44C NAIADACE^. (PONDWEED FAMILY.) 



the branches ; stipules small, united with the long and sheathing base of tlie 

 leaves ; spikes slender, interrupted, on long filiform peduncles ; achenium obovate, 

 smooth, slightly compressed, keeled on the back. — Fresh or brackish water, 

 West Florida, and northward. June - Aug. — Stems 2° - 3° long. Leaves 3' - 

 4' long. Spikes l'-2' long. 



2. P. pauciflorus, Pursh. Stem very slender, flattened, sparingly branched; 

 leaves scattered, thin, 3-nerved, sessile; stipules free from the leaves, connate, 

 sheathing ; spikes short-peduneled, 4 - 6-flowered, globose in fruit ; achenium 

 round-obovate, short-pointed, keeled and sinuate-toothed on the back. — Shallow 

 ponds, Georgia, and northward. July and Aug. — Stem 1°- 2° long. Leavus 

 1'- 2' long. Peduncles ^'- 1' long. 



^- •<— Leaves lanceolate or cordate ; stipules free^ sheathing. 



3. P. perfoliatus, L. Stem terete, branching, very leafy ; leaves ovate, 

 cordate, clasping, obtuse, many-nerved, those at the branches and peduncles 

 opposite ; spikes lateral and tenninal, oblong, densely many-flowered, on stent 

 peduncles 2-3 times as long as the leaves ; achenium obliquely obovate, rounded 

 on the back, short-pointed. — Fresh or brackish water. West Florida, and north- 

 ward. July - Sept. — Stems 1° - 2° long. Leaves 6" - 8" long. 



4. P. lucens, L. 7 Stems sparingly branched ; leaves lanceolate, acute, 

 contracted and sessile at the base, pellucid, 5 - 9-nerved, wavy on the margins ; 

 stipules (white) connate, rounded on the back ; spikes cylindrical, many-flow- 

 ered, on stout peduncles shorter than the leaves; achenium (immature) oval, 

 compressed, rounded on the back, short-pointed. — Fresh water, Apalachicola, 

 Florida. Aug. — Stems 2°-3° long. Leaves 2'-3' long, equalling the spikes. 



* * Leaves of iivo forms ; the immersed ones thin and pellucid, the floating ones long- 

 petioled and somewhat coriaceous. 



5. P. fluitans, Koth. Stem simple ; leaves many-nerved ; the floating 

 ones varying from oblong-lanceolate to ovate, acute at each end, or obtuse or 

 cordate at the base ; the others large, oblong, gradually or abruptly short- 

 potioled, undulate ; stipules connate and keeled on the back ; peduncles stout, 

 thickened upward ; spikes long, cylindrical, dense-flowered ; achenium smooth, 

 ] - 3-keeled on the back. — Fresh-water ponds and streams, Florida, and north- 

 ward. June - Aug. — Leaves 2' - 9' long. 



P. NATANS, L. probably occurs within our limits, but I have not seen 

 specimens. It may be known by longer-petioled (4'- 12') leaves, more slender 

 peduncles, and rounded stipules and achenia. 



6. P. heterophyllus, Schreber. Stem slender, branching ; floating leaves 

 small, thin, elliptical or oblong-linear, on filiform petioles ; immersed leaves long, 

 sessile, linear or lanceolate ; stipules connate, 2-ribbed; peduncles thickened up- 

 ward ; spikes narrowly cylindrical ; achenium smooth, slightly keeled on the back. 

 — Shallow ponds. North Carolina, and northward. July. — Floating leaves 1' - 

 2' long. Immersed leaves 4' - 6' long. 



7. P. hybridUS, Michx. Small; stems very slender, branched; floating 

 leaves lanceolate or elliptical, commonly acute at each end, shining and strongly 



