316 PLANT STKUCTUKES 



buoyancy is provided for by the development of bladder- 

 like floats. 



Conspicuous among hydrophyte associations may be 

 mentioned the following : (1) Free-swimming associations, 

 in which the plants are entirely sustained by water, and are 

 free to move either by locomotion or by water currents. 

 Here belong the " plankton associations," consisting of 

 minute plants and animals invisible to the naked eye, 

 conspicuous among the plants being the diatoms ; also the 

 " pond associations," composed of algas, duckweeds, etc., 

 which float in stagnant or slow-moving waters. 



(2) Pondweed associations, in Avhich the plants are an- 

 chored, but their bodies are submerged or floating. Here 

 belong the "rock associations," consisting of plants an= 

 chored to some firm support under water, the most conspic- 

 uous forms being the numerous fresh-water and marine 

 algae, among which there are often elaborate systems of 

 holdfasts and floats. The "loose-soil associations" are dis- 

 tinguished by imbedding their roots or root-like |)i'ocesses 

 in the mucky soil of the bottom (Figs. 281, 282). The wa- 

 ter lilies with their broad floating leaves, the pondweeds or 

 pickerel weeds with their narrow submerged leaves, are 

 conspicuous illustrations, associated with which are algae, 

 mosses, water ferns, etc. 



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

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

 rise above the surface. The conspicuous swamp associations 

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

 and reed-grasses (Figs. 283, 284), tall wand-like Monocoty- 

 ledons, usually forming a fringe about the shallow margins 

 of small lakes and ponds ; " swamp-moors," the ordinary 

 swamps, marshes, bogs, etc., and dominated by coarse 

 sedges and grasses (Fig. 282) ; /^swamp-thickets," consist- 

 ing of willows, alders, birches, etc. ; ** sphagnum-moors," in 

 which sphagnummoss predominates, and is accompanied by 

 numerous peculiar orchids, heaths, carnivorous plants, etc. ; 



