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, culms tufted, triangular, very slender, erect, 4-15' tall, roughish on the angles. Lower sheaths imbricated, one or more of them bearing short subulate blades, the upper one bearing a flat, narrowly linear blade shorter than the culm; spikelet solitary, terminal, ovoid, few-flowered, l 1/2"-2" long, subtended by a subulate involucral bract of less than its own length or somewhat longer; scales ovate, pale brown, acute or the outer one awned; bristles 3-6, filiform, upwardly barbed, as long as the achene or longer; style 3-cleft; achene oblong, brown, sharply 3-angled, smooth, obtuse.
Scirpus planifolius Muhl. Gram. 32. 1817.
Perennial, culms triangular, slender, tufted, rather weak, roughish on the angles, 6'-15' tall. Lower sheaths bearing short subulate blades, the upper with a flat narrowly linear rough-margined leaf about as long as the culm; spikelet solitary, terminal, ovoid-oblong, acute, several-flowered, subtended by a short involucral bract; scales ovate-lanceolate, yellowish-brown with a green midvein, which is extended beyond the acute apex into a sharp cusp; bristles 4-6, upwardly barbed, about equalling the achene; stamens 3; style 3-cleft, pubescent; achene oblong, 3-angled; smooth, light brown, rather obtuse.
Scirpus subterminaiis Torr. Fl. U. S. 1: 47. 1824.
Perennial, aquatic, culms slender, terete, nodulose, 1°-3 1/2° long. Leaves very slender, channeled, 6'-2° long, 1/4"-3/4" wide; spikelet solitary, terminal, oblong-cylindric, narrowed at each end, several-flowered, 3"-7" long, subtended by a subulate erect involucral leaf, 1/2'-2 1/2' long, thus appearing lateral; scales ovate-lanceolate, acute, membranous, light brown with a green midvein; bristles about 6, downwardly barbed, as long as the achene or shorter; stamens 3; style 3-cleft to about the middle; achene obovoid, 3-angled, dark brown, smooth, rather more than 1" long, obtuse, abruptly beaked by the slender base of the style.
In ponds and streams or sometimes on their borders, Newfoundland to the Northwest Territory and British Columbia, South Carolina, Pennsylvania, Michigan and Idaho. The so-called variety terrestris is an emersed form with erect culms and shorter spikelets. July-Aug.