Herbs rhizomatous or not rhizomatous; turions present or absent; tubers absent. Stems terete or compressed, nodes occasionally with oil glands; turions with extremely shortened internodes, divided into inner and outer leaves, inner leaves 1--10, rolled into a fusiform structure, unmodified, or shortened and oriented at 90° angles with respect to outer leaves, outer leaves 1--5 per side, mostly similar to vegetative leaves or occasionally corrugated near base. Leaves submersed or both submersed and floating, alternate to subopposite; stipules tubular, sheathing stem and young inflorescences, connate or convolute; submersed leaves sessile or petiolate, stipules either free from or adnate to base of leaf blades for less than ½ length of stipule, if adnate, then extending past adnation as a free ligule, blades pellucid, linear to orbicular, not channeled, flattened, subulate to obtuse at apex, acute to perfoliate at base, margins entire or serrate, rarely crisped, veins 1--35; floating leaves mostly petiolate, rarely subsessile, stipules free from base leaf blades, blades coriaceous, elliptic to ovate, acute to obtuse at apex, cuneate to rounded or cordate at base, margins entire, veins 1--51. Inflorescence a capitate or cylindrical spike or panicle of spikes, submersed or emersed; peduncle stiff. Flowers: gynoecium of 1 or 4 carpels. Fruit abaxially rounded or keeled, beaked, flattened to turgid; embryo coiled 1 or more times. x = 13 or 14.

Species ca. 100 (33 in the flora): nearly worldwide.

As Haynes (1975) has pointed out, Potamogeton is one of the most important genera in the aquatic environment, especially as food or habitat for aquatic animals. A few species become slightly weedy, but nothing really significant. In addition to being food for animals or pests, they are important in stabilizing substrates and removing particulate matter from the water column.

The genus has been divided into several sections and numerous subsections by several botanists, but predominantly by Hagström (1916). Haynes (1975, 1985) has published on three subsections in particular. 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 such an infrageneric classification here.

Hagström (1916) suggested that hybridization was common among members of the genus. In fact, he proposed numerous hybrids, using intermediate stem anatomy as evidence of such hybrid origin. We list all the hybrids that Hagström proposes for species that are known to occur in North America. Preston (1995), too, has considered hybridization to be common in the genus, and recognized 26 for the British Isles.

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 the erect stem. Two types of leaves exit, submersed leaves and floating leaves. The floating leaves have a well-developed epidermis on both the abaxial and adaxial surfaces, and a well-developed cuticle on at least the adaxial surface. These leaves may be similar in shape to that of the submersed leaves, or they may differ very much in shape. Submersed leaves are without a cuticle and usually are without a well-developed epidermis. All species have submersed leaves, whereas others also have floating leaves. Occasionally, individuals of floating-leaved species lose their submersed due to decay or way action. The leaves of Potamogeton may be sessile or petiolate and are divided into at least a blade and a stipule. The stipule may be adnate to the blade for 1/3 or less of the length of the stipule. Venation in the stipule is parallel, and the veins may appear coarse as distinct ridges on the stipule, in which case they are considered fibrous, or they may be much less obvious, even difficult to observe, in which case they are considered to be delicate. Stipule tissue between the veins of fibrous stipules usually decays, leaving strands of fibers, whereas the tissue between the veins and the veins 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. They are circular in outline and range in color from green to golden to white. They may be present at every node, but more commonly, they are present at most nodes, or possibly only occasionally. The glands (or nodal glands) are best observed with dried species, 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 mostly are globular in shape and axillary. Most species have either all emergent inflorescences or all submersed inflorescences, but not both. We have indicated these to be not dimorphic. Other species have both types of inflorescences on one plant, and we have indicated these to be 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. At the time the plants are in fruit, vegetative features are distinctive for the species. Consequently, vegetative features at time of fruiting are the condition included in the key. Features of the fruit that are important include presence or absence of lateral and abaxial wings, ribs, ridges, or keels. Generally, we refer to the character as "ribbed" if there appears to be a raised "vein" on a rounded surface, as "ridged" if there is a ridge running along the surface with an obtuse angle outermost, as "keeled" if there is a ridge with an acute angle, as "winged" if the ridge appears to have a wing on it distally.

Selected references: Fernald, M. L. 1932. The linear-leaved North American species of Potamogeton section Axillaries. Mem. Amer. Acad. Arts 17: 1--183. (also, Mem. Gray Herb. No.3). Haynes, R. R. 1975. A revision of North American Potamogeton subsection Pusilli (Potamogetonaceae). Rhodora 76: 564--649. Haynes, R. R. 1985. A Revision of the clasping-leaved Potamogeton (Potamogetonaceae). Sida 11: 173--188. Hellquist, C. B., C. T. Philbrick, & R. L. Hilton. 1988. The taxonomic status of Potamogeton lateralis Morong (Potamogetonaceae). Rhodora. 90: 15--20. Ogden, E. C. 1943. The broad-leaved species of Potamogeton of North America north of Mexico. Rhodora. 45: 57--105, 119-163, 171--214 (also, Contr. Gray Herb. CXLVII). Reznicek, A. A. and R. S. W. Bobbette. 1976. The taxonomy of Potamogeton subsection Hybridi in North America. Rhodora. 78: 650--673.

Key to Species