1. Stipular sheaths of submersed leaves adnate to leaf
blade base, the tip usually projecting as a
ligule............................2
1. Stipular sheaths of submersed leaves free from leaf blade base
or with only 1--5 adnate, ligule not obvious...............
3
2. Submersed leaves stiffish, conspicuously 2-ranked; blades lobed at junction with stipule, with
20--60 fine veins; fruit 2--3.3 mm wide, lateral keels without sharp tips, beak 0.7--0.9 mm
long......................................................................................................................1. Potamogeton robbinsii
2. Submersed leaves lax, not conspicuously 2-ranked; blades lacking basal lobes, with fewer
than 20 veins; fruit 0.9--2 mm wide, lateral keels with sharp tips, beak ca. 0.1 mm long
.......................................................................................................................2. Potamogeton diversifolius
3. Submersed leaf blades linear, thread-like or
ribbon-like, 0.1--10 mm
wide.............................................................
4
3. Submersed leaf blades broadly linear--oblong to lanceolate to
elliptic or nearly orbicular, 4--58
mm wide (occasional stranded plants lack submersed
leaves)......................................................................................
6
4. Fruit with a prominent abaxial keel 0.2--1.2 mm broad, with lateral keels; floating leaves often
present; lacunae prominent in submersed leaves................................................... 3. Potamogeton epihydrus
4. Fruit with an abaxial keel less than 0.2 mm broad, without lateral keels; floating leaves absent;
lacunae sometimes present but not prominent in most species.......................................................................... 5
5. Fruits with undulating abaxial keel or
ridge........................................................................4.
Potamogeton
foliosus
5. Fruits with abaxial surface
rounded..................................................................................
5. Potamogeton
pusillus
6. Leaf margins conspicuously serrate; stem flattened; fruit beak 2--3 mm long; turions commonly
formed and hard .....................................................................................................6. Potamogeton crispus
6. Leaf margins entire; stem terete; fruit beak 1 mm or less in length; turions rarely formed............................ 7
7. Submersed leaves clasping stem; floating leaves absent
............................................11. Potamogeton
perfoliatus
7. Submersed leaves petioled or sessile (not clasping),
floating leaves absent or
present............................................ 8
8. Submersed leaves 19--49-veined, distinctly arcuate................................... 7. Potamogeton amplifolius
8. Submersed leaves with 7-- 29 veins, not arcuate....................................................................................................................................................... 9
9. Stems conspicuously black-spotted; submersed leaf
blades crisped along margin; floating
leaves 15--21-veined
........................................................................................................8.
Potamogeton
pulcher
9. Stems inconspicuously spotted or lacking spots;
submersed leaf blades flat or rarely crisped
along margin; floating leaves
7--29-veined...............................................................................................................
10
10. Larger submersed leaves acute at apex but without sharp awl-like tip; fruit red to
reddish-brown ......................................................................................................9. Potamogeton nodosus
10. Larger submersed leaves acute at apex with a sharp awl-like tip; fruit grayish-green
to olive-green ...................................................................................................10. Potamogeton illinoensis