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See list of 10 species in this genus

The ability of certain species to form turions (i.e., winter buds) is a useful identification character. The turions appear as a mass of crowded, highly reduced leaves. Of the species with dissected leaves, only M. alternifolium, M. humile, and M. spicatum do not form turions. The other species do, these generally present on the plant from September or October through April. Reference: Aiken (1981).

  • 1a. Leaf blades (when present) entire, represented by small scales or scale-like bumps on the erect stems [Fig. 679]; bracts entire [Fig. 679]
  • 1b. Leaf blades pinnately divided [Figs. 677,678,680]; bracts entire or, more commonly, toothed or divided [Figs. 677,678,680]
    • 2a. Foliage leaves alternate, subopposite, or irregularly subverticillate; staminate flowers commonly with 4 stamens 0.4–1.1 mm long; flowers in the axils of submersed leaves or in emersed spikes
      • 3a. Leaves alternate (sometimes some subopposite, especially in submersed plants) 
 [Fig. 677]; fruit 0.7–1.2 mm long; mericarps rounded on the outer surface, without ridges or projections
      • 3b. Leaves typically both alternate and whorled on the same stem; fruit 1.3–2.5 mm long; mericarps with 2 tuberculate, longitudinal ridges on the outer surface
        • 4a. Flowers in emersed spikes, subtended by bract-like leaves; anthers 0.8–1.1 mm long; fruits 1.3–1.8 mm long; turions (i.e., winter buds) not formed
        • 4b. Flowers in the axils of submersed leaves; anthers ca. 0.4 mm long; fruits 2–2.5 mm long; turions present, formed in autumn
    • 2b. Foliage leaves regularly whorled [Figs. 678,680]; staminate flowers commonly with 8 stamens 1.2–2 mm long (only 4 stamens in M. heterophyllum or the staminate flowers missing); flowers in emersed spikes
      • 5a. Flowers subtended by bracts mostly 25–35 mm long with uniform linear divisions 4–8 mm long, the bracts ± similar in size and division to the submersed leaves; submersed leaves with petioles 5–7 mm long; plants dioecious, ours always carpellate
      • 5b. Flowers subtended by bracts less than 20 mm long, with marginal teeth or divisions (when present) shorter than 4 mm, the bracts conspicuously reduced in size and less divided than the submersed leaves; submersed leaves with petioles 0–2 mm long; plants monoecious
        • 6a. Flowers and their subtending bracts alternate (sometimes the lowest opposite); leaf blades with usually 3–7 pairs of narrow segments; spikes 2–5 cm tall; turions not formed
        • 6b. Flowers and their subtending bracts whorled [Fig. 680]; leaf blades with usually 6–20 pairs of narrow segments; spikes 4–15 (–37) cm tall; turions formed in late season (not formed in M. spicatum)
          • 7a. Carpellate bracts less than 2 times as long as the flowers or fruits [Fig. 678]; staminate bracts (i.e., the upper ones) with entire to denticulate margins
            • 8a. Middle leaves with mostly 6–12 segments on each side of the rachis; upper leaves rounded at the apex; stems of ± similar diameter below the inflorescence and near the stem base, green near the apex of vegetative shoots (except sometimes in late season when turions are being formed), ± erect; turions 
(i.e., winter buds) formed in the fall
            • 8b. Middle leaves with mostly 12–20 segments on each side of the rachis; uppermost leaves truncate at the apex; stems thicker below the inflorescence than at the stem base, up to twice as thick, usually red near the apex of vegetative shoots, curved to lie parallel with the water [Fig. 678]; turions 
not formed
          • 7b. Carpellate bracts usually 2 or more times as long as the flowers or fruits 
 [Fig. 680]; staminate bracts with serrulate to pinnatifid margins
            • 9a. Carpellate bracts prominently serrate to pinnately lobed [Fig. 680]; fruits 2–2.5 mm long; turions formed in the late summer and fall on the stems, clavate in outline
            • 9b. Carpellate bracts merely serrulate (sometimes pectinately lobed near the water line); fruits 1–1.5 mm long; turions formed in the fall at the base of the stems or on the rhizomes, not clavate in outline

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 Show photos of:   Each photo represents one species in this genus.