446 NAIADACE^E. (PONDWEED FAMILY.) 



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

 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 1' - 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-peduncled, 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. Leaves 

 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 terminal, oblong, densely many-flowered, on stout 

 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. ? 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 ttvo 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 sliort- 

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

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

 1 - 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 



