Scirpus yokoscensis Franchet & Savatier, Enum. Pl. Jap. 2: 543. 1878 ["yokoscencis"]; Eleocharis acicularis (Linnaeus) Roemer & Schultes var. longiseta Svenson; E. acicularis subsp. yokoscensis (Franchet & Savatier) T. V. Egorova; E. svensonii G. Zinserling.

Perennials. Stolons slender. Culms densely tufted, 2-12 cm tall, filiform. Leaf sheaths reddish, tubular, 0.5-1.5 cm. Spikelet pale purple, ovoid, ca. 3 × 2 mm, few flowered, apex obtuse. Glumes laxly imbricate, subdistichous at base of spikelet; basalmost glume empty, oblong, ca. 2 × 1 mm, 3-veined, amplexicaul for whole spikelet base, apex obtuse; fertile glumes greenish medially and purple on sides, ovate, ca. 1.8 × 1.2 mm, membranous, margin hyaline, apex acute. Perianth bristles 1-4, ca. 2 × as long as nutlet, retrorsely spinulose. Stigmas 3. Nutlet pale yellow, narrowly oblong, ca. 1 × 0.4-0.5 mm, obscurely 3-sided, longitudinally ridged, ridges separated by numerous cross-walls (trabeculae), apex contracted; persistent style base slightly inflated, conic, base ca. 1/3 as wide as nutlet. Fl. and fr. Apr-Nov.

J. Kern (Fl. Males., Ser. 1, Spermat. 7(3): 535. 1974) included Eleocharis yokoscensis within the circumscription of E. acicularis (Linnaeus) Roemer & Schultes as a synonym of E. acicularis var. longiseta, but we have observed and compared the characters of E. yokoscensis and E. acicularis and find they differ distinctly in characteristics of the nutlet and perianth bristles. The nutlet of the former has few transverse trabeculae (cross-walls) on the surface, the style base is ± long and narrow, and the perianth bristles are ca. 2 × as long as the nutlet. The nutlet of the latter has many transverse trabeculae on the surface, the style base is ± short and broad, and the perianth bristles are shorter than or as long as the nutlet, rarely slightly exceeding the nutlet.

Flooded fields, pond margins, moist and muddy soil; 300-3000 m. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, N Xinjiang, Yunnan, Zhejiang [India, Indonesia (Sumatra), Japan, Korea, Mongolia, Myanmar, Philippines, Russia (Far East), Vietnam].