Eleocharis tenuis (Willd.) Schult.

The name Eleocharis capitata (Linneaus) R. Brown was long misapplied to E. tenuis (H. K. Svenson 1939). Although the extremes of the three varieties are very different, intermediates are fairly common. Some plants are intermediate with E. elliptica.

Eleocharis tenuis belongs to the E. tenuis complex, which comprises species 16–21 and is restricted to North America, where it is widely distributed except for the Southeast and Southwest. Eleocharis occulta, E. bifida, and E. nitida are very distinct, have little variation, and have relatively restricted ranges; E. tenuis, E. elliptica, and E. compressa are difficult to separate, are highly variable, and have relatively large ranges (see also discussions under E. compressa and E. elliptica). Eleocharis occulta and E. bifida are evidently closely related to E. compressa; they are known only from unglaciated areas south of the limits of Pleistocene glaciation, while E. compressa has a broad range in glaciated regions. The cytotaxonomic study by L. J. Harms (1972) of E. tenuis, E. elliptica, and E. compressa included artificial interspecific and infraspecific hybrids as discussed under those species.

Eleocharis tenuis var. pseudoptera might also be treated as E. elliptica var. pseudoptera following L. J. Harms (1972) because it is intermediate between E. tenuis var. tenuis and E. elliptica var. elliptica in most characters except for the 4-angled, usually deeply sulcate culms. It appears to intergrade with E. elliptica. It is placed in E. tenuis because many plants, including an isotype, are more like E. tenuis than E. elliptica; because most plants key more easily to E. tenuis; and to continue the use of the traditional name. The achenes of the holotype (from Lancaster county, Pennsylvania) and some other specimens closely resemble those of E. elliptica but are not obviously persistent after the scales fall. The usually diagnostic tooth on the distal leaf sheath in E. tenuis var. pseudoptera is also characteristic of E. elliptica var. elliptica; but in E. tenuis var. pseudoptera it is more often present and usually longer and stouter than in E. elliptica. The culms of the holotype of E. tenuis var. pseudoptera and many other specimens are 4- to 5-angled, and in some specimens are very irregularly to 6-angled and often rigid and compressed. L. J. Harms (1972) transferred E. tenuis var. pseudoptera to E. elliptica because he counted the same 2n = 38 chromosome number in both E. elliptica and E. tenuis var. pseudoptera and produced artificial E. elliptica × E. tenuis var. pseudoptera hybrids in which meiotic pairing and pollen stainability were very high. A. E. Schuyler (1977) reported 2n = 39 for E. tenuis var. pseudoptera as well as 2n = 34 and 68 for a putative E. tenuis var. tenuis × E. tenuis var. pseudoptera hybrid.

Putative hybrids between Eleocharis compressa and E. erythropoda of the E. palustris complex in Ontario have been reported (P. M. Catling and S. G. Hay 1993), and I have observed putative E. compressa × E. erythropoda and E. elliptica × E. erythropoda hybrids in the field in southeastern Wisconsin. It seems possible that introgression from E. erythropoda is responsible for some of the variation of both E. compressa and E. elliptica, especially the frequent presence of some 2-fid styles and lenticular achenes in both species and some (rarely all) entire floral scales in E. compressa (S. G. Smith 2001).

