This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The habitat of this plant is lakes, pools, ponds, ditches, and slow streams. The plant has the pondweed habit. The stem is creeping below, simple, round in section. The leaves are leathery, elliptic. The floating leaves are alternate, ovate, elliptic to lance-shaped, long-stalked, the blade decurrent on the leaf-stalk, folded, the leaf-stalk jointed below the limb. The lower leaves are linear to lance-shaped, or awl-like. The stipules are long and narrow-pointed. The flowers, with parts in fours, are in a dense spike which rises above the water, on a stout stalk, with stalked, roundly rhomboidal sepals. The anther-cells are not parallel. The fruit is large, rounded, greenish, flattened, keeled on the back when dry, round when green. The plant is floating, flowering in June and July, and is a herbaceous perennial.
Potamogeton fluitans, Roth. = P. natatis x P. lucens. - As a hybrid between the two species cited the characters lie between the two, the blade of the upper floating leaves gradually merging into the leaf-stalk. This species or hybrid has been found in 5 vice-counties: W. Sussex, Surrey, Cambridge, Northants, and Hunts. It is also found in Ireland. It is very rare, and the habitat is canals. The plant is floating, and flowers in June and July, and is a herbaceous perennial.
The habitat of this plant is stagnant, peaty water, ditches, fens. The stem is creeping below, branched, with long runners from the axils. The leaves are all shortly-stalked, membranous, transparent, netted, blunt, the upper leaves opposite, nearly stalkless, elliptic, or rounded, the lower oblong, heart-shaped below. The stipules are short, broad, and blunt. The leaf-stalk is planoconvex above. The flowers are in long, cylindrical, dense spikes, on very slender equal stalks. The sepals are ovate. The anther-cells are nearly parallel. The fruit is green, very small, rounded on the back when fresh, keeled when dry, with an obsolete beak. The plant is floating, and flowers between June and September, being a herbaceous perennial.
The habitat of this plant is ditches, slow streams, ponds, canals. The stem is simple, round in section. The leaves are entire, leathery, short-stalked or stalkless, translucent, the lower submerged leaves stalkless, tinged with red, many-nerved, lance-shaped, narrowed both ends, netted, the upper leaves floating or erect, broader, leathery, inversely ovate, blunt, narrowed into a short leaf-stalk. The stipules are large, blunt, not winged. The flower-stalk is equal. The sepals are transversely oval. The spike is stout. The drupelets are ovate, with a long point, red, convex ventrally, semicircular dorsally, keeled. The plant is 1-4 ft. long, flowering between July and September, and is a herbaceous perennial.