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.
Potamogeton polygonifolius, Pourr.( = P. oblongus, Viv.). - The habitat of this plant is pools, ditches, or small streams and ponds, lakes and pools on heaths. The plant has the pondweed habit. The stem is creeping below. The lower submerged leaves are membranous, linear, lance-shaped, often very narrow. The upper leaves are rather leathery, floating, oblong- to elliptic or lance-shaped. All the leaves which are round or heart-shaped below are long-stalked. The leaf-stalks are not jointed, and bear leaves. The blade may run down the leaf-stalk. The spikes are short and irregular. The sepals are elliptic, transversely stalked. The fruit is a drupelet, red, with rounded margins, faintly keeled with lateral ridges. The plant is floating, and flowers in June and July, being a herbaceous perennial.