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..
Annual or perennial sedges. Culms simple, triangular, quadrangular, terete, flattened or grooved, the leaves reduced to sheaths or the lowest very rarely blade-bearing. Spikelets solitary, terminal, erect, several-many-flowered, not subtended by an involucre. Scales concave, spirally imbricated all around. Perianth of 1-12 bristles, usually retrorsely barbed, wanting in some species. Stamens 2-3. Style 2-cleft and achene lenticular or biconvex, or 3-cleft and achene 3-angled, but sometimes with very obtuse angles and appearing turgid. Base of the style persistent on the summit of the achene, forming a terminal tubercle. [Greek, referring to the growth of most of the species in marshy ground.]
About 140 species, widely distributed. Besides the following, some 20 others occur in The southern and western parts of North America. Type species: Scirpus palustris L.
1. Spikelet scarcely or not at all thicker than the culm; scales coriaceous.
Culm stout; spikelet many-flowered.
Culm terete, nodose.
Culm quadrangular, continuous.
Culm slender, triangular, continuous; spikelet few-flowered, subulate.
2. Spikelet manifestly thicker than the culm (except in No. 13); scales membranous.
*Style mostly 2-cleft; achene lenticular or biconvex.
Upper sheath scarious, hyaline; plants perennial by slender rootstocks.
Scales dark reddish-brown; achene 1/2" long.
Upper sheath truncate, oblique or toothed, not scarious.
Annual, with fibrous roots.
Achene jet black.
Culms 1'-3' tall; achene 1/4" long; bristles 2-4.
Culms 3'-10' tall; achene 1/2" long; bristles 5-8.
Achene pale brown.
Spikelet ovoid or oblong.
Tubercle narrower than the top of the achene.
Tubercle about as broad as the top of the achene.
Spikelet oblong-cylindric; tubercle broad, low.
Perennial by horizontal rootstocks.
Scales pale green to straw-color.
Scales brown to purple-brown.
Tubercle flattened-conic; spikelet thicker than the culm.
Tubercle swollen, bulb-like; spikelet not thicker than the culm,
**Style 3-cleft; achene 3-angled or turgid.
Achene reticulated or cancellate.
Spikelet compressed; culm filiform.
Spikelet terete; culm slender.
Achene transversely cancellate; bristles none.
Achene reticulated; bristles present, stout.
Tubercle conic, smaller than the achene.
Tubercle cap-like, as large as or larger than the achene.
Achene smooth or papillose.
Achene smooth, white; culms capillary.
Achene papillose or smooth, brown, black or yellow.
Tubercle depressed or short-conic.
Tubercle flat, covering the top of the black achene.
Tubercle ovoid-conic, acute, contracted at the base.
Achene 3-ribbed on the angles.
Achene obtuse-angled, not ribbed.
Culms filiform; scales obtuse.
Culms flat; scales acute.
Tubercle subulate or narrowly pyramidal.
Culms filiform, wiry, densely tufted, 4'-io' long.
Culms flattened, slender, 1°-2° long.