Key to Potamogeton

View taxon page for Potamogeton

(For a list of species in Potamogeton, use the above link.)

Jepson Manual glossary definitions can be seen by moving your cursor over words underlined with dots.

1. Leaves submersed and floating

2. Submersed leaves 

linear 

Elongate, with nearly parallel sides; narrower than elliptic or oblong.

, < 1 cm wide

3. Stipules fused to 

blade 

Expanded portion of a leaf, petal, or other structure, generally flat but sometimes rolled, cylindric, wavy, or cupped.

base; inflorescences of 2 kinds, lower submersed, spheric, upper emergent, 

cylindric 

Elongate, with parallel sides and, at any point, round in transverse section.

; fruit 

compressed 

Flattened side-to-side or front-to-back.

..... P. diversifolius (2)

3' Stipules free from blade base; inflorescences all emergent, ± cylindric; fruit ± compressed or not

4. Submersed leaves with a net-like band along midrib, ribbon-like, 2–10 mm wide ..... P. epihydrus (2)

4' Submersed leaves without a net-like band along midrib, not ribbon-like, either cylindric, often petiole-like, < 2 mm wide, or linear to lance-elliptic, 10–40 mm wide ..... P. natans

2' Submersed leaves linear or wider, generally 1–7 cm wide

5. Submersed leaves curved backward, generally folded along midvein, generally 

lanceolate 

Narrowly elongate, widest in the basal half, often tapered to an acute tip.

to 

ovate 

Egg-shaped (i.e., widest below the middle) in two dimensions (i.e., in one plane), as a leaf.

; floating leaves 27–50-veined ..... P. amplifolius (2)

5' Submersed leaves not curved backward, not folded along midvein, lanceolate to 

elliptic 

In the shape of a flattened circle or ellipse; wider than linear

to oblanceolate; floating leaves < 30-veined

6. Submersed leaves petioled

7. Submersed leaves microscopically 

serrate 

Having margins with sharp, fine to coarse teeth generally pointing tipward, not outward; margins with such teeth on such primary teeth are doubly serrate.

near tip, 

petioles 

Leaf stalk, connecting leaf blade to stem; sometimes more or less indistinct.

0.5–4 cm ..... P. illinoensis (4)

7' Submersed leaves 

entire 

Having margins that are continuous and smooth (i.e., without teeth, lobes, etc.).

, petioles 2–13 cm ..... P. nodosus (2)

6' Submersed leaves 

sessile 

Without a petiole, peduncle, pedicel, or other kind of stalk.

(petioles rarely <= 3 cm in P. gramineus)

8. Submersed leaves 3–9-veined; fruit 1.9–2.3 mm ..... P. gramineus (2)

8' Submersed leaves 7–19-veined; fruit 2.5–3.6 mm

9. Floating leaves 7–15-veined; submersed leaves 7–9-veined, entire ..... P. alpinus (2)

9' Floating leaves 13–29-veined; submersed leaves 7–19-veined, microscopically serrate near tip ..... P. illinoensis (4)

1' Leaves submersed

10. Leaves linear or ovate to lanceolate to elliptic to oblanceolate, sessile or petioled

11. Leaf blade serrate; fruit beak 2–3 mm ..... P. crispus

11' Leaf blade entire or microscopically serrate near tip; fruit beak < 2 mm

12. Leaf base not clasping

13. Petioles 0.5–26 cm

14. Leaf microscopically serrate near tip, tip acute-mucronate ..... P. illinoensis (4)

14' Leaf entire, tip 

acute 

Having a short-tapered, sharp tip, the sides convex or straight and converging at less than a right angle.

15. Leaves 19–50-veined, curved backward, generally folded along midvein ..... P. amplifolius (2)

15' Leaves 7–15-veined, not curved backward, not folded along midvein ..... P. nodosus (2)

13' Petioles 0

16. Leaves entire, 7–9-veined; fruit pedicelled ..... P. alpinus (2)

16' Leaves minutely to microscopically serrate near tip, 3–19-veined; fruit sessile

17. Leaf blade 3–9-veined, 2–10 cm, (3)10–27 mm wide; fruit 1.9–2.3 mm, beak 

erect 

Upright; vertically oriented.

, 0.3–0.5 mm ..... P. gramineus (2)

17' Leaf blade 7–19-veined, 6–20 cm, 15–50 mm wide; fruit 2.5–3.6 mm, beak erect to ± 

recurved 

Gradually curved downward or backward.

, 0.5–0.8 mm ..... P. illinoensis (4)

12' Leaf base clasping

18. Leaf lance-linear, 5–20 cm, tip hood-like, splitting when dry; stipules 2–10 cm, persistent; stems often zigzag; fruit 4–5 mm, keels 1(3) ..... P. praelongus

18' Leaf linear- to lance-ovate, 1–12 cm, tip flat, not hood-like, not splitting when dry; stipules < 2 cm, persistent as fibers; stems generally straight; fruit 2–4 mm, keels 0(1) ..... P. richardsonii

10' Leaves linear (linear to lanceolate in P. robbinsii), sessile

19. Stipules fused to blade base for ± 1/4 to 1/2 

stipule 

Appendage at base of petiole, generally paired, variable in form but often leaf- or scale-like, sometimes a spine.

length, or 1–15 mm, sheath-like basally, liguled.

20. Leaves thin, not conspicuously 2-ranked, vein 1, not fine ..... P. diversifolius (2)

20' Leaves ± stiff, 2-ranked, 3–4(8) mm wide, veins 20–60, fine ..... P. robbinsii

19' Stipules free from or fused to blade base for <= 1/5 stipule length, or 1–2 mm, not sheath-like basally, not liguled

21. Leaf veins 15–35; stems compressed, winged ..... P. zosteriformis

21' Leaf veins 1–13; stems ± compressed or cylindric

22. Rhizomes present, winter 

buds 

1. An incompletely developed, more or less embryonic shoot, usually covered with bud scales. 2. An unopened flower, often protected by sepals.

0; fruit 2.5–4.5 mm, 2–3.6 mm wide; stems compressed ..... P. epihydrus (2)

22' Rhizomes 0, winter buds present; fruit 1.4–2.7 mm, 1.1–2.2 mm wide; stems rounded to ± compressed

23. Fruit with wing-like keel; 

peduncles 

Stalk of an individual flower borne singly, not in an inflorescence, or of an entire inflorescence, or the corresponding structure in fruit; the stalk subtending an involucre (e.g., in Asteraceae, Polygonaceae).

0.3–1.1(3.7) cm; nodal glands 0 (faint)..... P. foliosus

24. Stipule veins in age persisting as fibers; inflorescence interrupted; fruit pale green, 1.4–1.7 mm, 1.1–1.2 mm wide, keel < 0.2 mm high, beak <= 0.2 mm ..... subsp. fibrillosus

24' Stipule veins in age decaying, occasionally persistent as delicate fibers; inflorescence generally not interrupted; fruit olive to green-brown, 1.5–2.7 mm, 1.2–2.2 mm wide, keel 0.2–0.4 mm high, beak 0.2–0.6 mm ..... subsp. foliosus

23' Fruit without keel; peduncles 0.5–8 cm; nodal glands present

25. Fruit widest at or below middle, sides convex (concave); peduncles per plant > 3; inflorescence continuous; stipules free ..... P. berchtoldii

25' Fruit widest above middle, sides concave; peduncles per plant 1–3; inflorescence interrupted; stipules fused below middle ..... P. pusillus

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