316 PLANT STRUCTURES 



buoyancy is provided for by the development of bladder- 

 like floats. 



Conspicuous among hydrophyte societies may be men- 

 tioned the following : (1) Free-swimming societies, 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 societies," consisting of minute 

 plants and animals invisible to the naked eye, conspicuous 

 among the plants being the diatoms ; also the " pond so- 

 cieties," composed of algae, duckweeds, etc., which float in 

 stagnant or slow-moving waters. 



(2) Pondweed societies, in which the plants are an- 

 chored, but their bodies are submerged or floating. Here 

 belong the " rock societies," consisting of plants anchored 

 to some firm support under water, the most conspicuous 

 forms being the numerous fresh-water and marine algae, 

 among which there are often elaborate systems of holdfasts 

 and floats. The "loose-soil societies" are distinguished 

 by imbedding their roots or root-like processes in the mucky 

 soil of the bottom (Figs. 281, 2S2). The water lilies with 

 their broad floating leaves, the pondweeds or pickerel weeds 

 with their narrow submerged leaves, are conspicuous illus- 

 trations, associated with which are algae, mosses, water 

 ferns, etc. 



(3) Swamp societies, 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 societies 

 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. ; 



