This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
There are certain spots, usually low-lying, or hollows at higher altitudes, that preserve a type of vegetation which exhibits a marked difference to that of the surrounding higher and drier ground. They may be the relics of former aquatic vegetation, of a marsh, a bog, or a wet heath, and as a whole are hygrophilous types. They may differ little in general character from a wet meadow, but usually contain some distinctive species that stamp the flora as something more specialized.
Their connection with aquatic vegetation is clear, but they may be members originally of an entirely distinct formation. Such plants are Meadow Rue, Lesser Spearwort, Creeping Yellow Cress, Land Yellow Cress, Great Yellow Cress, Hairy Bitter Cress, Bog Stitch-wort, Water Blinks, Waterwort, Square-stalked St. John's Wort, Great Hedge Lotus, Hyssop Loosestrife, Water Purslane, True Square-stalked Willow Herb, Least Marshwort, Cowbane, Water Parsnip, Marsh Cudweed, Bur Marigold, Meadow Thistle, Bastard Pimpernel, Brookweed, Tufted Scorpion Grass, Mudwort, Water Speedwell, Round-leaved Mint, Peppermint, Water Germander, Bistort, Water Pepper, Water Dock, Osier, Loose - flowered Orchid, Marsh Orchis, Spotted Orchid, Butterfly Orchid, Spring Snowflake, Loose-flowered Soft Rush, Common Hard Rush, Small Capitate Rush, Marsh Arrowgrass, Compressed Club Rush, Brown Club Rush, Oval-headed Sedge, Broad - leaved Water Sedge, Pink - leaved Sedge, Distant-spiked Sea Sedge, Cutgrass, Marsh Foxtail, Beard Grass, Tufted Hair-grass, Great Water Reed Grass, Flote Grass, Greater Fescue Grass, Meadow Barley. A number of these are members of different formations, but all agree in requiring a moist habitat.
The great difference between soil, which is a solid immobile substratum, and water, which is a liquid mobile solution, naturally enables one to distinguish as a rule between a land and a water plant at sight, even when the material is a dried herbarium specimen, since the difference in habitat is closely correlated with a marked difference in habit. The chief points of difference are the reduction in the roots, which are long and thread-like, or borne in whorls around a creeping rhizome, or subaqueous stem, or even absent.
The stems and branches also are slender, herbaceous, differing in structure, and show every sign of reduction, being seaweed-like in habit in the case of submerged types, and in erect half-submerged types seldom thick or woody. The leaves differ in form, texture, and arrangement, being adapted to a floating or streaming habit, or of the grass type in the reed swamp as a rule. Riparial types of plants are most nearly akin to terrestrial types.
The flowers also are seldom brilliant, and are frequently reduced or apetalous, many types of aquatic plants relying mainly on vegetative reproduction.
The adaptations to aquatic conditions in internal structure have been referred to. The adaptations in external form or leaf arrangement are exemplified by the finely-divided or dissected foliage of Water Buttercups, Water Dropwort, Water Violet, Milfoil, Hornwort, Bladderwort. The carbonic acid gas in water is more abundant but less available, hence the dissected type of leaf, which also offers less resistance to the current, and a greater relative surface exposed to the light, and for the absorption of oxygen. The ribbon type of leaf is an adaptation to the currents, and is found in Water Plantain, Arrowhead, Flowering Rush, etc.
Another type is the clustered awl-shaped leaf, as in Water Lobelia, Shoreweed, Pill-wort, etc. Floating leaves may be orbicular, as in some types of Water Buttercup, Water Lilies, Frogbit, or ribbon-shaped, as in Manna Grass, Bur Reed, Bulrush. The Water Buttercups and some other types have two types of leaves, adapted to either the submerged or floating position.