Pondweed is a common name for members of the family Potamogetonaceae [Gk potamos, "river"], which consists of the genus Potamogeton. Formerly, pondweeds were classified as family Zosteraceae, along with several other genera, eg, Ruppia (ditch-grass), Zannichellia (horned pondweed) and Zostera (eelgrass or grass-wrack). All these have representatives in Canada and are now generally regarded as distinct families. Pondweeds grow submerged in fresh or, occasionally, brackish water. The genus Zostera is a true marine aquatic and, in Canada, Z. marina is widely distributed on both the east and west coasts. Potamogeton species are perennial, herbaceous plants anchored in the mud at the bottom of shallow lakes and ponds by underground stems (rhizomes). The leaves, varying considerably in shape, are arranged alternately and all may be submerged or some may float. The flowers are small, often borne in elongated clusters that may be submerged, allowing for pollination by water, or emergent from the water for wind pollination. Pondweeds are widely distributed, but a majority of the approximately 100 species occur in the northern temperate region. About 30 species occur in Canada, several extending into arctic areas.