242 BRITISH PLANTS 
narrow leaves. Where the water varies in depth, the 
plants become arranged in zones, or associations. In 
the deeper part Scirpus lacustris is usually dominant ; 
next to this, on the landward side, is a zone of Phragmites 
communis. Other associations may be dominated by 
sedges (Carex), Typha latifolia (bulrush or reed-mace), 
and Equisetum limosum (horsetail). These plants, how- 
ever, have a wide range, and where the water is uniformly 
shallow the deeper-growing forms invade the shallower 
zones, and a mixed association is the result. The 
Phragmites-association is the commonest, and may ex- 
tend for miles along the banks of rivers, especially in the 
brackish water of estuaries. The dominant plant often 
occurs to the complete exclusion of all others. The 
“narrow upright leaf, combined with the cespitose habit, 
allows the plants to grow very close together, and produce 
a deep shade in which no broad-leaved plant can live. 
Where the vegetation is not very dense, dicotyledons 
and broad-leaved monocotyledons occur. Plants of this 
type which grow in the deeper water are : @nanthe fistu- 
losa and @. Phellandrium (water-dropworts), Sagittaria 
sagittifolia, Alisma Plantago (water-plantain), and Dama- 
sonium stellatum (star-fruit, confined to some of the 
South-eastern Counties). These plants approach the 
hydrophyte in many respects, especially @nanthe Phellan- 
drium, which possesses very finely-divided submerged 
leaves and broader aerial ones; whilst Sagzttaria has 
already been referred to as growing as a true aquatic, 
with very long submerged leaves, in rivers and streams. 
Nearer the bank, where the water is shallower, the 
plants have only a small part of their erect stems sub- 
merged. Narrow-leaved monocotyledons, not usually 
dominant, include: Iris Pseudacorus, Sparganium ramo- 
sum (bur-reed), Acorus Calamus (sweet sedge), Butomus 
umbellatus (flowering rush), Triglochin palustre (marsh 
arrow-grass), Phalaris arundinacea (reed canary-grass), 
Glyceria aquatica (reed manna-grass), Catabrosa aquatica 
(whorl-grass), and Alopecurus geniculatus (marsh fox- 
tail). Of broader-leaved forms, the following commonly 
occur: Ranunculus Lingua (spearwort), R. Flammula 
(lesser spearwort), Nasturtium officinale and N. amphibium 
(watercresses), Hypericum Elodes (marsh St. John’s-wort), - 
Apium nodiflorum, and A. inundatum (marshworts)—the 
