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..
Potamogeton natans L. Sp. PI. 126. 1753.
Stems 2°-4° long, simple or sparingly branched. Floating leaves thick, the blade ovate, oval or elliptic, 2'-4' long, 1-2' wide, usually tipped with a short abrupt point, rounded or subcordate at the base, many-nerved; submerged leaves reduced to phyllodes or bladeless petioles which commonly perish early and are seldom seen at the fruiting period; stipules sometimes 4' long, acute, 2-keeled; peduncles as thick as the stem, 2'- 4' long; spikes cylindric, very dense, about 2' long; fruit turgid, 2"-2 1/4" long, about 1 1/4" thick, scarcely keeled, narrowly obovoid, slightly curved on the face; style broad and facial; nutlet hard, more or less pitted or impressed on the sides, 2-grooved on the back; embryo forming an incomplete circle, the apex pointing toward the base.
In ponds and streams, Nova Scotia to British Columbia, New Jersey, Missouri and Nebraska. Also in Europe and Asia. Called also Tench-weed, Batter-dock, Deil's-spoons. July-Aug.
Potamogeton Oakesianus Robbins in A. Gray, Man. Ed. 5, 485. 1867.
Stems very slender, often much branched from below. Floating leaves elliptic, mostly obtuse, rounded or slightly subcordate at the base, 1'-2' long, 5"-9" wide, 12-20-nerved; petioles 2'-6' long; submerged leaves mere capillary phyllodes, often persistent through the flowering season; peduncles 1' - 3' long, commonly much thicker than the stem, mostly solitary; spikes cylindric,1/2'-1' long; stipules acute, hardly keeled; fruit obovoid, about 1 1/2" long, 1" thick, nearly straight on the face, 3-keeled, the middle keel sharp; style apical or subapical; sides of the nutlet not pitted, but sometimes slightly impressed; embryo circle incomplete, the apex pointing toward the base.
In still water, Anticosti to Wisconsin and New Jersey. Summer.
Potamogeton amplifolius Tuckerm. Am. Journ. Sci.
(II.) 6: 225. 1848. Potamogeton amplifolius ovalifolius Morong; A.
Benn. Journ. Bot. 42: 70. 1904.
Stems long, simple or occasionally branched. Floating leaves oval or ovate, abruptly pointed at the apex, rounded at the base, 2'-4' long, 1 1/4'-2' wide, many-nerved; petioles 3' - 5' long; submerged leaves mostly petioled, the uppermost often elliptic or oval, 3'-6' long, 1'-2 1/2' wide, the lowest lanceolate, often 8' long, with the sides closed and assuming a falcate shape; stipules tapering to a long sharp point, sometimes 4' long; peduncles thickened upward, 2'-8' long; spikes cylindric, 1-2' long; fruit 2"-2 1/2" long, 1 1/4" thick, turgid, the pericarp hard, obliquely obovoid, 3-keeled; face more or less angled; style subapical; embryo slightly incurved.
In lakes, Ontario to British Columbia, south to Georgia and Nebraska. July-Sept.