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.
Streams may be either upland or lowland. The former are the beginnings of rivers, and form torrents, cascades, and waterfalls, 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 the-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 swamp 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, pota-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 perhaps more frequent in streams than in rivers, the forms in the latter being larger. Rarely Water Lilies occur where the stream is wider at a bend. Manna Grass is abundant, and Flote Grass (Catabrosa aquatica) is not uncommon.
In what corresponds to the reed swamp may be found Meadow Rue, Water Cress, Great Yellow Cress, Purple Loosestrife, Great Water Chickvveed, Great Hairy Willow Herb, Water Bedstraw, Fleabane, Coltsfoot, Butterbur on the banks, Scorpion Grass, Water Betony, Musk, Brooklime, Mints, Gipsywort, Skullcap, Yellow Flag, Bur Reed, Water Plantain, Sedges, and Phalaris arundinacea.
The ditch is largely artificial, and thus no measure of the true aquatic vegetation of a district even on the small scale on which it is developed. It is thus of relatively modern date. But it is of importance as showing the types of vegetation that have once flourished in the district, for ditches afford the last resorts of the hygrophilous types that have been driven from the wet meadows, once marshes, etc.; whilst a few are relics of true aquatic vegetation.
The types of aquatic plants found in ditches are restricted in character. They include chiefly plants that grow in the reed swamp, and in fact the whole ditch may be filled up with such plants as Marsh Marigold, Water Cress, Great Water Chickweed, Great Hairy and other Willow Herbs, Water Bedstraw, Fleabane, Hemp Agrimony, Coltsfoot, Water Figwort, Marsh Thistle, Scorpion Grass, Brooklime. In some ditches Frogbit used to grow. Gipsywort may occur here and there, and Amphibious Knotgrass, the terrestrial riparial type. It is rarely that such large plants as Yellow Flag, Phalaris arundinacea, and Reed grow in ditches, indicating vestiges of more extensive aquatic vegetation. Wood Club Rush is more frequent. Duckweed may fill up a stagnant ditch almost entirely. Frequently Marshwort (Sium erectum), numerous Sedges, and some moisture-loving Grasses, as Manna Grass, Poa trivialis, etc, may be found in a ditch. Starwort, and even Canadian Waterweed, Water Violet, Milfoil, and Hornwort may be found in ditches of a particular type.