AQUATIC VEGETATION 231 
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. Nymphea alba and Nuphar lutea (water-lilies). 
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. Prlularia 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 Ceratophyllum 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 floating, as in the 
other species of duckweed (Lemna minor, L. gibba, etc.), and 
Hydrocharis Morsus-rane (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 
