72 NAJABACEAE (L'0^D\VEEL> FAMILY) 



m. Plant without winter-buds. 

 Leaves bi-friandular at base. 



Stipules 1-2 cm. long, persistent 29. P. rntilun. 



Stipules less than 1 cui. long, scarcely persistent . 25. P. pusillus. 

 Leaves glandless at base. 

 Spikes short-peduncled, axillary; leaves broader than 



the diameter of the steins 30. I\ folionuH. 



Spikes long-ped uncled, terminal : leaves narrower than 



the diameter of the stems 31. P. confervoides. 



r. Stipules united with the sheathing base of the leaf; spikes inter- 

 rupted z. 

 z. Leaves at most 3 mm. wide, entire. 

 Stigma broad and depre.ssed, sessile. 

 Stigma nearly central, the ventral face of the fruit curved ; 



leaves filiform, taper-pointed 34. P.filiformis. 



Stigma neai-ly in line with the straightish ventral face of 

 the fruit ; leaves narrowly linear, with blunt or rounded 



tips 35. /*. interior. 



Stigma capitate, tipping the definite style. 



Fruit not keeled 36. 7*. pectinafit.r. 



Fruit prominently keeled SI. P. ititerrupfus. 



a. Leaves 4^8 mm. wide, ciliate-serrulate 38. P. Robbinnii. 



1. P. natans L. Stpm simple or sparingly branched; Jloatinr/ leaves 2.5-10 

 cm. long, elliptical or 'ovate, somewhat cordate at base, obtuse but witli a blunt 

 point, 21-29-nerved, flt^xible at base, as if jointed to the petiole ; upper sub- 

 mersed leaves lanceolate, early perishing, the lower (later in the season) very 

 slender (7-18 cm. long, barely 2 mm. wide) ; upper stipules very long, aentp ; 

 peduncle about the thickness of the stem ; spikes 3-6 cm. long; fruit obliquely 

 obovoid ; sides of the turgid seed with a small deep impression in the middle; 

 embryo coiled into an incomplete elliptical ring. — Ponds and quiet streams, 

 common. July-Sept. (Widely distr. in temp, and subtrop. regions.) 



2. P. Oakesianus Robbins. Stem more slender, much branched; floating 

 leaves smaller (2-5 cm. long), ovate- or oblong-elliptical, obtuse, fewer (17-23)- 

 nerved ; lowest submersed ones almost capillary (barely 1 mm. wide), continu- 

 ing through the flowering season ; spikes shorter (1.5-3 cm. long), on peduncles 

 much thicker than stem; fruit smaller and more acute ; sides of the seed not at 

 all impressed ; curvature of the embryo nearly circular, its apex directed to a 

 point above its base. — Ponds, and especially pools and quiet streams, local, 

 Anticosti to n. N. Y. and N J. July-Sept. 



3. P. polygonif51ius Pourret. Stem slender, freely creeping, and sending up 

 short leafy branches; floating leaves elliptic-lanceolate to cordate-ovate, rather 

 thin, 2.5-9 cm. long, 1-4 cm. broad, 11-33-nerved, not apparently jointed to the 

 petioles; submersed leaves (when present) lanceolate, short, mostly exceeding 

 the petioles ; stipules blunt, 2-4 cm. long ; spikes 2-4 cm. long, very slender ; 

 fruit plump, 3-keeled, 1.5-2 mm. long. — Shallow pools. Sable I., N. S. and Nfd. 

 Aug. (Greenl., Kurasin, Afr., Au.str.) 



4. p. epihydrus Kaf. Stems compressed^ often simple from the creeping 

 rootstocks ; floating leaves chiefly opposite (3-7.5 cm. long, 1-2.5 cm. broad), 

 \\-21 -nerved, oblong, tapering into a short petiole, the lower gradually narrow- 

 ing and passing into the submersed ones, which are very numerous and apjiroxi- 

 mate, conspicuously 2-ranked (5-13 cm. long, 2-(5 mm. wide), b-1-nerved, the 

 lateral nerves slender and nearly marginal, the space within the inner nerves 

 coarsely rellular-rcticnlatcd ; stipules very obtuse; spikes numerous, about the 

 length of the thickened peduncle ; fruit round-obovoid, flattish, 3-keeled when 

 di'y, 2.5-3.5 mm. long ; seed distinctly impressed on the sides ; curvature of the 

 embryo transversely oval. (P. pensylvanicus Willd. ; P. Nuttallii C. & S ) 

 — Still or flowing water. July-Sept. 



Var, cayugensis (Wiegand) Benn. Stouter ; j^oa^mf/ leaves 5-8 cm. long, 

 2-3.5 cm. wide, 29-4\ -nerved ; submersed ones less distichous, 1.2-2.2 dm. 

 long, 0.5-1 cm. wide, ^^-I'^^-Jierved ; fruit 3.5-4.5 mm. long. — N. B. and Que. to 

 Wash., s. tocenrr N. Y., Mich., and la. (Japan.) 



5. P. alpinus Ball)is. Steins mostly simple ; floating leaves (often wanting) 

 3. .5-8 cm. lonir, rather thin, inedge-obl anceolate, narrowed info a short petiole, 

 1 1-21-nerved ; submersed leaves almost sessile, lanceolate and lance-oblong. 



