PLANT ASSOCIATIONS 



941 



many algae and some higher plants) and of forms attached to the bot- 

 tom ; of the latter some forms are submersed, some have floating leaves, 

 and still others are in part emersed. Aquatic plants or hydrophytes, 

 especially those that are submersed, have many noteworthy structural 

 peculiarities that have been separately noted on previous pages, but 

 which may here be summarized. 



The root systems commonly are reduced, both in length and in amount 

 of branching, and root hairs are absent, at least in the water. True 

 water roots are hairless, and may possess root pockets. The chloren- 

 chyma is spongy and but slightly differentiated, and usually the plastids 

 are large and motile. The leaves of submersed plants are very thin and 

 often are finely dissected. Air chambers are capacious, often exceeding 

 the tissues in actual volume. Stomata are absent in submersed leaves 

 and are present only on the upper surfaces of floating leaves; where 

 present in floating or in emersed leaves, they have but slightly cutinized 

 walls and are almost always open. Protective features are few or want- 

 ing; for example, cutin and cork rarely are developed below the water 

 surface, hairs are scarce, and the cell sap has a low osmotic pressure; 

 the absence of protective structures is not disadvantageous, since ab- 

 sorption is easy, and below the water level, transpiration is slight or even 

 absent. Leaves equal or surpass roots in importance as absorptive 

 organs. Submersed organs usually are slime-covered, the slime har- 

 boring commensalistic communities of bacteria and other low organ- 

 isms. The aerial surfaces of floating organs usually are wax-coated and 

 thus are not readily wetted. Conductive and mechanical tissues are 

 greatly reduced. Vegetative reproduction is highly developed, both 

 through the fragmentation of ordinary shoots, and through the develop- 

 ment of winter buds. In the algae, reproduction and dispersal are 

 facilitated by zoospores and by motile gametes. Among the higher 

 plants, flowers and seeds are less abundant than in most habitats. 



Swamps. Various swamp stages in turn follow the primitive pond 

 associations, bulrushes, cat-tails, and reeds often being among the first 

 emersed plants, and sedges are often prominent later. In mesophytic 

 climates, thickets (as of willows and alders) soon appear, and they in 

 turn are replaced by forest vegetation. The structural features of swamp 

 plants are in part like those noted above, especially in the matter of re- 

 duced root systems and prominent air chambers, but in general they are 

 not unlike those seen in mesophytes; particularly is this true of leaf 

 thickness, stomata, chlorenchyma, and protective structures, The 



