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Rhizomes absent. Cauline stems terete to slightly compressed, without spots, 18--150 cm; glands present on at least some nodes, green, gold, brown, or rarely white, to 0.5 mm diam. Turions common, soft, lateral or terminal, 0.9--3.2 cm ´ 0.3--1.8 mm, soft; leaves ± 2-ranked; outer leaves 1--3 per side, base not corrugate, apex subulate to obtuse; inner leaves rolled into hardened fusiform structure. Leaves submersed, ± spirally arranged, sessile, delicate to coarse; stipules persistent, inconspicuous, connate or convolute, free from base of blade, brown to green or white, not ligulate, 0.31--0.92 cm, rarely appearing fibrous, not shredding at tip, apex obtuse; blade pale green to olive-green, rarely somewhat reddish, linear, not arcuate, 0.9--6.5 cm ´ 0.2--2.5 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, subulate to obtuse, lacunae absent or present, in 0--5 rows each side of midrib; veins 1--3(--5). Inflorescences unbranched, submersed or emersed; peduncles not dimorphic, axillary or terminal, erect, rarely recurved, filiform to slightly clavate, 0.5--6.2(--6.6) cm; spikes not dimorphic, capitate to cylindric, 1.5--10.1 mm. Fruits sessile, green to brown, ovoid to obovoid, turgid to concave, not abaxially or laterally keeled, 1.5--2.2 ´ 1.2--1.6 mm; beak erect, 0.1--0.6; sides without basal tubercles; embryo with less than 1 full spiral.
Submerged perennial or annual. Stolons absent or present towards the end of the growing season, thread-like, sometimes developing terminal or axillary bulb-like tubers. Stems up to 100 cm or more long, flattened to almost terete, usually repeatedly branched, sometimes developing terminal or axillary tubers. Leaves submerged; stipules axillary, closed but splitting open later, translucent, 0.4-1.8(-3.2) cm long, persistent or decaying; blades sessile, linear, flattened, (1.5-)2-8.5(-11) cm long, (0.3-)0.8-2.5(-3.1) mm wide, green to brown, 1-, 3-or 5-veined, midrib with poorly developed gas-filled channels along each side or gas-filled channels confined to the basal half of the leaf, or gas-filled channels absent, bases wedge-shaped, margins entire, tips acuminate to subobtuse. Peduncles slender, (0.6-)1-4.5(-8) mm long, usually shorter than the leaves, flexible at first becoming firm in fruit. Spikes cylindrical, 0.4-1.3 cm long in fruit, with 2-7 flowers, ± contiguous. Drupelets usually 4, asymmetrically obovoid, 1.8-2.7(-3.3) mm long, beak less than 0.6 mm long, dorsal keel indistinct.
Stems very slender, to 1.5 m, with numerous branches distally, with or without nodal glands; rhizome scarcely developed; lvs all submersed, narrowly linear, 1–7 cm × 0.2–2.5 mm, 1-or 3(5)-veined, with 0–5 rows of lacunar cells on each side of the midvein; stipular sheaths axillary, free, to 1 cm, delicate, the margins connate or merely overlapping; winter-buds commonly produced,1–3 cm, the inner lvs rolled into a hard, fusiform structure; peduncles filiform to slightly clavate, axillary orterminal, mostly erect, 0.5–6 cm; spikes capitate to cylindric, 2–10 mm, with 1–4 whorls of fls; body of the frovoid to obovoid, usually plump, not keeled, 1.5–2.2 mm, the beak 0.1–0.6 mm; 2n=26. Common and widespread in both acid and alkaline waters from Nf. to Alas., s. to Fla. and Mex. Highly variable. (P. berchtoldii; P. panormitanus; P. tenuissimus) The most narrow-lvd phase, with subulate, 1-nerved lvs 0.2–0.6 mm wide, considered to be restricted to N. Engl. and s. Que., has been called var. gemmiparus Robbins. (P. gemmiparus)
Submerged herbs arising from winter-buds or seed and forming no rhizome or a poorly developed one. Roots long and fine. Stems up to 100 cm, branching from near the base, internodes fairly equal in length, thin, slightly striate and compressed. Leaves all submerged, firm, translucent, linear, usually c. 20-40 mm long and 1-2 mm broad, apex obtuse, with 2 lateral longitudinal veins joining the midrib abruptly near the tip at unequal heights; areas between the lateral and central veins without a distinct lacunar system, sometimes with a narrow dense band of small cells flanking the midrib; stipular sheaths 6-17 mm, tubular, splitting later, usually semi-persistent, pale brown. Spikes axillary, usually shorter than the leaves; scape thin, becoming firm in fruit, 1-4 cm long; flowers 2-4 per whorl with up to 4 whorls per spike. Drupe 2-2.5 mm long and 1-1.5 mm broad, pale olive green, obovoid, smooth, ventral face convex, dorsal more strongly convex, with a faint brown keel when dry, beak short and straight.
