AQUATIC VEGETATION. 



they often catch debris, and eventually fix a soil upon which 

 shore vegetation, then land herbs and shrubs, and finally trees 

 grow. 



1116. Structural types of limnetic plants. There are certain 

 types of plant form and structure in relation to the water envi- 

 ronment recognized. These are as follows: 



1. The quillwort type. Plants submerged, rooted to the 

 soil, with rosette habit and mostly cylindrical terete leaves. 

 This type is represented by the Isoetes, or quillwort formation, 

 the Pilularia formation, water-lobelia formation, etc. 



2. The water-lily type. Plants floating with leaves and long 

 flexile petioles or stems arising from creeping or subterranean 

 perennial rootstocks. Examples : white water-lily formation (Cas- 

 talia odorata = Nymphaea odorata), yellow water-lily formation 

 (Nymphaea advena = Nuphar ad vena), water-fern formation 

 (Marsilia formation), floating pond weed formation (Potamoge- 

 ton natans formation), etc. 



3. The pondweed type. Plants entirely submerged, rooting to 

 the soil, with long slender floating stems. Examples: most of 

 the pondweeds (Pondweed formations), Zannichellia formation, 

 Naias formation, Heteranthera formation, Myriophyllum forma- 

 tion, etc. 



4. The duckweed type. Free floating plants with short stems, 

 or "fronds." Examples: duckweed or duckmeat (Lemna) for- 

 mation, Riccia formation (Riccia natans, fluitans, etc.), Salvinia 

 formation, Azolla formation, etc. 



5. The river-weed, or fluvial, type. Plants fastened to stones 

 in streams, entirely submerged, stems slender, flexible, frond- 

 like, leaves not differentiated. Examples: river-weed (Podo- 

 stemon) formation, Fontinalis (fluvial mosses) formation. Among 

 the algae fluvial forms like Cladophora, Lemanea, etc., belong here. 



III. Marine, or Pelagic, Plant Societies. 



1117. By far the larger number of the fixed, or benthonic, 

 marine plants are lithophytes (attached to rocks), and the larger 

 number of these are algae. The larger ones are attached by 



