Herbs, perennials or rarely annuals, rhizomatous or not rhizomatous, caulescent; turions absent or present. Leaves alternate or subopposite, all submersed or both submersed and floating, sessile or petiolate, divided into sheath and blade; sheaths not persisting longer than blades, not leaving a circular scar when shed, ligulate, mostly not auriculate or rarely auriculate; intravaginal squamulae as > 2 scales. Inflorescence spikes, capitate spikes, or panicle of spikes, terminal or axillary, not subtended by a spathe, pedunculate, peduncle following fertilization not elongating, not spiraling. Flowers perfect, without subtending bracts; perianth present, tepals 4 in 1 series; stamens [2 or] 4, in 1 series epitepalous, anthers separate, dehiscing by vertical slits, pollen spherical; carpels 1 or 4, mostly not stipitate, rarely short-stipitate, separate, ovules orthotropous, marginal. Fruit drupaceous. Seeds 1, embryo curved.

Genera 3, species ca. 100 (2 genera, 37 species in the flora): near worldwide.

The family has historically been considered to consist of two genera, Potamogeton and Groenlandia. Recent molecular evidence (D. H. Les, unpublished), combined with existing morphological evidence, indicates that Potamogeton s.l. actually represents two separate lineages. We have chosen to recognize these lineages at the generic level, Potamogeton s.s. and Stuckenia. Consequently, we accept three genera in the family, Potamogeton, Stuckenia, and Groenlandia."

Members of the Potamogetonaceae have been variously combined with members of the Zosteraceae, Cymodoceaceae, and Zannichelliaceae, Najadaceae to comprise a family called either the Zosteraceae, Najadaceae, or Potamogetonaceae. The Potamogetonaceae, as here interpreted, are separated from these other families by their perfect flowers, lack of spathe-like bract, and in some species, the presence of turions.

Aquatic vascular plants are known for their phenotypic plasticity (Haynes 1974). This plasticity may result from varied environmental conditions in which the populations grow or, as Haynes (1975) pointed out, from morphological changes in the individuals of a population during the growing season. We have observed that individuals in fruit, however, have a relative consistent morphology with a species. Regardless of this phenotypic plasticity, collections of the Potamogetonaceae in particular and aquatic vascular plants in general are often taken regardless of the presence of reproductive structures. Haynes (1978) noted that reproductive features are the most important in separating the species of Potamogeton in particular, and we include the entire family here. The keys, however, may not always utilize reproductive features, but they are based on fruiting individuals. We, therefore, strongly recommend that no one collect specimens of Potamogetonaceae that are lacking in reproductive structures.

We here consider the fruits of the Potamogetonaceae as drupaceous, indicating they are like drupes. The fruits do have endocarps but do not have any fleshy mesocarps. Mesocarps do exist, but they never become fleshy. Consequently, the fruits are not true drupes, hence drupaceous.

Many species of Potamogetonaceae undergo extensive vegetative reproduction, which may either be by turions or stem fragmentation. Turions, which have often been called winter buds for Potamogeton, are excellent modes of vegetative reproduction. These structures are produced at the stem tips and eventually fall to the substrate, either by breaking off of the stem or by the stem itself falling to the substrate. The turions survive an unfavorable season, usually winter in North America, and germinate to grow into new plants the next growing season. Since the unfavorable season is usually winter, the turions have been called winter buds. We prefer not to use that term since at least one species, Potamogeton crispus, is known to produce turions in the early summer, and the turions survives the unfavorable season, which is summer in this instance, germinating in the fall. The plant then survives the winter as a young individual, usually only a few cm long, even under ice, and begins growth as the water warms the following spring. Winter buds is certainly not the correct term for P. crispus. Hence, we use turion for all such structures, regardless of the unfavorable season.

Selected references: Cronquist, A., A. H. Holmgren, N. H. Holmgren, J. L. Reveal, P.K. Holmgren. 1977. Intermountain Flora, Vascular plants of the intermountain west, U.S.A. Vol. 6. The Monocotyledons. Columbia Univ. Press., N.Y. 584pp. Hagström, J. O. 1916. Critical researches on the Potamogetons. Kungl. Svenska Vetenskapsakad. Handl. 55(5): 1--281. Haynes, R. R. 1978. The Potamogetonaceae in the Southeastern United States. J. Arnold Arbor. 58: 161--170. Les, D. H. 1983. Taxonomic implications of aneuploidy and polyploidy in Potamogeton Potamogetonaceae). Rhodora. 85: 301--323. Mason, H. L. 1957. A flora of the marshes of California. Univ. California Press. Berkeley. 878pp. Preston, C. D. 1995. Pondweeds of Great Britain and Ireland. Botanical Society of the British Isles, Handbook No. 8. London. 350 pp.

Key to Genera