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Annual or perennial, rhizomatous or stoloniferous herbs. Culms erect, terete to trigonous, usually leafy only at the base. Leaves basal, 3-ranked, rarely without blade; blades linear, rarely lanceolate or elliptic; ligule absent. Involucral bracts leaf- like. Inflorescence terminal, simple to decompound, umbel-like with ultimate branches terminating in 1 or more spikes or finger like cluster of spikelets. Spikelets compressed, quadrangular, or subterete, linear to oblong or elliptic, 1-many flowered; rachilla straight or zigzag. Glumes 2-ranked, usually 2 basal ones empty. Flower bisexual, the uppermost of the spikelets often male or barren. Perianth segment absent. Stamens 1-3. Stigmas (1-) 3, style continuous with ovary. Nutlets usually trigonous, triquetrous or dorsiventrally compressed.

Key to the species

Serial Key Reference
1a. Culm terete, septate C. articulatus
1b. Culm trigonous, at least in distal part, not sepatate 2
2a. Spikelets digitate or fasciculate on a very short rachis, or inflorescences capitate or globose 3
2b. Spikelets in spikes terminating rays 14
3a. Leaf blades reduced; Primary rays and foliose involucral bracts and primary rays up to 20 C. alternifolius
3b. Leaf blades well developed; primary rays and involucral bracts up to 5 4
4a. Inflorescences capitate or anthelate, with very short rays 5
4b. Inflorescences anthelate, with elongate rays, sometimes with secondary and tertiary branching 7
5a. Culm base bulbous; spikelets falling from rachis as a single unit while glumes remain attached to winged rachilla C. dubius
5b. Culm base not bulbous; spikelets rachilla remaining attached to rachis as glumes and nutlets fall with maturity 6
6a. Spikelets 3- or 4-flowered; nutlet with spongy base of style C. cephalotes
6b. Spikelets 8-flowered or more; nutlet without enlarged spongy apex C. michelianus
7a. Nutlet lenticular, with face toward rachilla C. serotinus
7b. Nutlet 3-sided (sometimes dorsiventrally compressed) 8
8a. Glumes ca. 2 mm; nutlet 0.7-1.5 mm 9
8b. Glumes ca. 1(-1.5) mm; nutlet 0.2-0.3 mm 10
9a. Spikelets slightly turgid; glumes 7-veined or more C. laxus
9b. Spikelets compressed; glumes 3-5-veined C. platystylis
10a. Glumes papery, apex truncate and with a recurved awn; nutlet oblong-obovoid to oblong C. cuspidatus
10b. Glumes membranous, apex obtuse and mucronate or muticous; nutlet obovoid or ellipsoid 11
11a. Perennials; stamens 3; nutlet less than half as long as subtending glume 12
11b. Annuals; stamens 1 or 2; nutlet half or more as long as subtending glume C. difformis
12a. Lower bracts exceeding the anthela; nutlets oblong-ellipsoid, half to three fifth as long as glume C. pulcherrimus
12b. All bracts shorter the anthela; nutlets obovoid to subglobose, one thirds as long as glume 13
13a. Leafy involucral bracts usually longer than inflorescence; glumes laxly imbricate, apex recurved; anther apices not white setose; nutlet 0.2-0.3 mm wide. C. tenuispica
13b. Leafy involucral bracts usually shorter than inflorescence; glumes densely imbricate, apex straight; anther apices white setose; nutlet 0.4-0.5 mm wide. C. haspan
14a. Rachilla not winged; style short 15
14b. Rachilla winged; style usually long 20
15a. Perennials, with long stolons; spikelets slightly turgid; glumes abaxially not keeled, margin involute, apex obtuse or rounded 16
15b. Annuals, without rhizomes or stolons; spikelets compressed; glumes abaxially keeled, margin not involute, apex mucronate to cuspidate 18
16a. Culms with short blade or nearly bladeless; rachis of spikes glabrous; spikelets linear; glumes elliptic to oblong. C. malaccensis
