 

 Many grasses thrive, however, in 

 these shifting sands (Plate I), and 

 some, like the beach grass (Am- 

 mophila) of the Atlantic coast and 

 the Great Lakes, will continue to grow upward as the sand is 

 piled about them by the winds, until they have risen to a level 

 of a hundred feet above the starting point. 



The water of peat bogs contains little mineral matter, and 

 only a very scanty supply of nitrogen, in the form of nitrates 

 dissolved in it. The bog plants, therefore, must either get on 

 with an exceptionally small supply of nitrogen, or they must 

 get it from an unusual source. The peat mosses adopt the 

 former alternative, while the sundews, the pitcher plants, and 

 some other species adopt the latter and derive their nitrogen 

 supply largely from insects which they catch, kill, and digest. 



FIG. 3(38. A halophyte 

 (Salicornia) 