Plants perennial, mat-forming; rhizomes evident, 0.4–2 mm thick, soft to hard, cortex persistent, longer internodes 2–10 mm, scales persistent or decaying to fibers, 5–10 mm, papery, finely to coarsely fibrous. Culms terete or usually with 4 or 5(–6) angles, often sulcate; 5–90 cm × 0.2–0.5(–0.8) mm, firm to soft. Leaves: distal leaf sheaths persistent, not splitting, proximally dark red (or yellow-brown), distally green or stramineous or red, membranous, apex often reddish, obtuse to acute, often callose, often with tooth to 0.2(–0.9) mm. Spikelets ovoid, 3–6 × 1.5–2 mm, apex obtuse to acute; proximal scale amplexicaulous, apex entire; subproximal scale with flower; floral scales appressed in fruit, 20–60, 5–6 per mm of rachilla, medium to dark brown, midrib region often paler, ovate, 1.5–2.5 × 1 mm, apex rounded (to acute), entire, rarely shallowly notched, carinate in distal part of spikelet. Flowers: perianth bristles absent or sometimes 1–3, stramineous to pale brown, slender, to equaling achene, obscurely retrorsely spinulose; stamens 3; anthers brown, 0.8–1.8 mm; styles 3-fid. Achenes falling with or before scales, lemon yellow, dark yellow, medium brown, or green, obpyriform, trigonous, angles evident, sometimes prominent, 0.6–0.9 × 0.45–0.7 mm, finely to coarsely rugulose and usually alveolate (cancellate) at 10–20X, 6–10(–14) sharp horizontal ridges in each vertical series. Tubercles brown, pyramidal and to as high as wide to greatly depressed-apiculate, often rudimentary, 0.05–0.3 × 0.25–0.4 mm.

Scirpus tenuis Willdenow, Enum. Pl. 1: 76. 1809

Eleocharis tenuis (Wilid.) Schultes in R. & S Syst. Veg. Mant. 2 : 89. 1824.
(Virginia.) Not Eleocharis capitata R. Br. 1810. 138. 1791. (New York.) Not S. filiformis Burm. f.
1918. Not Eleocharis capitata R. Br.
1920. 1932.
202. 1932. (Arkansas.)
(New Jersey.)
Scirpiis capifatus L. Sp. PI. 48. 1753. Scirpus filiformis Lam. Tab. Encyc. 1 :
1768. Scirpus tenuis Willd. Enum. 76. 1809. (Pennsylvania.) Isolepis filiformis R. & S. Syst. Veg. 2: 106. 1817. Eleogiton filiformis A. Dietr. Sp. PI. 2 : 96. 1833. Eleocharis filiformis Kunth, Enum. PI. 2 : 146. 1837. Eleocharis capitata (L.) sensu Blake, Rhodora 20 : 24.
1810. Trichophyllum tenuis Farwell, Rep. Mich. Acad. 21 : 359. Eleocharis capitata var. typica Svenson, Rhodora 34 : 199 Eleocharis capitata var. pseudoptera Weath. in Svenson, Rhodora 34 Eleocharis capitata var. verrucosa Svenson, Rhodora 34 : 202. 1932, Eleocharis tenuis var. pseudoptera Svenson, Rhodora 41 : 65. 1939. Eleocharis tenuis var. verrucosa Svenson, Rhodora 41 : 66. 1939.
Culms capillary or filiform, 0.5-4 dm. high, usually quadrangular with slightly concave sides or five-angled, erect from a thickened, creeping, ligneous rootstock; stolons thickened, elongate, covered w^ith acute, brown or reddish scales ; sheaths truncate at the apex, with a short mucro ; spikelets ellipsoid to ovoid, acute or blunt, 3-10 mm. long, 20-30-flowered ; scales ovate, obtuse or acute, reddish-brown to black, with a scarious margin and green keel, the lowest scale suborbicular and larger ; styles trifid ; stamens 3 ; achene obovoid, 0.8-1 mm. long, trigonous, olivaceous, alveolate, with wavy transverse bands formed by the projecting angles of the vertically elongated cells ; style-base brownish, pyramidal or depressed ; bristles 2 or 3, rarely persisting, light brown, less than half as long as the achene. Type locality : Pennsylvania.
Distribution : Cape Breton to South Carolina, west to Illinois, Missouri, Oklahoma, and Texas.

Eleocharis tenuis là loài thực vật có hoa trong họ Cói. Loài này được (Willd.) Schult. mô tả khoa học đầu tiên năm 1824.