42 FLORA. 



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, 5~7-nerved, obtuse at the apex. 



16. P. Mysticus. 

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



2r. P.foliosus. 

 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. zosteraefolius. 

 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, 3~5-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. pusillus. 



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. diversifolius. 

 Submerged peduncles none, or at most hardly 2 mm. long. 



30. P. Spirillus. 

 With submerged leaves only. 



Stigma broad and sessile. 31. P. filiformis. 

 Style apparent ; stigma capitate. 



Fruit without keels or obscurely keeled. 32. P. pectinatus. 

 Fruit strongly 3-keeled. 



Leaves entire, 3~5-nerved. 33. P, interruptus. 



Leaves minutely serrulate, finely many-nerved. 34. P. Robbinsii. 



1. Potamogeton natans L. COMMON FLOATING PONDWEED. (I. F. f. 142.) 

 Stems 0.6-1.3 m - l n g> simple or sparingly branched. Floating leaves thick, the 

 blade ovate, oval or elliptic, 5-10 cm. long, 2-5 cm. wide, usually with a short 

 abrupt tip, rounded or subcordate at the base, many-nerved; submerged leaves 

 reduced to phyllodes which commonly perish early; stipules sometimes 10 cm. 

 long, acute, 2-keeled; peduncles as thick as the stem. 5-10 cm. long; spikes very 

 dense, about 5 cm. long; fruit turgid, 4-4.5 mm - ' on g' about 2.5 mm. thick, scarcely 

 keeled, narrowly obovoid, slightly curved on the face; style broad; nutlet hard, 

 pitted or impressed on the sides, 2 -grooved on the back ; embryo forming an 

 incomplete circle, the apex pointing toward the base. In ponds and streams, 

 throughout N. Am. except the extreme north, extending into Ivfex. Also in Europe 

 and Asia. July-Aug. 



2. Potamogeton Oakesianus Robbins. OAKES' PONDWEED. (I. F. f. 143.) 

 Stems very slender, often much branched from below. Floating leaves elliptic, 

 mostly obtuse, rounded or subcordate at the base, 2-5 cm. long, 10-20 mm. wide, 

 !2-2O-nerved; petioles 5-15 cm. long; submerged leaves mere capillary phyllodes, 

 often persistent; peduncles 2-8 cm. long, commonly thicker than the stem; spikes 

 cylindric, 12-25 cm. long; stipules acute, hardly keeled*; fruit obovoid, about 

 3 mm. long, 2 mm. thick, nearly straight on the face, 3-keeled, the middle keel 

 sharp; style apical or subapical; sides of the nutlet not pitted, but sometimes 



