Stipules axillary and free from the leaf. 
Submerged leaves blad if 
Nutlets deeply pitted. pua 
Nutlets not pitted. а А a. 
Submerged leaves with a proper blade. 
Submerged leaves of 2 kinds, lanceolate and oval or oblong. 
Uppermost broadly oval or elliptical, lowest lanceolate. 
3. P. amplifolius. 
Uppermost lanceolate and pellucid, lowest oblong and opaque. 
4. Р. pulcher. 
Submerged leaves all alike, capillary or Nice az 
I-nerved or nerveless. 27. Р. Vaseyi. 
3-nerved. 28. P. lateralis. 
Submerged leaves all alike, linear. 
early the same breadth throughout, obtusely pointed, coarsely cellular-retic- 
ulated in the middle. 5. Р. Nuttalhi, 
Broader at base, acute, without cellular-reticulation. 
10. P. AeteropAy из. 
Submerged leaves all alike, lanceolate. 
Fruit strongly embossed or dentate on the keels. 9. P. Mexicanus. 
Keels of fruit even. 
Uppermost leaves petioled, lowest sessile. 6. alpinus. 
All petioled. 
Floating leaves large, broadly elliptical, rounded or subcordate at base. 
12. P. Illinoensis. 
Floating leaves narrowly elliptical, sloping at base. 
7. Р. lonchites. 
Floating leaves mostly obovate or oblanceolate, sloping at base, 
8. P. Faxoni. 
All sessile or subsessile. 
Fruit only 1 line long, obscurely 3 keeled. 11. Р. spathule fi 
JET 5 . 
Fruit 114 lines long, distinctly 3-keeled. 
With submerged leaves only. 
Without propagating ча or glands. 
13. P. angustifolius. 
road blades, mostly lanceolate or ovate, many nerved. 
Leaves subsessile or shortly petioled, mostly acute or cuspidate, 
I 
4. P. lucens, 
M 1 3 1 na 11 = ы 
T I , obtnse ti t the apex. 
IS. P. prelongus. 
Leaves meeting around the stem, very obtuse at the py not cucullate, 
5. А E 
Leaves with narrow blades, linear or oblong-linear, pee 
таза жаса алы 
g linear, 5-7 nerved, obtuse at the apex. 
