PLANT ASSOCIATIONS 



441 



easily explained by a concrete illustration of succession often 

 exhibited in the history of a pond or lake which becomes grad- 

 ually filled with soil and decaying vegetation. The plants 

 comprising the first association in such a habitat are wholly 

 hydrophytic and consist of free masses of algse and of shore 

 plants such as flags (7m), bulrushes (Scirpus), and arrowhead 



: 



-V--X" 



v 



FIG. 287. Zonation of grass (Deschampsia), bulrushes (Scirpus), and 

 pines (Pinus ponderosa) 



After Clements 



(Sagittaricf). As the pond or lake becomes shallower by the 

 accumulation of vegetable remains and the iiiwash of soil the 

 shore plants encroach more and more upon the water area, fol- 

 lowed by grasses and sedges which convert the old shore line 

 into a marsh or bog. This marsh or bog may form a permanent 

 plant association for a considerable period or it may be converted 

 by willows, alders, and similar shrubs into a thicket, and then by 

 poplars, ash, maples, and oaks into a typical mesophytic forest. 



