560 NAIADACE.E. (PONDWEED FAMILY.) 



^ ^_ ^_ Stems slender or filiform, much branched; fioatincj leav:is 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 flat, 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 a lance-oblong or broad-linear proper 

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

 apex (|- 1^' long, j-f" wide) ; stipules early lacerate ; submersed flowers usu- 

 ally so'itary on very short erect peduncles ; fruit with the back either ivinged 

 and with 4-5 distinct teeth or ivingless 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. hybridus, Michx. Floating leaves oval to lance-oblong (the largest 

 10" long, 6" wide), often acute, longer than the flliform 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 -7'' long; submersed spikes 1-4-flowered, theh jjed uncles (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-flowered, on stout peduncles. 



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

 2-5' long, rather thin, wedge-oblanceolate, narrowed into a short petiole, W- 

 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, 17 -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. piilcher, Tuckerm. Stem simple, black-spotted ; leaves of three 

 kinds; floating ones becoming very large (4^ by 3-i'), 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 



