$88 RELATION TO ENVIRONMENT. 



sand, humus, or morass; 3d, the percentage of humus. Varia- 

 tions in exposure to tides, wave action, drainage, and winds 

 affect not only the topography of the shore, but also the develop- 

 ment of the plant formations. 



In general, then, there are three different kinds of strand as 

 regards the physical character of the ground: 

 i st. Rocky shores, or Lithophytic. 

 2d. Sandy shores, or Psammophytic. 

 3d. Loamy shores, or Humiphytic. 



The first two condition open growth of vegetation and approach 

 the desert type, while the last one conditions a close growth. 



The first and third types will be treated of under the head 

 of the various rock and swamp societies (Chapter LV). 



1079. Formation of sandy shores. The sand of the sea or 

 lake shores is formed by the action of the beating waves or cur- 

 rents on rocky shores; or the grinding action of rocks on the 

 beds of streams or under moving glaciers. From streams this 

 is washed down to the seas or lakes, where, if the shore is not too 

 precipitate, it is deposited where the wave action may throw it 

 up on the shore and form the sandy beach. So wave action on 

 rocky sea or lake coasts grinds the rock down into sand in places, 

 which is again washed up on the shore to form the sandy beach. 

 When the sand accumulates on the shore, at the upper edges, 

 during calm periods of the water, it dries out and then is easily 

 swept inland by the wind, when it forms fields or hills of sand 

 known as sand-dunes. The vegetation of sandy shores can 

 then best be studied under two heads: ist, the vegetation of the 



beach or strand; and 2d, the vegetation of sand-dunes. 



\ 



II. The Vegetation of the Beach or Strand. 



1080. Divisions of the strand. Sandy shores are sometimes 

 spoken of as the true strand, or beach. There are certain natu- 

 ral divisions in the topography of the beach, due largely to the 

 action of the water, and parallel with the coast line, and the 

 plant formations are found to bear a distinct relation to these 



