560 NAIADACE^E. (PONDWEED FAMILY.) 



M- H- -t- Stems slender or filiform, mack branched; floating leaves sometimes 

 wanting ; stipules adnate to the base of the leaf; spikes of two kinds, one 

 emersed, cylindrical and many-flowered, on a club-shaped peduncle, the other 

 submersed, globular and few-flowered ; fruit fiat, cochleate, with thin or 

 scarcely any flesh and a thin nutlet ; embryo spiral. 



6. P. Spirillus, Tuckerm. Floating leaves oval to lance-oblong and 

 lanceolate (the largest 10" long, 4" wide), usually obtuse, about equalling the 

 rather dilated petioles, with 5 - many nerves beneath deeply impressed ; upper 

 submersed leaves either with or without ? lance-oblong or broad-linear proper 

 blade ; the numerous lower ones narrow-linear, tapering toward the obtuse 

 apex (f-l'long, J-" wide) ; stipules early lacerate ; submersed flowers usu- 

 ally so'itary on ven/ short erect peduncles ; fruit with the back either winged 

 and with 4-5 distinct teeth or wingless and entire; embryo coiled If turns. 

 Rivers, and even far up small streams, N. Eng. to Va., west to Mich, and 

 Mo. June - Aug. Stem less slender than in the next. 



7. P. hybridllS, Michx. Floating leaves oval to lance-oblong (the largest 

 10" long, 6" wide), often acute, longer than the filiform petioles, with about 

 5-7 nerves beneath deeply impressed ; submersed leaves very numerous, al- 

 most setaceous (1-3' long, very rarely \" wide); stipules obtuse ; emersed 

 spikes 4 -1" long; submersed spikes 1 -4-flowered, their peduncles (of their 

 own length) frequently recurved ; fruit minute, about 8-toothed on the. margin; 

 embryo coiled 1 turns. Shallow stagnant waters, N. Brunswick to Fla., west, 

 to Mich., Mo., and N. Mex. June -Aug. 



* * Submersed leaves lanceolate, rarely oval or linear, membranaceous ; spikes 

 dense, many-flotrered, on stout peduncles. 



8. P. rufescens, Schrad. Stem simple ; floating leaves (often wanting) 

 2-5' long, rather thin, u-edge-oblanceolate, narrowed into a short petiole, 11- 

 17-nerved ; submersed leaves almost sessile, lanceolate and lance-oblong, smooth 

 on the margin, fewer-nerved ; stipules broad, hyaline, obtuse, upper ones 

 acuminate; spike 1-2' long, often somewhat compound; fruit obovate, len- 

 ticular, pitted when immature, with an acute margin and pointed with the 

 rather long style; embryo incompletely annular. In streams or ponds, N- 

 Brunswick to N. J., west to Minn, and Tex. Aug., Sept. (Eu.) 



9. P. fluitans, Roth. Stem often branching below ; floating leaves thin- 

 nish, lance-oblong or long-elliptical, often acute, long-petioled, 1 7 - 23-nerved ; 

 submersed leaves very long (3 - 12', by 2 - 12" wide), lanceolate and lance-linear, 

 7- 15-nerved, coarsely reticulated; peduncles somewhat thickened upward; 

 fruit obliquely obovate, obscurely 3-keeled when fresh, and distinctly so when 

 dry, the middle one winged above and sometimes with 3-5 shallow indenta- 

 tions; the rounded slightly curved face surmounted by the short style; nutlet 

 with the sides scarcely impressed ; upper part of the embryo circularly in- 

 curved. (P. lonchites, Tuckerm.) In streams or rarely in ponds, N. Bruns- 

 wick to N. J., west to Minn, and Iowa. Aug., Sept. (Eu.) 



10. P. pulcher, Tuckerm. Stem simple, black-spotted; leaves of three 

 kinds; floating ones becoming very large (4 by 3^'), roundish-ovate and cor- 

 date or ovate-oblong, 25-37-nerved, all alternate; upper submersed ones (3-5) 

 usually lanceolate, acute at base and very long-acuminate, 10- 15-nerved, very 

 thin, cellular each side of the midrib, undulate, short-petioled ; lowest (2-^4 



