PONDWEED FAMILY 23 



1. P. natans L. Stem 6-15 dm. long, mostly simple; floating leaf-blades 

 oval or ovate, abruptly short-pointed, rounded or cordate at the base, thick, 

 21-29-nerved, 5-10 cm. long, 2.5-5.5 cm. wide; submerged leaves bladeless 

 and early perishing; peduncles 5-10 cm. long; spike cylindric, 3-5 cm. long, very 

 dense; fruit 4-4.5 mm. long, 2-5 mm. thick; stone 2-grooved on the back. Ponds 

 and streams: N.S. -N.J. Calif. Alaska; Eurasia. Plain -Mont. Jl-S. 



2. P. amplifolius Tuckerm. Stem occasionally branched; floating leaf- 

 blades (occasionally wanting) ovate or oval, acute at the apex, rounded or sub- 

 cordate at the base; submerged leaves mostly short-petioled ; the blades of the 

 upper ovate or elliptic, 7-15 cm. long, 3-6 cm. wide, thin and shining; those of 

 the lower lanceolate, often 1 dm. long, about 25-nerved, generally conduplicate 

 and falcate; peduncles thickened upwards, 5-20 cm. long; spike cylindric, 2.5 

 cm. long; fruit 4-5 mm. long, 2.5 mm. thick, 3-keeled. Lakes: N.B. ^Ga. 

 Neb. B.C. Plain. Jl-S. 



3. P. americanus Cham. & Schlecht. Stem much branched, 1-2 m. long; 

 floating leaf-blades rather thick, elliptic, pointed at each end, 5-15 cm. long, 

 1.5-3 cm. wide, 11-23-nerved; submerged leaf-blades pellucid, thin, 1-3 dm. 

 long, 0.5-2.5 cm. wide, 7-15-nerved, lanceolate to linear-lanceolate, with distinct 

 petioles; peduncles thickened upwards, 5-7 cm. long; spike cylindric, 3-5 cm. 

 long; fruit about 4 mm. long, obliquely obovoid, 3-keeled on the back. P. 

 lonchites Tuckerm. P. fluitans Am. auth. Ponds and slow streams: N.B. Fla. 

 -Calif. B.C. Plain. Je-O. 



4. P. alpinus Balbis. Stem slender, simple or branched; floating leaf- 

 blades rather thin, oblanceolate or spatulate, or sometimes oblong, 11-17- 

 nerved, 5-12 cm. long, sometimes wanting; submerged leaves thin, semi-pellucid, 

 oblong to linear-lanceolate, 7-30 cm. long, 5-20 mm. wide, 7-17-nerved, sessile 

 or the upper short-petioled; spike cylindric, 2-4 cm. long; fruit obovoid-lenticular, 

 reddish, 2-5 mm. long, 3-keeled. P. rufescens Schrader. Ponds and streams: 

 N.S.N.J. Colo. Calif. Alaska; Eu. PlainMont. Jl.-Au. 



5. P. angnstifolius Berch. & Presl. Stem slender, branching; floating leaf- 

 blades somewhat coriaceous, elliptic, 4-10 cm. long, 12-35 mm. wide, many- 

 nerved; submerged leaves lanceolate or oblanceolate, thin, acute or cuspidate, 

 5-15 cm. long, 5-30 mm. wide, 7-17-nerved; peduncles thicker than the stem, 

 6-15 cm. long; spike cylindric, 2.5-5 cm. long; fruit obliquely ovoid, 2.5-4 mm. 

 long, 2 mm. thick, 3-ribbed; style short, blunt. P. Zizii Roth. Lakes and 

 streams: Me. Fla. Tex. Calif. B.C.; Mex., C. Am., and Eu. Plain 

 Submont. Jl-Au. 



6. P. epihydrus Raf. Stem slender, simple, compressed, 3-18 dm. long; 

 floating leaves opposite; blades elliptic or obovate, 4-9 cm. long, 6-17 mm. 

 wide, many-nerved, obtuse, short-petioled; submerged leaf -blades linear or 

 linear-lanceolate, 5-15 cm. long, 2-4 mm. wide, coarsely reticulate along the 

 midrib, 5-nerved; spike cylindric, 1-6 m. long; fruit rounded-obovoid, 2.5-4 

 mm. long, 2-3 mm. thick, 3-keeled; style short, terminal. P. Nuttallii Cham. 

 & Schlecht. Ponds and streams: Newf. S.C. Iowa B.C. Plain. Je-Au. 



7. P. heterophyllus Schreb. Stem slender, much branched, sometimes 

 3 m. long; floating leaf-blades oval, pointed at the apex, narrowed, rounded, or 

 subcordate at the base, 1.5-6 cm. long, 8-28 mm. wide, 9-19-nerved; submerged 

 leaves linear or linear-lanceolate, pellucid, 1-5 cm. long, 2-18 mm. wide, 3-9- 

 (mostly 7-) nerved, acuminate or cuspidate; peduncles often thickened upwards; 

 spike cylindric, 1-2 cm. long; fruit rounded or obliquely obovoid, 2-3 mm. long, 

 1-2 mm. thick, indistinctly 3-keeled; style short, apical. A form with the sub- 

 merged leaves linear, flaccid, 5-12 cm. long, 2-6 mm. wide, is P. heterophyllus 

 graminifolius . Ponds or lakes, and slow streams: Lab. Fla. 'Utah -Calif. 

 B.C.;Eu. Plain Submont. Jl-S. 



8. P. diversifolius Raf. Stem very slender, much branched; floating leaf- 

 blades coriaceous, oval or elliptic and obtuse, or lanceolate-oblong and acute, 

 1-2.5 cm. long, 4.5-12 mm. wide; submerged leaves linear-setaceous, 2-4 cm. 

 long, 1 mm. wide or less; stipules of the floating leaves free, those of the submerged 



