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Stems up to 150 cm. long, up to 2 mm. in diameter, mostly armed with spines.. Leaves 3.6-65 mm. long, fleshy, acute, 1.1-5.7 mm. wide including teeth on both sides (or 0.4-2.5 mm. excluding the teeth); margin on each side serrulated with (0-)4-17(-40) conspicuous spiny teeth on broad triangular excrescences, the spine-cell resting upon several elongate brown cells; teeth up to 2 mm. long, the ratio of teeth-length to leaf-width being 0.4-2.5; abaxial side of midrib with (0-)2-8(-40) spines, similar to those on the margin and on the stem; septa and fibres always absent; sheath 1.5-5 × 1-5.5 mm. (ratio = 0.4-1.2), rounded, entire or serrulated with 1(-3) spine-cells on each side.. Inflorescences axillary, solitary.. Male flower 2-5 (including spathe-neck) × 0.8-3 mm.; neck of the spathe 0.5-1 mm., tapering at the top, bearing brownish spine-cells at the apex; inner envelope protruding 0.2-0.7 mm. above the anther; anther 1.2-4 × 0.7-2.5 mm.. Female 2-5 mm. long; ovary 1-3.5 × 0.3-1.9 mm.; style and stigma 0.3-1.6 mm.; stigma (2-)3(-4)-lobed.. Fruit with persistent membranous pericarp and remnants of style.. Seed ovate, slightly asymmetrical, 1.9-7.5 mm. × 0.8-3.3 mm. (ratio = 1.2-3.5); testa pitted with areoles, arranged irregularly; areoles irregular in shape and dimensions.
Stems branched distally, 6--45 cm ´ 0.5--4 mm; internodes 0.3--11 cm, usually with prickles. Leaves spreading to ascending with age, 0.5--3.9 cm, stiff in age; sheaths 2--4.4 mm wide, apex acute; blade 0.4--4.5 mm wide, margins coarsely serrate, teeth 8--13 per side, apex acute, with 1 tooth, teeth multicellular; midvein with prickles abaxially. Flowers 1 per axil, staminate and pistillate on different plants. Staminate flowers in distal to proximal axils, 1.7--3 mm; involucral beaks 2-lobed, 0.3--0.7 mm; anthers 4-loculed, 1.7--3 mm. Pistillate flowers in distal to proximal axils, 2.5--5.7 mm; styles 1.2--1.7 mm; stigmas 3-lobed. Seeds not recurved, reddish brown, ovoid, 2.2--4.5 ´ 1.2--2.2 mm, apex with style situated at center; testa dull, 10--15 cell layers thick, pitted; areoles irregularly arranged, not in distinctive rows, not ladderlike, 3--4-angled, longer than broad, end walls slightly raised. 2n = 12 (Europe).
Stems 30-100 cm or more tall, 1-4.5 mm in diam., mostly armed with spines, spines sometimes absent except apically. Leaves 1.5-3 cm × 2-3.5 mm, fleshy, abaxial side of midvein usually with spines; sheath ca. 3 mm, entire or serrulate with 1-3 spine cells on upper side without auricle, leaf acute at apex, with 2-10 conspicuous teeth on each side of margin; teeth 1-2 mm. Plants dioecious; flowers yellowish green. Male flowers ca. 5 × 2 mm; spathe with a short neck; anther 4-thecous. Female flowers 2-4.5 mm; style ca. 1 mm; stigmas 2-or 3-lobed. Fruit elliptic to obovoid-elliptic, 4-6 × 3-4 mm. Seeds ovoid; testa pitted; areoles polygonal, irregularly arranged. Fl. and fr. Sep-Nov. 2n = 12*, 24, 48, 60*.
Submerged, dioecious, robust annual, to 3 m long. Stems to 2 mm diam.; internodes spinescent. Leaves linear, flat, to 5 cm long, to 5 mm wide (including spines), acute; septa and fibres always absent; margins with numerous large multicellular spine-tipped teeth to 2 mm long; midrib usually with similar teeth; sheath without auricles. Male flowers spatheate, 3–4 mm long; anther 1–3 mm long, tetrasporangiate. Female flowers espatheate, 3–4 mm long; ovary 1–3.5 mm long; style and stigma 0.3–1.6 mm long; stigma usually 3-lobed. Seed usually 3–6 mm long, with remnants of style attached; testa with irregularly arranged areoles.
A water plant. It grows as an annual herb. It grows up to 3 m high. The leaves are 5 cm long by 0.5 cm wide. There are large, spine tipped teeth along the edge. The leaves form a sheath at the base. The flowers are of one sex and occur singly in the axils of leaves. Male flowers have a leafy spathe at the base. The flowers are 3-4 mm long. They have one stamen. The female flowers do not have the leafy spathe. They are 3-4 mm long. Flowering and fruiting take place beneath the water surface.
Dioecious; stems 0.5–4.5 dm, 0.5–4 mm thick, often prickly; lvs 0.5–4 cm × 0.5–4.5 mm, spreading or ascending, prickly along the midvein beneath, coarsely serratewith 8–13 multicellular teeth projecting 0.5–1 mm on each side; anthers dithecal, with 4 microsporangia; seeds 2.2–4.5 mm, reddish-brown, ovoid, pitted, with irregular areolae; 2n=12. Brackish or highly alkaline water of ponds and lakes; irregularly cosmop.; in our range from N.Y. and Pa. to Wis., Ill., and Minn.