Vertical shoots short-lived or annual, seasonal; stem filiform, terete, to 1 m long, much branched, producing numerous axillary and apical turions, nodal glands present; horizontal shoots usually not developed. Floating leaves absent, involucral leaves rarely spathulate; submersed leaves narrowly linear, sessile, lamina 50(-80) by (0.5-)1-1.5 (-2) mm, 3-veined, midrib bordered by 1-3 rows of lacunae, margin entire, apex acute; stipules axillary, convolute or connate at base, 6-11(-20) mm, truncate, membranous. Spikes cylindrical, contiguous or sometimes the lowest flower remote, 5-13 mm; peduncle 10-15 mm, not thickened. Flowers 4-7, usually with 4 carpels. Fruit 2-2.5 mm, olive-green. Stem anatomy: Stele of the circular type. Endodermis of O-type. Interlacunar bundles absent, subepidermal bundles present. Pseudo-hypodermis absent. Chromosome number: 2n = 26.
Plants annual, submerged in fresh water. Rhizome absent. Stems terete to slightly compressed, ca. 0.8 mm in diam., sparsely to densely branched; nodal glands absent or inconspicuous; turions markedly reduced, axillary, usually narrowly fusiform. Stipules axillary, free, 5-12 mm, connate, membranous and translucent, tubular for most of their length when young, but splitting with age, not fibrous-persistent, green or greenish brown when dry; leaves sessile, linear, 2-6 cm × 0.6-2.3 mm, 3-veined with lateral veins inconspicuous, mostly without rows of lacunae bordering midvein, midvein distinctly thickened toward leaf base, apex acute to acuminate. Spikes with 2-4 whorls of opposite flowers, lax. Carpels 4. Fruit obliquely obovoid, 1.8-2.2 mm, abaxial keel indistinct, with a short beak at tip. Fl. and fr. May-Oct. 2n = 26.
Annual, occasionally perennial herb, hydrophyte, height depending on depth of water; stem branching from near base, internodes equal in length, slightly striate and compressed. Leaves submerged, firm, translucent, linear, 20-40 x 1-2 mm, apices obtuse. Spikes axillary; flowers 2-4 per whorl. Flowering time Oct.-Mar. Fruit a pale olive-green, obovoid drupe; beak short, straight.
Perennial or annual, long-rooted herb. Leaves all submerged; stipules closed and tubular at base when young; blade linear, usually 2 mm wide, arising directly from node, flat in transverse section; midrib occupying 15-35% of leaf width. Flowers: in continuous or sometimes interrupted spikes, 2-4 in a whorl, up to 4 whorls per spike; Nov.-Mar. Fruit 2.0-2.5 mm long.
Submerged, long-rooted herb. Stems up to 1 m long. Leaves all submerged, very narrow, usually 2 mm wide. Lacunar system bordering midrib not developed or very narrow. Stipule tubular, pale brown. Fruit smooth.
Submerged aquatic. Leaves linear. Flowers green, inconspicuous.
