446 NAIADACE^E. (POXDWEED FAMILY.) 



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

 haves ; spikes slender, interrupted, on long filiform peduncles ; aehenium 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 \'-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-pedunclcd, 4 - G-flowered, globose in fruit; aehenium 

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

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

 1 ' - 2' long. Peduncles %' - 1 ' long. 



-t- -•- 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 terminal, oblong, densely many-flowered, on stout 

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

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

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



4. P. lueens, L. ? 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 ; aehenium (immature) oval, 

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

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



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

 petioled and somewhat coriaceous. 



5. P. fluitans, Roth. 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- 

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

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

 1 -3-keeled on the back.— Fresh-water ponds and streams, Florida, and north- 

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



P. \ a tans, 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. 



G. P. heterophyllus, Schrelier. Stem slender, branching ; floating leaves 

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

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

 ward ; spikes narrowly cylindrical ; aehenium smooth, slightly keeled on the hack. 

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



2' long. Immersed Leaves 4'-G' long. 



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

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



