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 compressus L. Sp. PI. 127. 1753. Potamogeton zosteraefolius Schum. Enum. PI. Saell. 50. 1801.
Stems much flattened, sometimes winged, widely branching. Leaves linear, obtuse and mucronate or short-pointed at the apex, 2'-12' long, 1"-2" wide, with 3 principal nerves and many fine ones; stipules scarious, obtuse, finely nerved, soon perishing; peduncles 1 1/2'-4,' long; spikes cylindric, about 1/2' long, 12-15-flowered; fruit obovoid with a broad base, about 2" long, 1 1/4"-1 1/2" thick, 3-keeled on the back, the lateral keels rather obscure; face arched, beaked with a short recurved style; embryo slightly incurved. The plant is propagated by the terminal leaf-buds, which sink to the bottom, and rest during the winter.
Potamogeton Hillii Morong, Coult. Bot. Gaz. 6: 290. 1881.
Stems slightly compressed, slender, widely branching, l°-2° long. Leaves linear, acute or cuspidate, or often almost aristate, 1'-2 1/4' long, 1/2"-1 1/4" wide, 3-nerved, the lateral nerves delicate and nearer the margins than the midrib; stipules whitish, many-nerved, obtuse, 3"-5" long; peduncles about \' long, erect or slightly recurved, more or less clavate; spikes capitate, 3-6-fruited; fruit obliquely obovoid, obtuse at the base, about 2" long, 1"-1 1/4" thick, 3-carinate on the back, the middle keel sharp and more or less undulate, flat on the sides, face slightly arched; style nearly facial, short; embryo coiled.
Potamogeton foliosus Raf. Med. Rep. (II.) 5:
354. 1808. Potamogeton pauciflorus Pursh, Fl. Am. Sept.
Sci. (II.) 7: 354. 1849. Potamogeton foliosus niagarensis (Tuckerm.)
Stems flattened, much branched, 1°-3° long. Leaves 1-3' long, 1"- 1" wide, acute, 3-5-nerved, not glandular at the base; stipules white, hyaline, obtuse or sometimes acute, 6"-10" long; peduncles more or less clavate, erect, about 1/2 long; spikes 4-12-flowered; fruit lenticular or nearly orbicular, about 1" in diameter, 3-keeled on the back, the middle keel winged, sinuate-dentate, often with projecting shoulders or teeth at each end, the face strongly angled or arched, sharp, often with a projecting tooth at the base; style apical.
In ponds and streams, New Brunswick to British Columbia, south to Florida, New Mexico and California. July-Aug.