PLANT SOCIETIES 



451 



638. Edaphic plant societies. These are societies the plants 

 of which are chiefly controlled by the peculiar conditions of the 

 soil. There are a number of different kinds of edaphic plant 

 societies determined by the character of the physiographic areas. 

 First. Sphagnum moors. These are formed in shallow basins 

 originally with more or less water. The growth of the sphagnum 



Fig. 410. 



Perennial rosette plant from alpine flora of the Andes, showing short stem, rosette of 

 leaves, and large flower. (After Schimper.) 



moss along with other vegetation and its partial decay in the 

 water builds up ground rapidly so that in course of time the 

 pond may be completely filled in. This filling in proceeds from 

 the shore toward the center, and in the early stages of course 

 there would be a pond in the center. The partial decay of vege- 

 tation creates an excess of humus acid which retards absorption 

 by the roots. The conditions are such, then, as require aerial 

 structures for retarding the loss of water, and plants growing in 

 such moors are usually xerophytes. Some of the plants are 

 identical with those growing in the arctic tundra. Second. 

 Sand strand or beach* The quantity of sand with very little or 

 no admixture of humus or plant food makes it difficult for plants 

 to obtain a sufficient amount of water even where rainfall is 

 abundant. The same may be said of the sand dunes farther 

 back from the shore. The plants of these areas are then usually 



* See Chapter LIV of the author's College Text-book of Botany. 



