Polypogon viridis (Gouan) Breistr. (syn. Agrostis viridis, Agrostis semiverticillata), water beard grass, water blue grass. Aquatic perennial herb, evergreen, clonal, without obvious rhizomes, fibrous–rooted, several–stemmed at base, clumped to cespitose, erect to decumbent, 10—75 cm tall; shoots glabrous; with adventitious roots at lower nodes.
Stems (culms) cylindric, < 2 mm diameter, tough, internodes 35—90 mm long; internodes hollow.
Leaves alternate distichous, simple with sheath; sheath open, shallowly ridged, with only outer margin membranous, without lobes (auricles) at top; ligule membranous, truncate, 1.2—4.5 mm long, often with 1 or 2 splits to base, irregularly toothed, minutely scabrous; blade linear–lanceolate, 25—80(—200+) × 2.5—5 mm, the widest near base, flat, minutely toothed and scabrous on margins, long–acuminate at tip, parallel–veined with midrib raised on lower surface, upper surface minutely scabrous along veins.
Inflorescence spikelet, in terminal, open panicles, panicle lanceolate in outline, 20—70 × 6—30 mm, many–branched, interrupted with distichous semi–whorls of 3—10+ ascending to spreading lateral branches, of many spikelets, spikelet with 1 floret, bracteate, glabrous, lacking awns; lower main axis concealed by ascending branches, striped (corrugated), glabrous or sparsely minutely scabrous; major branches spreading, to 25(—40) mm long, striped, with axillary pulvinus, branchlets minutely scabrous.
Spikelet erect, not compressed, breaking below glumes; glumes 2, subequal, lanceolate, 1.2—1.6 mm long, at anthesis > spikelet, green with membranous margins often aging purplish, truncate to obtuse at tip, short–strigose and minutely scabrous along midvein; rachilla not prolonged above palea; floret subsessile; lemma membranous, lanceolate, 0.9—1.2 mm long, 5–veined, 5–toothed; palea membranous, 0.7—1 mm long, lemma slightly > palea, 2–veined, not 2–keeled, 2–toothed.
Flower bisexual; perianth (lodicules) 2, membranous, lanceolate, ± 0.3 mm long; stamens 3, free; filaments threadlike, initially < 0.5 mm long increasing to become exserted, whitish; anthers exserted from top of lemma, dorsifixed, dithecal, 0.3—0.5 mm long, pale yellow, longitudinally dehiscent; pollen whitish; pistil 1, 0.7—0.9 mm long; ovary superior, obovoid, ± 0.3 × 1.5 mm, glabrous, 1–chambered with 1 ovule; styles 2, ascending, narrowly separated 0.1 mm, 0.4—0.5 mm long, conspicuously feathery nearly to base (plumose).
Fruit achene (caryopsis), oblanceoloid or ovoid to ellipsoid slightly compressed, 0.6—0.8 × 0.3—0.4 mm, orangey brown (reddish), glabrous, with shriveled remnants of stigmas persisting at tip, often slipping easily spikelet; fruit wall finely winkled to form irregular longitudinal ridges.
A. C. Gibson & B. A. Prigge