Eleocharis fallax Weath.

Eleocharis ambigens is usually treated as a synonym of E. fallax because some specimens from Massachusetts to Georgia have some 3-fid styles and some obscurely trigonous achenes. A specimen of E. ambigens from Louisiana has trigonous achenes. Eleocharis ambigens is very similar to and perhaps better included in E. uniglumis (S.-O. Strandhede 1967). Intermediates with E. macrostachya occur in Louisiana and Texas. The depressed tubercles are diagnostic of E. ambigens. Eleocharis ambigens is sometimes confused with E. montevidensis, which is readily distinguished by its floral scales with broadly rounded apices.

The taxonomic status of Eleocharis fallax is problematic. I have seen only the type, from 1919 from a pond on Cape Cod, Massachusetts; a specimen from 1955 from Inverness County, Nova Scotia; and a specimen from the Passaic River marshes in New Jersey. It is reportedly extirpated from the type locality (M. L. Fernald 1950). Another specimen from Cape Cod is typical of E. fallax except for some lenticular achenes. I have not seen a voucher for the material, reportedly collected by H. K. Svenson from a brackish marsh on Plum Island, Essex County, Massachusetts, from which S.-O. Strandhede (1967) counted the chromosome number. It seems possible that these collections are of hybrid origin from E. elliptica and either E. erythropoda or E. uniglumis var. halophila (M. L. Fernald 1950). The plants are similar to E. erythropoda except for their trigonous, rougher achenes and sometimes fibrous rhizome scales, which suggests introgression from E. elliptica. Because some specimens of E. ambigens, including the type, have mixtures of lenticular and trigonous achenes, E. fallax is often treated as conspecific with E. ambigens, from which it differs only in its 3-fid styles, trigonous, more rugulose achenes, and higher tubercles. It seems best to recognize E. fallax as a species pending further research because the plants bear many, apparently normal achenes and are not exactly intermediate between their putative parents, and to avoid using a name for the widespread E. ambigens that may apply only to rare hybrids.

Plants perennial, mat-forming; rhizomes evident, long, 1–2 mm thick, firm, cortex persistent, longer internodes 1–3 cm, scales persistent, 5–10 mm, membranous, not fibrous. Culms terete, often with to 16 blunt ridges when dry, 25–80 cm × 0.5–1.5 mm, firm, internally spongy. Leaves: distal leaf sheaths persistent, not splitting, proximally dark red, distally red to stramineous or green, often callose, thinly papery (to membranous), apex dark red-brown, obtuse to subtruncate, tooth sometimes present on some culms, to 0.3 mm. Spikelets ovoid to lanceoloid, 5–23 × 2–3(–4) mm, apex acute (to obtuse); proximal scale amplexicaulous, entire; subproximal scale with flower; floral scales often spreading in fruit, 10–60, 3–4 per mm of rachilla, medium brown, rarely red-brown, midrib regions mostly stramineous to green, in proximal part of spikelet ovate, in distal part lanceolate, 2.5–3.5 × 1.7 mm, entire, apex acute to rarely obtuse, often carinate in distal part of spikelet. Flowers: perianth bristles 2–4(–5), brown, slender to stout, usually very unequal, rudimentary to equaling achene; stamens 3; anthers brown, 1.5–2 mm; styles 2-fid, sometimes some 3-fid. Achenes not persistent, dark yellow or stramineous, obovoid to obpyriform, all biconvex or some (very rarely all) compressed trigonous, angles obscure, 1–1.5 × 0.9–1.25 mm, apex rounded, neck absent (to very short), finely rugulose at 10–20X, with 20 or more horizontal ridges in vertical series and/or finely cancellate at 10–30X. Tubercles brown or proximally whitish, pyramidal, depressed, rarely some as high as wide, 0.15–0.3(–0.5) × (0.3–)0.4–0.6 mm. 2n = 44, 45, 46.

Plants perennial, mat-forming; rhizomes evident, long, 1–2 mm thick, firm, cortex persistent, longer internodes 5–10(–20) mm, scales sometimes fugaceous, 5–7 mm, papery (to membranous), sometimes fibrous. Culms terete, often with to 12 blunt ridges when dry, 30–75 cm × 0.5–1.5 mm, firm, internally spongy. Leaves: distal leaf sheaths persistent, not splitting, proximally dark red, distally stramineous or green, papery to membranous, apex dark red brown, obtuse, not callose, tooth rarely present on some culms, 0.2 mm. Spikelets ovoid or subspheric, 5–12 × 2–3(–4) mm, apex acute to obtuse; proximal scale amplexicaulous, entire; subproximal scale with flower; floral scales deciduous, often spreading in fruit, 10–35, 3 per mm of rachilla, red-brown to blackish brown, midrib regions sometimes green, ovate, 2.5–3 × 1.7 mm, apex entire, obtuse to acute, often carinate in distal part of spikelet. Flowers: perianth bristles 1–5, brown, stout, very unequal, rudimentary to equaling achene; stamens unknown [absent from specimens]; styles 3-fid or some 2-fid. Achenes not persistent, dark yellow or medium brown, obovoid to obpyriform, compressed trigonous, or some thickly biconvex, angles evident, 1.1–1.5 × 0.95–1.25 mm, apex rounded, neck very short (to absent), finely rugulose at 10–20X, 20 or more horizontal ridges in vertical series, finely cancellate at 20–30X. Tubercles whitish to brown, pyramidal, not depressed, as high as wide, 0.3–0.5 × 0.4–0.6 mm. 2n = 42.

Que.; Conn., Fla., Ga., La., Mass., Miss., N.J., N.C., R.I., S.C., Tex., Va.

N.S.; Mass., N.J.

Fruiting spring–summer.

Fruiting late summer.

Coastal (rarely inland) fresh to brackish pond shores, marshes, disturbed places; 0–10m.

Coastal, fresh to brackish pond and lakeshores, marsh; of conservation concern; 0–10m.

Eleocharis fallax Weath. Rhodora 24: 23. 1922
Heleocharis nervosa Kiikenth. Repert. Sp. Nov. 23: 192. 1926. (Cuba.) Eleocharis ambigcns Fernald, Rhodora 37 : 394. 1935. (Virginia.)
Loosely stoloniferous or subcespitose with purplish-castaneous stolons 1.5-2 mm. thick; culms subrigid, pale, 2.5-7 dm. high; sheaths castaneous or purplish, coriaceous and subtruncate at the apex; spikelets lanceolate or narrowly ovoid, acute, 4-10 mm. long, loosely few-flowered ; lowest scale amplexicaul, much elongated at the base ; fertile scales lanceolateovate, acute or subacute, pale brown with a hyaline margin ; achene ellipsoid-obovoid, convex to obtusely 3-angled, yellow, often becoming castaneous when mature, 1.2-1.7 mm. long (excluding the pyramidal tubercle which is much smaller than the achene), the surface prominently punctate-reticulate ; bristles variable in length.
Type locality : Fresh and brackish springy border of Dinah's Pond, Yarmouth, Massachusetts.
Distribution: Along the coast, southern Massachusetts to Louisiana; Texas (Matagorda County, Bechtold) ; Cuba.

Eleocharis fallax là loài thực vật có hoa trong họ Cói. Loài này được Weath. mô tả khoa học đầu tiên năm 1922.