NAIADACE.E. (POXDWEED FAMILY.) 471 



3. P. pusillus, L. Stem filiform, slightly compressed ; leaves narrowly 

 linear, acute or acuminate, biglaudular at the base ; stipules free ; peduncles 

 half as h)ng as the leaves, 4- 8-flowered; achenia obliquely obovate, turgid, 

 rounded on the back; embryo hooked at the apex. — Ponds, North Carolina, 

 and northward. 



■*- •*- Leaves lanceolate or cordate: stipides free, sheathing. 



4. P. perf oliatus, L. Stem terete, branching, very leafy ; leaves ovate, 

 cordate, clas])ing, obtuse, many-nerved, those at the brandies and ])eduncles 

 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 brackisii water, West Florida, 

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



5. P. lucens, L. Stem branching ; leaves short-jjetioled, oblong-lanceo- 

 late, niucrouate-acute ; stipules long, connate ; peduncles stout, longer than 

 the leaves ; achenia roundish, compressed, slightly 3-keeled. — Lakes and 

 ponds, Florida (?), and northward. 



6. P. Zizii, Mert. & Koch. 1 Stems sparingly branched ; leaves lanceo- 

 late, acute, contracted and sessile at the base, pellucid, 5 - 9-nerved, wavy on 

 the margins; stipules (white) connate, rounded on the back; spikes cylindri- 

 cal, mauy-tlowered, on stout peduncles shorter than the leaves ; achenium 

 (immature) oval, compressed, rounded on the back, short-pointed. — Fresh 

 water, Apalachicola, Florida. August. — Stems 2° -3° long. Leaves 2' -3' 

 long. 



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

 long-petioled and somewhat coriaceous. 



7. P. amplifolius,. Tuck. Stem simple, floating ; leaves (mostly absent) 

 large, oblong or ovaldaueeolate, acutish, long-petioled; the submerged ones 

 lanceolate, undulate ; stijjules very long, pointed ; peduncles stout, fruit 

 obliquely obovate, bluntly keeled. — Ponds on the mountains of Georgia, and 

 northward. 



8. P. Pennsylvanicus, Cham. Stem slender, branching; floating 

 leaves oppo.site, thin, elliptical or oblong-linear, on filiform ])etioles; immersed 

 leaves long, sessile, linear or lanceolate ; stipules coiniate, 2-ribbed ; peduncles 

 thickened upward; spikes narrowly cylindrical; achenium smooth, slightly 

 keeled on the back. — Shallow ponds, North Carolina, and northward. July. 

 — Floating leaves l'-2' long. Immersed leaves 4' -6' long. 



0. P. hybridus, Michx. Small; stems very slender, branched ; float- 

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

 strongly impressed-nerved, longer than the filiform petioles; immersed leaves 

 filiform, scattered ; spikes oval or oblong, short-peduncled ; achenium nearly 

 circular, concave on the sides, rugose or tuberculate, and 1 -3-ridged on the 

 back ; embryo coiled. — Shallow ponds. June - August. — Floating leaves 

 6" - 8" long, commonly 5-nerved. 



