182 PLANT STUDIES 



132. Associations. — The hydrophyte associations may be 

 put into two great divisions : 



1. True hydrophytes, in which the contents and tem- 

 perature of the water are favorable to plant activity. 

 Among such associations may be mentioned the following : 

 (1) Free-swimming associations, in which the plants are 

 entirely sustained by water, as the " pond associations," 

 composed of algas, duckweeds, etc., which float in stagnant 

 or slow-moving waters. 



(2) Pondweed associations, in which the plants are 

 anchored, but their bodies are submerged or floating. 

 Here belong the " rock associations," consisting of plants 

 anchored to some firm support under water, as the alga? ; 

 and the "loose-soil associations," which imbed their roots 

 in the mucky soil of the bottom (Fig. 163), the water 

 lilies and pickerel weeds being conspicuous illustrations. 



(3) Swamp associations, in which the plants are rooted 

 in water, or in soil rich in water, but the leaf-bearing stems 

 rise above the surface. The conspicuous swamp associa- 

 tions are " reed swamps," characterized by bulrushes, cat- 

 tails, and reed-grasses (Figs. 164, 167) ; " swamp-moors," 

 the ordinary swamps, marshes, bogs, etc., aud dominated 

 by coarse sedges and grasses (Fig. 163) ; and " swamp- 

 thickets," consisting of willows, alders, birches, etc. 



2. Xerophytic hydrophytes, in which the contents and 

 temperature of the water are unfavorable to plant activity, 

 and the structures of the plants are adapted to reduce 

 transpiration. This results in such xerophytic structures 

 as are displayed by the true xerophytes (see §144). Here 

 belong the " sphagnum moors " (Fig. 191), in which sphag- 

 num moss predominates, and is accompanied by numerous 

 peculiar orchids, heaths, carnivorous plants, etc. ; " swamp- 

 forests," where tamarack, spruce, pine, etc., are the pre- 

 vailing trees; "mangrove swamps," of the flat tropical sea- 

 coasts; and "salt marshes," the extensive meadow-like ex- 

 panses of coarse sedges and grasses near the sea-coast. 



