Herbs: rhizomes present or absent; tubers absent; turions present or absent. Stems terete or compressed, nodes occasionally with oil glands; turions with extremely shortened internodes, divided into outer and inner leaves; outer leaves 1--5 per side, similar to vegetative leaves or occasionally corrugate near base; inner leaves 1--10, rolled into fusiform structure, unmodified, or shortened and oriented at 90° angles to outer leaves. Leaves submersed or both submersed and floating, alternate to nearly opposite; stipules connate or not, if not, then convolute, tubular, sheathing stem and young inflorescences. Submersed leaves sessile or petiolate; stipules either free from or adnate to base of leaf blade for less than ½ length of stipule, if adnate, then extending past adnation as free ligule; blade translucent, linear to orbiculate, not channeled, flattened, base acute to perfoliate, margins entire or serrate, rarely crispate, apex subulate to obtuse; veins 1--35. Floating leaves petiolate, rarely nearly sessile; stipules free from base of leaf blade; blade elliptic to ovate, leathery, base cuneate to rounded or cordate, margins entire, apex acute to obtuse; veins 1--51. Inflorescences spikes or panicles of spikes, submersed or emersed, capitate or cylindric; peduncles stiff, if long enough then projecting inflorescence above surface of water. Flowers: pistils 1 or 4. Fruits abaxially rounded or keeled, flattened to turgid, beaked; embryo coiled 1 or more times. x = 13 or 14. Potamogeton is one of the most important genera in the aquatic environment, especially as food or habitat for aquatic animals (R. R. Haynes 1975). A few species become slightly weedy, but not significantly so. Plants of Potamogeton are important in stabilizing substrates and removing particulate matter from the water column. The genus has been divided into several sections and numerous subsections (predominantly by J. O. Hagström 1916; see also R. R. Haynes 1975, 1985 for in-depth coverage of three subsections). After studying thousands of specimens over at least five continents, we believe that recognition of the many infrageneric categories is unwarranted. Consequently, we are not including infrageneric classification here.
Hybridization is common among members of the genus (J. O. Hagström 1916). Numerous hybrids were proposed, using intermediate stem anatomy as evidence of hybrid origin. We list all the hybrids that Hagström proposed for species that occur in North America. An additional 26 hybrids have been recognized for the British Isles (C. D. Preston 1995). Vegetative and reproductive morphology varies considerably in the genus. Two types of stems occur, rhizomes and erect stems. Some species have both, others have only erect stems. Two types of leaves exist, submersed and floating. Floating leaves have well-developed epidermis abaxially and adaxially, and well-developed cuticle at least adaxially. Floating leaves may be similar in shape to that of the submersed, or they may differ considerably. Submersed leaves have no cuticle and do not have well-developed epidermis. All species of Potamogeton have submersed leaves; some also have floating leaves. Occasionally, individuals of floating-leaved species lose their submersed leaves because of decay or wave action. Leaves of Potamogeton may be sessile or petiolate and are divided into at least blade and stipule. The stipule may be adnate to the blade for 1/3 or less the length of the stipule. Venation in the stipule is parallel, and veins may appear coarse as distinct ridges on the stipule (fibrous), or they may be much less obvious, even difficult to observe (delicate). Stipular tissue between veins of fibrous stipules decays, leaving strands of fibers, whereas veins and the tissue between them decay in delicate stipules.
Many species have oil glands on the stem at the node of submersed leaves. These glands are especially common on species with sessile leaves. Circular and ranging from green to golden to white, they are present at most nodes, sometimes at all, or possibly only occasionally present. The glands (or nodal glands) are best observed with dried specimenses, a good light source, and magnification of at least 15´, although they can be observed under less ideal conditions.
Inflorescences may be either emergent or submersed. Emergent inflorescences are elongate and almost always terminal on the stem, whereas submersed inflorescences are globular and axillary. Most species have either emergent inflorescences or submersed inflorescences, but not both (monomorphic). Other species have both types of inflorescences on one plant (dimorphic).
All specimens should be collected when in fruit. Fruiting characteristics are extremely important in the genus, although they are not always given in the key. Vegetative features during fruiting are distinctive for the species; consequently, they are included in the key. Important features of the fruit include presence or absence of lateral and abaxial wings, ribs, ridges, or keels. Here, 'ribbed' indicates a raised 'vein' on a rounded surface; 'ridged,'; a ridge with an obtuse angle; 'keeled,' a ridge with an acute angle; and 'winged,' a ridge that appears to have a wing distally.