NAIADACE.E. (PONDWEED FAMILY.) 561 



near the base of the stem) thicker, plane, oral or oblony with a rounded base, 

 or spatulate -oblong, on longer petioles; stipules rather short and obtuse; pe- 

 duncles thicker than the stem ; fruit with a rounded back and angular face, 

 pointed, distinctly 3-keeled when fresh, sharply so when dry ; nutlet with two 

 deep dorsal furrows, and a sinus below the angle in front ; sides flat ; embryo 

 circularly much incurved above. Ponds, Vt. to Ga. and Mo. July, Aug. 



11. P. amplifolius, Tuckerm. Stems simple, of very variable length ; 

 Jlnutinij leaves (sometimes wanting) large, oblony or lance-ovate, sometimes 

 slightly cordate at base, abruptly acutish, 30 - 50-uerved, on rather long peti- 

 oles ; submersed leaves often very large (reaching 7' by 2'), lanceolate or oval, 

 acute at each end, usually much recurved, undulate, mostly on short petioles ; 

 stipules very long and tapering to a point, soon becoming loose; peduncles 

 thickened upward, in deep water much elongated; fruit very large (over 2" 

 long), rather obliquely obovate, 3-keeled, with a broad stout beak; nutlet 

 slightly impressed on the sides ; upper part of the embryo curved into a ring. 

 Ponds and rivers, X. Eng. to N. J., west to Minn, and Kan. Aug., Sept. 



12. P. Illinoensis, Moroug. Stem stout, branching towards the sum- 

 mit ; floating leaves opposite, oval or ovate (2-5' long by l|-2' broad), 19 - 

 25-nerved, rounded or subcordate at base, with a short blunt point at apex, on 

 short petioles ; submersed leaves rather few, oblong-elliptical, acute at each end, 

 usually ample (largest 8' by H') ; stipules coarse, obtuse, stronr/ly bicarinate 

 (2' long) ; peduncles often clustered at the summit (2-4' long), thickening 

 upward ; fruit round ish-obovate (If -2" long), 3-keeled on the back, middle 

 keel prominent ; nutlet flattened and slightly impressed on the sides, obtuse 

 or pointed at base; apex of embryo directed transversely inward. Streams 

 and ditches, western N. Y. to 111., Iowa, and Minn. Very near the last. 



13. P. heterophyilus, Schreb. iStem slender, eery branching below; 

 floating leaves mostly thin, variable, but with a short blunt point, - 15-nerved, 

 usuallv 1-2' long and G - 9" wide ; submersed ones usually lanceolate or linear- 

 lanceolate, acuminate or cuspidate, narrowed toward the base, about 7-nerved 

 on the stem and 3-uerved on the branches ; upper ones petioled, lower sessile ; 

 stipules obtuse, loose ; peduncles somewhat thickened upward ; fruit small, 

 roundish, compressed, scarcely keeled ; embryo annular above. (P. gramineus, 

 Fries.) Still or flowing water, common. Varies exceedingly in its sub- 

 mersed leaves, peduncles, etc.; the var. GRAMINIFOLIUS (Fries), growing in 

 rapid streams, with stems much elongated and less branched, and the flaccid 

 submersed leaves 2-7' long by 2- 10" wide. 



Var. ( ? ) myriophyllus, Bobbins. Sending up from running rootstocks 

 many short repeatedly dichotomous and densely leafy stems ; fertile stems 

 very slender; floating leaves small, delicate, lance-oblong, on long filiform 

 petioles ; submersed stem-leaves larger, early perishing ; those of the branches 

 (deep-green) linear-oblauceolate, very small (f-1' long), acute, sometimes 

 minutely serrulate; spike slender, loosely-flowered, much shorter than the 

 thickened peduncle. Apponaug Pond, R. I., without fruit. 



13 a . P. Zizii, Mert. & Koch. Resembling P. luceus, but smaller, much 

 branched at base ; upper leaves coriaceous or subcoriaceous, long-petioled and 

 sometimes emersed, the others subsessile, all usually numerous, undulate and 

 shining; peduncle elongated. (P. lucens, var. minor, Nolle. Also P. gramin- 

 eus, var. (?) spathulajformis, Robbins ; P. spathaeformis, Tuckerm. ; "P. vari- 



36 



