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 very small or very large sedges, with leafy culms or the leaves reduced to basal sheaths. Spikelets terete or somewhat flattened, solitary, capitate, spicate or umbellate, subtended by a 1-several-leaved involucre or the involucre wanting in some species. Scales spirally imbricated all around, usually all fertile, the 1 or 2 lower sometimes empty. Flowers perfect. Perianth of 1-6, slender or rigid, short Or elongated, barbed, pubescent or smooth bristles, or none in some species. Stamens 2 or 3. Style 2-3-cleft, not swollen at the base, wholly deciduous from the achene, or its base persistent as a subulate tip. Achene triangular, lenticular or plano-convex. [Latin name of the Bulrush, said to be from sirs, the Celtic word for rushes.]
About 150 species of wide geographic distribution. Besides the following, some 10 others occur in the southern and western United States. Type species: Scirpus lacustris L.
Spikelet solitary, terminal, bractless or subtended by a single bract or short leaf.
No involucral bract.
Culms 1'-2' high; achene smooth; plant of saline soil.
Involucral bract present, erect.
Bract shorter than or but little exceeding the spikelet; plants not aquatic.
Culm terete; leaf of upper sheath subulate; bristles smooth.
Culms triangular; leaf of upper sheath linear; bristles upwardly barbed.
Leaves shorter than the culm; scales acute.
Leaves about as long as culm; scales cuspidate or awned.
Bract at least twice as long as the spikelet; plant aquatic.
Spikelets normally more than 1, usually several or numerous, often appearing lateral; involucral bract only 1.
Spikelets few, 1-12, appearing lateral.
Culms not sharply 3-angled; achene plano-convex; annuals.
Achene strongly transversely rugose.
Achene smooth or very slightly roughened.
Culms sharply 3-angled; plants perennial by rootstocks.
Achene plano-convex; bristles shorter than or equalling the achene.
Spikelets acute, much overtopped by the slender involucral leaf; scales awned.
Spikelets obtuse; involucral leaf short, stout; scales mucronulate.
Achene 3-angled, ridged on the back.
Bristles longer than the achene; involucral leaf erect.
Bristles as long as the achene; involucral leaf abruptly bent.
Spikelets several or numerous, umbelled; tall sedges.
Culm sharply triangular, equalled by the long leaves.
Culm terete; leaves reduced to sheaths.
Styles 2-cleft; achene lenticular.
Achenes 1" long, nearly as long as the scales; spikelets ovoid.
Achenes about 1 1/2" long, distinctly shorter than the scales; spikelets oblong-cylindric.
Style 3-cleft; achene trigonous.
Spikelets several, spicate.
3. Spikelets several, capitate or umbellate, large; involucral leaves 2 or more.
Achene lenticular or plano-convex; spikelets sessile or some stalked.
Scales short-awned; achene lenticular.
Scales long-awned; achene plano-convex.
S. robust us.
Achene trigonous; spikelets mostly long-stalked.
Achene sharply and nearly exactly trigonous.
Achene with one face broader than the other two.
Achene obovoid-orbicular; leaves 1"-2" wide; spikelets ovoid.
Achene obovoid; leaves 4"-8" wide; spikelets narrowly cylindric.
4. Spikelets very numerous in compound umbels or unibelled heads, small; involucral leaves several; tall sedges.
Bristles downwardly barbed; spikelets in umbelled heads. ______
Spikelets ovoid or oblong, 1 1/2"-21/2" long.
Bristles equalling or slightly exceeding the achene; leaves 3"-8" wide.
Style 3-cleft; achene 3-angled; bristles 6.
Spikelets 3-8 in each head; bristles barbed throughout; scales obtuse.
Spikelets 8-20 in each head; bristles not barbed below; scales acute or awned.
Plant dark green; scales acute.
Plant pale; scales rough-awned.
Style 2-cleft; achene plano-convex; bristles 4.
Bristles flexuous, twice as long as the achene; leaves 2"-3" wide. 27.
Spikelets cylindric, 3"-s" long; style 3-cleft.
Bristles smooth or slightly pubescent; umbel mostly decompound.
Bristles shorter than or scarcely exceeding the scales.
Bristles about as long as the achene; scales subacute.
Bristles much longer than the achene; scales mucronate.
Bristles much exserted beyond the scales when mature.