1	Perianth bristles plumosely fringed		(2)
+	Perianth bristles retrorsely scabrous		(3)
2 (1)	Culms terete; perianth bristles longer than nutlet; stigmas 2.		1 S. subulatus
+	Culms 3-angled to obtusely 3-angled just below inflorescence; perianth bristles ± as long as nutlet; stigmas 3.		2 S. litoralis
3 (1)	Glumes apex emarginate and with a 1-veined costa excurrent into an awn; nutlet smooth; culms scattered; leaves developed		(4)
+	Glumes apex acute to obtuse and without an awn; nutlet mostly rugose; culms tufted (except S. lineolatus); leaves absent		(7)
4 (3)	Involucral bracts much overtopping inflorescence, 1.5-2.5 cm; creeping rhizomes not red.		3 S. ehrenbergii
+	Involucral bracts usually shorter than inflorescence, 1.5-5.5 cm; creeping rhizomes red		(5)
5 (4)	Culms sharply 3-angled but sometimes basally terete and only 3-angled apically; creeping rhizomes slender.		4 S. triqueter
+	Culms terete; creeping rhizomes stout		(6)
6 (5)	Spikelets 4-7(-10) mm; glumes abaxially with reddish resinous spots; stigmas 2; nutlet plano-convex.		5 S. tabernaemontani
+	Spikelets 6-15 mm; glumes abaxially smooth; stigmas usually 3; nutlet compressed 3-sided.		6 S. lacustris
7 (3)	Culms 3-angled (3-5-angled in S. trapezoideus and S. monocephalus), sometimes winged		(8)
+	Culms terete		(10)
8 (7)	Perianth bristles longer than nutlet, retrorsely scabrous; nutlet compressed 3-sided.		7 S. mucronatus
+	Perianth bristles shorter than nutlet, retrorsely scabrous or not; nutlet 3-sided or biconvex		(9)
9 (8)	Nutlet 3-sided; perianth bristles glabrous.		8 S. chuanus
+	Nutlet biconvex; perianth bristles retrorsely scabrous.		9 S. trapezoideus
10 (7)	Plants with a creeping rhizome; culms scattered		(11)
+	Plants without a creeping rhizome; culms densely tufted		(12)
11 (10)	Glumes oblong to elliptic, margin not ciliate; perianth bristles as long to 2 × as long as nutlet.		10 S. lineolatus
+	Glumes broadly ovate, margin ciliate; perianth bristles absent or 1-3 and very short.		11 S. monocephalus
12 (10)	Culms 56-120 cm tall		(13)
+	Culms 15-30(-?40) cm tall		(14)
13 (12)	Involucral bracts often deflexed; culms without transverse septa; stigmas 2.		12 S. clemensii
+	Involucral bracts erect; culms with many transverse septa; stigmas 3.		13 S. articulatus
14 (12)	Nutlet 3-sided, distinctly rugulose; stigmas 3		(15)
+	Nutlet biconvex, obscurely rugulose or smooth; stigmas 2		(17)
15 (14)	Perianth bristles very well developed, slightly longer than nutlet, retrorsely scabrous.		20 S. chen-moui
+	Perianth bristles very short and glabrous or absent		(16)
16 (15)	Culms 2.5-3.5 mm thick; involucral bract 10-30 cm; perianth bristles very short, glabrous.		21 S. schoofii
+	Culms ca. 1 mm thick; involucral bract 1.5-12 cm; perianth bristles absent.		22 S. supinus
17 (14)	Glumes oblong, 2-3 mm; nutlet 1-1.5 mm		(18)
+	Glumes broadly ovate, oblong-ovate, or elliptic, 3.5-5.5 mm; nutlet 1.6-2.5 mm		(19)
18 (17)	Spikelets yellow, 7-20 mm; glume margin apically ciliate; nutlet obscurely rugulose.		18 S. jingmenensis
+	Spikelets brownish green, 4-7 mm; glume margin entire; nutlet subsmooth.		19 S. komarovii
19 (17)	Glumes pale green to brownish green, oblong-ovate, 4-5.5 mm, apex acuminate; nutlet broadly ellipsoid, ca. 2.5 mm; stigmas 2; perianth bristles 4, longer than nutlet.		17 S. wallichii
+	Glumes brown to dark brown, broadly ovate to ovate, 3.5-4 mm, apex acute; nutlet broadly obovoid to obovoid, 2-2.5 mm; stigmas 3; perianth bristles 5 or 6, shorter than or as long as nutlet		(20)
20 (19)	Culms spaced in row along rhizome; involucral bracts horizontal; glumes 3-3.2 mm.		16 S. fuscorubens
+	Culms tufted; involucral bracts ± erect; glumes to 4.5 mm		(21)
21 (20)	Culms 0.7-1 mm thick, rigid, shiny; spikelets ovoid, 4-6 mm wide; nutlets trigonous; stigmas 3.		15 S. hotarui
+	Culms 1.5-7 mm thick, subrigid, opaque; spikelets elliptic to subcylindric, 3.5-5 mm wide; nutlets plano-convex; stigmas 2.		14 S. juncoides