THE LARGER AQUATIC VEGETATION 197 



and the depth of water for a species varies much according to 

 clearness and other factors which may be incidental to a particular 

 region. 



It is possible to make a general grouping which will indicate the 

 important conditions and varieties of environment. To some one 

 of the groups thus established any aquatic plant may be assigned. 



I. Plants without attachment. 



(a) Plankton, free swimming, microscopic. 



(b) Macroscopic, possibility of attachment by accidental 



lodgment, as Lemna, Ceratophyllum, filamentous algae 

 common on plants in coves. 

 II. Plants attached to substratum. 



(a) Submerged, algae as Chara and Cladophora, and phan- 



erogams as VaUisneria, Elodea. 



(b) Partially submerged, usually with floating or emersed 



leaves, as Nymphaea, Bidens beckii. 



III. Swamp plants or marsh forms with roots and rhizomes under 

 water but leaves usually emersed though able to 

 endure inundation and temporary submersion. 



Sometimes representatives of each of these three classes may be 

 found in one small lake, especially if the water gradually deepens 

 from a marsh or low shore as in Lake St. Clair where the wholly 

 submerged species become so abundant as to form an aquatic 

 meadow. Potamogeton perfoliatus, P.foliosus, P. zoster aefolius, Val- 

 lisneria, Naias, Chara, Nitella, Elodea, and Myriophyllum may be 

 found in dense patches covering the bottom. In such formations 

 the struggle for space must be severe and from observations during 

 three summers on Lake Erie I should say that Vallisneria is a con- 

 queror. Naias flexilis may be found in distinct formations in 

 which other species are very infrequent but being an annual the 

 creeping rootstocks of Vallisneria may easily invade its territory. 



In many of the small inland lakes the water plants are important 

 agents as soil collectors. The aquatic meadows tend to filter the 

 water so that suspended matter sinks to the bottom. As the lake 

 gradually becomes filled and the water grows more shallow a suc- 

 cession of plant societies occurs. The aquatic meadows yield to the 



