LAKES, RIVERS, ETC. 



247 



stability of aquatic formations is the drainage 

 of land. It may, of course, incidentally pro- 

 duce fresh aquatic formations, as ditches and 

 drains (and artificial canals are of the same 

 type). But in very low-lying areas the forma- 

 tion of dikes or drains, and the withdrawal of 

 large sheets of water and their conduction into 

 definite and restricted areas, destroys aquatic 

 vegetation. This is what has occurred in the 

 Fens, causing the disappearance of wide areas 

 of aquatic, marsh, and fen vegetation. 



Lakes, Ponds, Pools, and Reservoirs. En- 

 closed tracts of water are as a rule still 

 waters. But there may be an inflowing 

 and outflowing stream, regulated frequently 

 by a sluice. The disturbance of such waters 

 is effected as a rule solely by the wind, or by 

 springs from beneath or at the sides. Such 

 tracts of water are normally sweet, though 

 stagnant, but may be rendered foul by the 

 percolation of sewage, by cattle, by waterfowl, 

 or by the decomposition of leaves in the case 

 of lakes, &c., in wooded areas. 



Lakes are either upland or lowland, and 

 highland with a peaty bottom, or lowland and 

 non-peaty. 



Ponds when stagnant may have a floating 

 association of Lesser Duckweed only, with 

 marginal wet-soil types, Rushes, Sedges. A 

 clearer pool may have either Stonewort or 

 Nitella on the bottom, Canadian Watervveed, 

 Pondweeds, Water Lily, Water Buttercup, 

 Manna Grass, Horned Pondweed, Celery- 

 leaved Crowfoot, Amphibious Knotgrass, and 

 be lined with Sedges, Rushes, Skullcap, &c. 



The zonation typical of the aquatic vegeta- 

 tion is in a lake, pond, &c., cyclic or elliptic. 

 A highland lake as mentioned contains few 

 types, whilst a lowland lake richer in mineral 

 constituents contains, in addition to the above, 

 such plants as Hippuris (Mare's-tail), Horn- 

 wort, Milfoil, Bladderwort, Water Soldier. 

 Starwort, &c., submerged or nearly sub- 

 merged; whilst floating forms include also 

 Frogbit, Arrowhead, &c. ; and in the mar- 

 ginal reed swamp also, Purple Loosestrife, 

 Great Hairy Willow Herb, Creeping Jenny, 

 Marsh Woundwort, Great Water Dock, Reed 

 Mace, Bulrush, and Reeds. 



Reservoirs are artificial or natural, and are 

 usually of lowland type with little or no peat, 

 but occasionally along the margin, Shore- 

 weed, Pillwort, &c., may be found. Hippuris, 

 or Mare's-tail, is common in such aquatic 

 habitats. 



Rivers, Canals, &c. The aquatic vegetation 

 of rivers differs little from that of streams, 

 except in the greater width of the former, and 

 the usually greater depth of the water. There 

 are quickly-flowing rivers poor in mineral 



salts, and slow-flowing rivers richer in mineral 

 salts in solution. The former are upland and 

 the water hard as a rule, the latter lowland 

 and the water soft. 



Some peculiar types are more at home in 

 slow-flowing water than in still waters; but 

 they exhibit different adaptations, having 

 ribbon-shaped leaves in this case, as in the 

 Arrowhead; and so does the common Pond- 

 weed of the pond in running water. Here 

 also grow Ranunculus circinahis, Water 

 Dropwort, Canadian Waterweed, Bur Reed, 

 Shining Pondweed, Perforate Pondweed, 

 Bulrush, and at the margin Brooklime, 

 Water Cress, Scorpion Grass, Marshwort, 

 Great Hairy Willow Herb, Starwort, &c. 

 Few floating-leaved types occur except where 

 the marginal vegetation deflects the course 

 and force of the current, as Water Lily, 

 Duckweed, Amphibious Knotgrass. In tin; 

 reed swamp many plants occur, as Meadow 

 Rue, Meadow Sweet, Water Betony, Sweet 

 Flag, Bur Reed, &c. Such plants occur also 

 in canals. 



In swift-flowing streams with no lime-salts 

 in solution occur Ranunculus Lenormuuli&w\ 

 R. hederaccus, with Manna Grass, Starwort, 

 &c. ; and in those with lime salts in solution 

 other types occur. 



Streams. Streams may be either upland or 

 lowland. The former are the beginnings of 

 rivers, and form torrents, cascades, and water- 

 falls, in which chiefly ferns luxuriate, and 

 lower cryptogams. In the lowlands streams 

 form the tributaries of rivers, being fed by 

 ditches and springs, between which they are 

 intermediate. The stream may, as in tin- 

 case of a river, form a boundary between 

 meadows or pastures, and is thus frequently 

 planted with low bushes; and in this way 

 there is little or no scope for the reed su amp 

 association, and in its place grow shade plants 

 or hygrophilous types. 



There is usually too little water in a stream 

 for the lowest zones of aquatic vegetation, but 

 occasionally Stoneworts may be found, though, 

 as they prefer standing water, and streams 

 are frequently quick-flowing, they are not 

 general in streams. But in the lowest zones 

 there is frequently a close growth of the 

 Canadian Waterweed. 



The chief pondweeds in a stream differ from 

 those in the river, being composed of such 

 species as Opposite-leaved Pondweed, I\>ta- 

 mogeton crispus, small forms which do not 

 require deep water. These have floating 

 leaves. The Amphibious Knotgrass is less 

 common in streams than in rivers, and so are 

 the Duckweeds. Water Buttercups are per- 

 haps more frequent in streams than in rivers, 



