This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Perennial marine or fresh-water plants with floating or submerged leaves, or both. Leaf-blades petioled or sessile, capillary or expanded into a proper blade, or rarely reduced to terete phyllodes. Flowers perfect or monoecious, in sessile or peduncled spikes, or in clusters in the axils of the leaves. Perianth none, but flowers sometimes enclosed in a hyaline sheath. Androecium of 1-4 stamens. Anthers extrorse, 1-2-celled, the connective sometimes becoming perianth-like. Gynoecium of 1-4 distinct, 1-seeded carpels. Fruits mostly nut-like or drupelike, sessile or stipitate. Endosperm wanting.
Flowers perfect; stamens more than 1.
Stamens 4; fruit sessile.
Stamens 2; fruit stalked.
Flowers monoecious; stamen 1.