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 (Car ex), 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 caespitose 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 : (Enanthe fistu- 

 losa and (E. PTiellandrium (water-drop worts), 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 (Enanthe PJiellan- 

 drium, which possesses very finely-divided submerged 

 leaves and broader aerial ones ; whilst Sagittaria 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 (spear wort), J?. Flammula 

 (lesser spearwort). Nasturtium officinale and N. amphibium 

 (watercresses), Hypericum Elodes (marsh St. John's-wort), 

 Apium nodiflorum, and A. inundatum (marshworts) the 



