AQUATIC VEGETATION 237 



distinct association. The deepest parts of lakes do not 

 receive sufficient light to support green plants, and the 

 only vegetation consists of colourless bacteria. In a 

 lowland lake or pond, travelling from the deeper to the 

 shallower water, the following zones may be met with : 



1. Chara and Nitella (stoneworts), often forming a 

 dense pasture. 



2. Elodea canadense. 



3. Potamogeton (several species) and Myriophyllum. 



4. Nymphcea alba and Nuphar lutea (water-hlies). 



5. Water-crowfoots. 



6. Glyceria fluitans, an aquatic grass, and Zannichellia 

 palustris (horned pondweed), a grass-like plant, both 

 with narrow floating leaves. 



7. Polygonum amphibium. 



Where the water is uniformly deep, as in shallow ponds, 

 ditches, and rhines, these plants may occur together, 

 forming a mixed association. Other rooted aquatics 

 present, but not usually in such quantity as to con- 

 stitute a separate association, are : Hippuris vulgaris 

 (mare's-tail), in which some of the very narrow whorled 

 leaves on the flowering shoots are above the surface of 

 the water ; Hottonia 'palustris (water- violet), entirely sub- 

 merged, with finely-cut leaves ; Callitriche aquatica 

 (water-starwort), with narrow, widely-separated sub- 

 merged leaves, and a rosette of crowded, floating, broader 

 leaves : this plant is amphibious, and can also grow on 

 wet mud, with all the leaves in the air. Pilularia globu- 

 lifera (pillwort), a water-fern, and Littorella lacustris 

 (shore-weed), which have awl-shaped leaves, are also as 

 commonly found on the wet margins of lakes as sub- 

 merged in water. 



Living amongst the rooted plants may be found many 

 free-floating forms. These are either entirely submerged, 

 as in Geratophyllum demersum (hornwort) and Utricularia 

 (bladderwort, Fig. 46), which possess very finely-divided 

 leaves, and Lemna trisulca (ivy-leaved duckweed), with 

 short, narrow leaves ; or the leaves are fioating, as in the 

 other species of duckweed (Lemna minor, L. gibba, etc.), and 

 Hydrocharis Morsus-rancB (frogbit. Fig. 109), which has the 

 appearance of a miniature water-lily. In small ponds these 

 free-floating plants with floating leaves, especially the duck- 

 weeds, may become dominant, and prevent the growth 



