Potamogetonaceae

Aquatic non-woody (herbaceous) smooth perennials, with or without rhizomes, flowers pollinated by wind or rarely by water. Leaves usually alternate (opposite or whorled in Horned Pondweed (Zannichellia palustris), stalked or not, simple, with parallel veins or only a central vein. Leaves in 2 vertical rows on opposite sides of the stem, all underwater or with some also floating, the floating leaves if present often very different in shape and texture from underwater leaves; leaves in some species have one or more rows of colourless air chambers on both sides of the central vein; base of leaf sheathing stem; edges of leaves smooth, sometimes wavy, or sometimes with teeth. Plants have ligules that are sheathing and resemble stipules.

Flowers at top of stem or in leaf axils, forming spikes, these sometimes head-like and sometimes arranged in panicles, sometimes fleshy. Flower spikes with no bracts, often raised a bit above the water on a stalk that does not get longer or become spiral after fertilization. Each flower radially symmetrical and producing pollen and seeds. Sepals and petals indistinguishable from one another (tepals), small, inconspicuous, fleshy, firm, separate, a total of 4 in each set. Stamens usually 4 (rarely 2), opposite the tepals and attached to them at the base. Carpels 4, separate, forming 4 pistils (rarely only 1), rarely on very short stalks; ovary located above point of attachment of other flower parts (superior); style 1 per pistil and short, or none; stigma 1 per pistil, attached directly to the ovary or nearly so, shaped like a funnel in Horned Pondweed. Fruits resemble stone fruits, with 1 seed.