The Jepson Manual: Higher Plants of California

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Names are linked to treatments from the Manual


Key to POTAMOGETON

N.B.: couplets are linked, e.g., 1. is linked to 1'.

  1. Plants with both submersed and floating leaves; floating leaves generally elliptic to ovate, generally leathery; submersed leaves linear to ovate, membranous
 
    2. Submersed leaves lanceolate to ovate, generally 1–7 cm wide, sometimes linear
 
      3. Submersed leaves generally lanceolate to ovate, curved, generally folded along midrib, sessile or short-petioled, blade veins 20–40; floating leaves elliptic to ovate, blade veins 30–50
P. amplifolius
      3' Submersed leaves linear to lanceolate, not curved, flat, upper long-petioled, lowermost sessile (all sessile in P. gramineus); all leaf blades with < 30 veins
 
        4. Submersed leaves with 8+ veins, long-petioled
2 P. nodosus
        4' Submersed leaves generally with 7 veins, all or lowermost sessile
 
          5. Margin of submersed leaves finely serrate near tip
2 P. illinoensis
          5' Margin of submersed leaves entire
 
            6. Leaves reddish; submersed leaf width = floating leaf width; submersed leaves ± all on central stem; floating leaves short-petioled
2 P. alpinus subsp. tenuifolius
            6' Leaves green; submersed leaves narrower than floating leaves; submersed leaves on short, axillary branches; floating leaves long-petioled
P. gramineus
    2' Submersed leaves linear, < 1 cm wide
 
      7. Stipules fused to leaf base; inflorescences of 2 kinds, lowermost spheric, submersed, short-peduncled, uppermost cylindric, emergent, long-peduncled; fruit clearly compressed — lateral keels low
P. diversifolius
      7' Stipules free from leaf base; inflorescences all emergent, ± cylindric; fruit plump to slightly compressed
 
        8. Submersed leaves ribbon-like, 2–10 mm wide, with a clearly net-like band along the midrib
P. epihydrus subsp. nuttallii
        8' Submersed leaves without a net-like band, cylindric, often petiole-like, < 2 mm wide, or linear to elliptic-lanceolate, 10–40 mm wide
 
          9. Floating leaf blade oblong to widely elliptic, base slightly lobed, jointed to petiole; fruit smooth or faintly keeled, sides concave
P. natans
          9' Floating leaf blade elliptic to ovate, base tapered to petiole, not jointed to petiole; fruit 3-keeled, sides flat
2 P. nodosus
  1' Plants with all leaves submersed
 
    10. Leaves lanceolate, elliptic, oblong, or ovate, sessile, base tapered to clasping (short-petioled in P. illinoensis)
 
      11. Leaf blade margin finely serrate; fruit with long, slender beak 2–3 mm
P. crispus
      11' Leaf blade margin entire (finely serrate near tip in P. illinoensis); fruit short-beaked
 
        12. Leaf base tapered, not clasping
 
          13. Upper leaves generally sessile; blade margins entire; leaves reddish
2 P. alpinus subsp. tenuifolius
          13' Upper leaves short-petioled; blade margin finely serrate near tip; leaves green
2 P. illinoensis
        12' Leaf base clasping
 
          14. Blades lanceolate to ovate, 5–20 cm, tip hood-like, splitting when dry; stipules 2–8 cm, persistent, whitish; stems zigzag; fruit 4–5 mm, sharply 3-keeled
P. praelongus
          14' Blades narrowly ovate, 1–12 cm, tip not hood-like, flat, margin crinkly, not splitting when dry; stipules < 2 cm, becoming fibrous; stems straight; fruit 2–4 mm, faintly 3-keeled
P. richardsonii
    10' Leaves linear
 
      15. Stipules fused to leaf base, sheath-like below, sheath 1+ cm
 
        16. Leaves 2–8 mm wide; stipules fused above junction with blade, ligule-like
 
          17. Leaf margin entire; ligule entire; leaf blade not appendaged at junction with sheath; fruit smooth or faintly keeled
P. latifolius
          17' Leaf margin finely serrate; ligule generally cut to fibrous; leaf blade often appendaged at junction with sheath; fruit clearly keeled
P. robbinsii
        16' Leaves linear to thread-like, < 3 mm wide; stipules free above junction with blade
 
          18. Stigma sessile; fruit 2–2.7 mm, not beaked; sheath partly closed; stem branched below, few-branched above
P. filiformis
          18' Stigma raised on minute style, head-like, persisting in fruit as minute beak; fruit 2.5–5 mm; sheath open to base; stem with many, forked branches above
P. pectinatus
      15' Stipules free from leaf or fused to leaf base for 1–2 mm
 
        19. Leaves 2–5 mm wide, veins 9–35, somewhat rigid; stems clearly flattened, winged, width at least 1/2 leaf width
P. zosteriformis
        19' Leaves < 3 mm wide, veins 1–7, weak; stem not flattened or winged
 
          20. Keel of fruit back wing-like, wavy; glands at leaf base 0 or faint
P. foliosus
            21. Stipules coarsely veined and ciliate, rigid, breaking apart into fibers; inflorescence generally interrupted; fruit pale green, 1.4–1.7 mm, 1.1–1.2 mm wide, keel on back generally < 0.2 mm high, beak < or = 0.2 mm
var. fibrillosus
            21' Stipules finely veined, not coarsely ciliate, decaying early; inflorescence generally not interrupted; fruit olive to greenish brown, 1.5–2.7 mm, 1.2–2.2 mm wide, keel on back generally 0.2–0.4 mm high, beak 0.2–0.6 mm
var. foliosus
          20' Fruit back rounded, not keeled; glands at base of some or all leaves 2, prominent
P. pusillus
            22. Fruit sides concave, tip beaked at top of central axis; inflorescence clearly interrupted, with 2–4 whorls; stipules fused below middle
var. pusillus
            22' Fruit sides rounded, tip oblique, beak to 1 side of central axis; inflorescence with 1–2 whorls; stipules free, flat or margins inrolled
var. tenuissimus


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