16b. Culms with long blades; rachis of spikes hispidulous; spikelets narrowly ovoid; glumes ovate or broadly ovate 17
17a. Rachis of spikes laxly hispidulous or glabrous; spikelets ca. 3 mm wide; glumes obtuse at apex C. procerus
17b. Rachis of spikes densely hispidulous; spikelets 1.5-2.5 mm wide; glumes mucronate at apex C. pilosus
18a. Rachis of spikes short; spikelets densely arranged, subcapitate; glumes densely imbricate; nutlet half as long as subtending glume 19
18b. Rachis of spikes long; spikelets laxly arranged; glumes laxly imbricate; nutlet nearly as long as subtending glume C. iria
19a. Glumes mucronate; stigmas one sixth as long as style C. compressus
19b. Glumes muticous; stigmas as long as style C. corymbosus
20a. Mature spikelets disarticulating at base of each glume, with each nutlet clasped by rachilla wings; rachilla wings yellowish and spongy. C. odoratus
20b. Mature spikelets either falling as a unit or rachilla remaining attached to rachis and glumes falling; rachilla wings hyaline and not spongy 21
21a. Spikes cylindric, with numerous spikelets 22
21b. Spikes broadly ovoid, turbinate, ellipsoid, oblong or orbicular, with up to 10 spikelets (rarely more in C. eleusinoides) 27
22a. Mature spikelets not deciduous with glumes and nutlets falling from rachilla 23
22b. Mature spikelets deciduous and falling from rachis with glumes and nutlets attached 25
23a. Spikes but slightly compressed, subterete, at right angles to rachis when ripe; glumes muticous or apiculate C. digitatus
23b. Spikes distinctly compressed, from suberect to spreading at right angles from rachis when ripe; glumes with short but distinct mucro 24
24a. Spikes pedunculate; spikelets distichous, laxly arranged; anthers linear C. exaltatus
24b. Spikes not pedunculate; spikelets in several rows, densely arranged; anthers oblong C. imbricatus
25a. Spikelets 1-3-flowered; glumes tightly included around nutlet 26
25b. Spikelets 4-10-flowered; glumes not tightly included around nutlet C. compactus
26a. Leaf blade margin smooth; spikes cylindric, 2-3.5 × 0.6-1 cm, with parallel sides; anthers 0.6-0.9 mm; nutlet ca. 0.5 mm wide C. cyperoides
26b. Leaf blade margin scabrous; spikes cylindric, oblong, or ovoid, 1.2-1.8 × 0.8-1.5 cm, with sides narrowed toward base; anthers 1.0-1.5 mm; nutlet 0.6-0.9 mm wide C. cyperinus
27a. Rhizomes with stolons and tubers; glumes densely imbricate 28
27b. Rhizomes without stolons or tubers; glumes laxly spreading 30
28a. Scales 3.5-4.5 mm long, with yellowish golden sides and greenish midrib C. strigosus
28b. Scales 1-3 mm long, often marked with reddish or dark brown 29
29a. Stolons soft, white; glumes yellow or light brown, sides with 3-4 long, distinct nerves C. esculentus
29b. Stolons strong, ligneous; glumes brown to reddish or blackish brown, sides not with 3-4 distinct nerves C. rotundus
30a. Culms slender; spikes broadly ovoid; spikelets subquadrangular, spreading; glumes 3-4 mm long 31
30b. Culms stout; spikes oblong, cylindric, ellipsoid, globose; spikelets slightly compressed, suberect; glumes ca. 2 mm long 32
31a. Leaves scacely one-third as long as the stem; bracts shorter than the anthela C. macer
31b. Leaves more than half as long as the stem; bracts longer than the anthela C. tenuiculmis
32a. Spikelets laxly arranged, spreading after anthesis, 0.8-4.0 cm × ca. 1 mm; glumes dark blood-red C. distans
32b. Spikelets densely arranged, not spreading after anthesis, 0.4-1.1 cm × 1.5-2.0 mm; glumes reddish brown or yellowish brown 33
33a. Anthela raylets radially spreading; spikes 6 cm or more; spikelets 1.0-1.5 mm wide C. nutans
33b. Anthela raylets suberect; spikes less than 6 cm; spikelets ca. 2 mm wide C. eleusinoides