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Key to Najas

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1 Lower side of the midvein of the leaves prickly; internodes prickly; leaves coarsely and raggedly serrate, the tooth sinuses often halfway or more to the midvein; plants dioecious; [subgenus Najas]
1 Lower side of the midvein of the leaves smooth; internodes smooth; leaves more finely serrate, the tooth sinuses < 0.2× of the way to the midvein; plants monoecious; [subgenus Caulinia].
  2 Leaf-teeth multicellular, evident at 10× magnification, 5-22 per side; leaves either ascending to spreading, or becoming recurved late in the season; seed-coat pitted.
    3 Style offset to the side of the seed apex; [section Caulinia]
    3 Style centered at summit of seed apex.
      4 Leaves remaining ascending late in the season; seeds straight, 0.7-1.5 mm long × 0.3-0.5 mm wide; [section Americanae]
      4 Leaves becoming recurved late in the season; seeds slightly to strongly curved, 1.5-3.0 mm long × 0.5-0.7 mm wide.
        5 Areoles of the seed coat longer than broad, never ladderlike; [of GA and FL]; [section Americanae]
        5 Areoles of the seed coat distinctly wider than long, in ca. 12-18 ladder-like rows; [widespread]; [section Caulinia]
  2 Leaf-teeth unicellular, not or barely evident at 10× magnification, 18-100 per side; leaves spreading to ascending; seed-coat smooth or pitted, the areoles (if present) longer than wide or about as long as wide; [section Americanae].
          6 Seeds smooth, glossy, obovate, broadest above the middle; anthers 1-locular.
             7 Seeds (0.4-) 0.6-0.8 (-1.0) mm wide, > 3.2× as long as wide
             7 Seeds (0.7-) 0.85-1.3 (-1.9) mm wide, < 3.1× as long as wide
          6 Seeds pitted, dull, cylindric, fusiform, or elliptic, broadest at the middle; anthers 1- or 4-locular.
               8 Anthers 4-locular; leaf teeth 20-100 per leaf side.
                 9 Stems 0.1-0.8 mm in diameter; leaf blades with 50-100 teeth per side
                 9 Stems 1.0-2.0 mm in diameter; leaf blades with 20-40 teeth per side
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