GENUS i. 



PONDWEED FAMILY. 



75 



Nearly the same breadth throughout, obtusely pointed, coarsely cellular-reticulated in 



the middle. 5. P. epihydrus. 



Broader at base, acute, without cellular-reticulation. 9. P. heterophylliis. 



Submerged leaves all alike, lanceolate. 



Uppermost leaves petioled, lowest sessile. 6. P. alpinus. 



All the leaves petioled. 



Floating leaves large, broadly elliptic, rounded or subcordate at base. 



1 1 . P. illinoensis. 



Floating leaves narrowly elliptical, tapering at base. 7. P. americanns. 



Floating leaves mostly obovate or oblanceolate, tapering at base. 



8. P. Faxoni. 

 All the leaves sessile or subsessile. 



Fruit only i line long, obscurely 3-keeled. -i-e -P. varians. 



Fruit i l / 2 lines long, distinctly 3-keeled. 12. P. angustifolius. 



With submerged leaves only. 



Without propagating buds and without glands. 



Leaves with broad blades, mostly lanceolate or ovate, many-nerved. 



Leaves subsessile or short-petioled, mostly acute or cuspidate. 13. P. lucens. 

 Leaves semi-amplexicaul, obtuse and cucullate at the apex. 14. P. praelongus. 

 Leaves meeting around the stem, very obtuse at the apex, not cucullate. 



15. P. perfoliatus. 

 Leaves with narrow blades, linear or oblong-linear, several-nerved. 



Leaves oblong-linear, s-/-nerved, obtuse at the apex. 16. P. mysticus. 



Leaves narrowly linear, 3-nerved, acute at the apex. 21. P. foliosits. 



Leaves with narrow blades, capillary or setaceous, i -nerved or nerveless. 



17. P. confervoides. 



With propagating buds or glands, or both. 

 With buds, but without glands. 



Leaves serrulate, 3-7-nerved. 18. P. crispus. 



Leaves entire, with 3 principal and many fine nerves. 19. P. compressus. 



Commonly with glands, but no buds. 



Stems long-branching from the base ; leaves lax, flat, 3-nerved, abruptly acute or 



cuspidate. 20. P. Hillii. 



Stems simple ; leaves strict, revolute, 3-s-nerved, acuminate. 24. P. rutilus. 

 With both buds and glands. 



Glands large and translucent ; buds rare. 22. P. obtusifolius. 



Glands small, often dull ; buds common. 



Leaves linear, 5-7-nerved. 23. P. Friesii. 



Leaves linear, 3-nerved. 27. P. pasillus. 



Leaves capillary, i -nerved or nerveless. 28. P. gemmiparus. 



Stipules adnate to the leaves or petioles. 



With both floating and submerged leaves. 



Submerged peduncles as long as the spikes, clavate, often recurved. 29. P. diver sifolius. 

 Submerged peduncles none, or at most hardly a line long. 30. P. dimorphus. 



With submerged leaves only. 



Stigma broad and sessile. 



Sheath of stipule less than 4" long. 

 Sheath of stipule more than 7" long. 

 Style apparent ; stigma capitate. 



Fruit without keels or obscurely keeled. 

 Fruit strongly 3-keeled. 



Leaves entire, 3-s-nerved. 



Leaves minutely serrulate, finely many-nerved. 



31. P. filiform is. 



32. P. interior. 



33. P. pectinatus. 



34. P. interruptus. 



35. P. Robbinsii. 



i. Potamogeton natans L. Common Floating Pond weed. Fig. 174. 



Potamogeton natans L. Sp. PI. 126. 1753. 



Stems 2-4 long, simple or sparingly branched. 

 Floating leaves thick, the blade ovate, oval or ellip- 

 tic, 2'~4' long, i '-2' wide, usually tipped with a short 

 abrupt point, rounded or subcordate at the base, 

 many-nerved ; submerged leaves reduced to phyllodes 

 or bladeless petioles which commonly perish early 

 and are seldom seen at the fruiting period ; stipules 

 sometimes 4' long, acute, 2-keeled ; peduncles as 

 thick as the stem, 2'~4' long; spikes cylindric, very 

 dense, about 2' long; fruit turgid, 2"-2j" long, about 

 li" thick, scarcely keeled, narrowly obovoid, slightly 

 curved on the face ; style broad and facial ; nutlet 

 hard, more or less pitted or impressed on the sides, 

 2-grooved on the back; embryo forming an incom- 

 plete circle, the apex pointing toward the base. 



In ponds and streams, Nova Scotia to British Co- 

 lumbia, New Jersey, Missouri and Nebraska. Also in 

 Europe and Asia. Called also Tench-weed, Batter- 

 dock, Deil's-spoons. July-Aug. 



