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 filiformis Pers. Syn. I: 152. 1805. Potamogeton filiformis Macounii Morong; Macoun, Cat. Can. PI. 4: 88. 1888.
Stems from a running rootstock, slender, 3°-20° long, filiform above, stout and thick towards the base. Leaves numerous, 2'-12' long, 1/4"-1/2" wide, 1-nerved with a few cross veins; sheaths about 2" long and the free part of the stipule shorter, scarious on the edges; flowers on long, often recurved peduncles, 2-12 in each whorl, the whorls \'-\' apart; fruit 1"-1 1/2" long, slightly less than 1" wide, the sides even, the back not keeled, the face nearly straight or obtusely angled near the top; stigma nearly or quite sessile, remaining on the fruit as a broad truncate projection. Embryo a complete spiral, the curved apex pointing inside the base.
In ponds and lakes, Anticosti to western New York and Montana. August.
Potamogeton interior Rydb. Fl. Colorado 13. 1906.
Potamogeton marinus occidentalis Robb. Bot. King's Exp. 339. 1871.
Potamogeton filiformis occidentalis A. Benn. Ann. Con-serv. Jard. Genev. 9: 102. 1905.
Stem slender, much branched and longer than in the preceding; leaves all submerged, capillary or narrowly linear, with an acute or more or less pungent apex, 2'-6' long, 1/4"-2/3" wide, mostly 1-nerved; stipules adnate to the leaf-bases, the sheath at least 7" long, the free part shorter; spikes few-flowered, often interrupted, 1/2'-3 1/2' long; peduncles as thick as the stem, 1 1/2-7' long; nutlets sometimes slightly pitted, without keels or inconspicuously 1-keeled; style almost invisible; embryo an incomplete spiral, the straight apex pointing directly towards the base.
Ontario to the Northwest Territory, south to Utah and Colorado.
Potamogeton pectinatus L. Sp. PI. .127. 1753.
Stems slender, much branched, 1°-3° long, the branches repeatedly forking. Leaves setaceous, attenuate to the apex, 1-nerved, 1'-6' long, aften capillary and nerveless; stipules half free, 1/2'-l' long, their sheaths scarious on the margins; peduncles filiform, 2'-12' long, the flowers in verticils; fruit obliquely obovoid, with a hard thick shell, 1 1/2"-2" long, 1"-1 1/4" wide, without a middle keel, but with obscure lateral ridges on the back, plump on the sides and curved or occasionally a little angled on the face; style straight or recurved, facial; embryo apex pointing almost directly toward the basal end.
In fresh, brackish or salt water, Cape Breton to British Columbia, south to Florida, Texas and California. Also in Europe. Pondgrass. July-Aug.