Rich. ex Thuill., Fl. Env. Paris , ed. 2: 178 (1799)
T: [Paris] Dans l’etang de St. Hubert, L.C.M.Richard; lecto: P-LAM, fide S.Snogerup, in P.H.Davis, Fl. Turkey 9: 20 (1986); iso: G-DC, G-DEL; [Paris] in ripis arenosis stagni de St. Hubert, L.C.M.Richard; [residual] syn: P [with a note: herb. Richard].
Annuals, 2–14 (–24) cm tall, caespitose. Stem erect or ascending, rarely rooting at the nodes, 0.15–0.8 (–1.3) mm in diam., with a few basal sheaths and 0–1 (–2) cauline leaves. Leaves short, cauline, 1–10 cm long, 0.15–0.7 (–1.5) mm in diam., unitubular, perfectly septate but septa rarely distinct externally; auricles scarious, c. 1–2 mm long, obtuse to subacute. Lower bracts 1–2, leaf-like, usually 1–5 (–8) cm long, usually equalling or exceeding inflorescence. Inflorescence of 1–7 (–12) heads; heads semiglobose or obconical, rarely sparsely subglobose in outline, 2–8 (–12)-flowered. Flowers usually cleistogamous. Tepals 3–6 (–7) mm long, equal or subequal, subobtuse to apiculate, green to reddish, later greenish brown; margin narrow, scarious, c. 0.1–0.2 mm wide; inner tepals more abruptly tapering at apex. Stamens 3–6, often unequal (inner often smaller or lacking), 1/3–1/2 as long as tepals; anthers 0.3–0.7 mm, 1/3 as long as to equalling (0.5–) 1.0–1.2 mm filaments; style 0.1–0.2 mm long; stigmas 0.5–1 mm long. Capsule unilocular, 2.5–3.5 (–4) mm long, shorter than tepals, narrowly ellipsoid to subpyramidal or narrowly ovoid, subabruptly narrowing into c. 0.2–0.5 mm rostrum, stramineous or pale brownish, thin-walled. Seeds ovoid to ellipsoid, c. 0.3–0.45 × c. 0.2 mm, shortly apiculate, reticulate, c. 18–22-striate, finely densely reticulate; appendages absent. 2n = 40, fide S.Snogerup, Bot. Not. 116: 155 (1963).