Aquatic herbs, submerged or with floating leaves, occasionally subterrestrial. Stems elongated. Leaves alternate or opposite (rarely in whorls of 3), sheathing at the base. Flowers hermaphrodite, ebracteate, in pedunculate spikes. Perianth of 4 unguiculate, sepaloid segments. Stamens 4, adnate at the base to the perianth-segments; anthers sessile, opening by longitudinal slits. Carpels (1-)4, free or shortly connate at the base, 1-ovulate; styles usually short. Fruitlets drupaceous or achenial. Seeds without endosperm; embryo unciform or spiral (coiled more than 1 complete turn).
1 Leaves all opposite or rarely in whorls of 3; fruitlets
achenial ................................................................................... 2. Groenlandia
1 Leaves alternate, or the involucral ones subopposite;
fruitlets drupaceous ............................................................... 1. Potamogeton
1. Potamogeton L.
By J.E. Dandy.
Leaves alternate, or the involucral ones subopposite; sheaths adnate to the leaf-base, ligulate, or free from the leaf-base and stipuliform. Carpels (1-)4, free or shortly connate at the base. Fruitlets drupaceous, with bony endocarp and softer exocarp; embryo unciform or spiral.
Hybridization is frequent within each of the two subgenera, but does not occur between them, their methods of pollination being different. With the exception of P. ? zizii (5 ? 6), which often bears well-formed fruitlets, all the hybrids are sterile, entirely without fruit or with occasional malformed fruitlets. The hybrids frequently form vigorous, persistent clonal populations, and, especially in rivers and canals, may become dispersed over long distances by means of vegetative winter-buds or detached fragments. As the hybrids show varying degrees of intermediacy between the parent species it is not practicable to include them in the key, but some of the more frequent are mentioned in the text and others are included in the index; the distinguishing features of most of them are given by J. E. Dandy in C. A. Stace, Hybridization and the Flora of the British Isles 444-459. London. 1975.
The floating leaves which occur in some of the species and hybrids are always petiolate, with an expanded lamina which is usually opaque but occasionally translucent. Some of the floating-leaved species and hybrids can grow subterrestrially in dried-up situations or in Sphagnum-bogs, in which case the floating leaves become aerial.
Potamogeton species are found in a wide variety of fresh-water habitats - in deep or shallow water, stagnant to fast-flowing; a few species, especially those of Subgen. Coleogeton, can flourish also in brackish water. Some of the species are more or less restricted to base-rich water, but the majority are tolerant of a considerable range of water conditions. The broad-leaved species (Sect. Potamogeton) vary greatly in the size and shape of the leaves according to the type of water, and leaf-dimensions are of little diagnostic value in this group.
Sect. POTAMOGETON. Rhizome creeping, often producing winter-buds; stems terete. Leaves submerged or floating, mostly convex-sided or some reduced to narrowly linear phyllodes; stipules open. Spikes many-flowered, cylindrical, dense. Carpels 4, free. Fruitlets short-beaked; embryo unciform.