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Key to families | Table of families and genera

Specimen numbers are hyperlinked to records in the Consortium of California Herbaria data view where possible. Taxa are hyperlinked to entries in the Jepson Interchange via the "[Online Interchange]" link.


Annual, perennial herb, rhizomed or not, tubers 0, winter buds generally 0. Stem: simple or branched, cylindric to compressed; nodal glands present or 0. Leaf: submersed, also floating or not, generally flat, generally green, entire to finely serrate; submersed sessile or not, linear to round, tip rounded to acuminate, veins 1–35; floating 0 or generally petioled, elliptic to ovate, leathery; stipules of submersed leaves free from blade base or fused to it < 1/2 stipule length. Inflorescence: generally floating to emergent; peduncle stiff. Fruit: generally beaked, abaxially with 0 or 1 central keel, 0 or 2 lateral, sometimes lower keels, sides concave to convex.
± 88 species: generally temperate northern hemisphere. (Greek: river neighbor, from aquatic habitat) Other taxa in TJM (1993) moved to Stuckenia.
Unabridged references: [Haynes 1974 Rhodora 76:564–649; Haynes 1985 Sida 11:173–188; Wiegleb 1988 Fedde Rep Sp Nov Regni Veg 99:249–266; Wiegleb & Kaplan 1998, Folia Geobot 33: 241–316.]

Key to Potamogeton

1. Leaves submersed and floating

2. Submersed leaves linear, < 1 cm wide

3. Stipules fused to blade base; inflorescences of 2 kinds, lower submersed, spheric, upper emergent, cylindric; fruit compressed ..... 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 to ovate; floating leaves 27–50-veined ..... P. amplifolius (2)

5' Submersed leaves not curved backward, not folded along midvein, lanceolate to elliptic to oblanceolate; floating leaves < 30-veined

6. Submersed leaves petioled

7. Submersed leaves microscopically serrate near tip, petioles 0.5–4 cm ..... P. illinoensis (4)

7' Submersed leaves entire, petioles 2–13 cm ..... P. nodosus (2)

6' Submersed leaves sessile (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

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, 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, 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 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 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 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


Citation for the whole project: Jepson Flora Project (eds.) [year] Jepson eFlora, [accessed on month, day, year]
Citation for an individual treatment: [Author of taxon treatment] [year]. [Taxon name] in Jepson Flora Project (eds.) Jepson eFlora, [URL for treatment]. Accessed on [month, day, year].
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