This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
We have pond-weeds in abundance, but the Potamogetons are the pond-weeds par excellence. There is scarcely a piece of water in this country, be it river, lake, pond, canal, or intermittently dry ditch, but has one or more species growing there. The genus is a very difficult one, such as it is impossible to do more than show the general characters of here. Hooker and Bennett, in their revision of the genus, give twenty-one British species with a number of connecting sub-species and varieties. The one figured here is the Oblong Pond-weed (P. polygoni-folius), with narrowly egg-shaped floating leaves, and narrower submerged leaves. All have long leaf-stalks. The floating leaves always present the upper side to the air, and are always perfectly dry. The flowers are greenish and unattractive, collected into a slender spike. Individually they consist of a four-parted perianth, four stamens, four carpels. There is a species (P. natans) with broader floating leaves and narrow submerged leaves. A broader still is P. plantagineus, with clearer leaves and more slender leaf-stalks. P. crispus, P. densus, P. perfoliatus, P. praelongus, etc., have only submerged leaves, which are more or less oblong.
Potamogeton polygonifolius. - Naiadeae. -