72 NAJADACEAE (PCXNDWEED FAMILY) 



a?. Plant without winter-buds. 

 Leaves bi-glandular at base. 



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



Stipules less than 1 cm. long, scarcely persistent . 2f>. /'. 

 Leaves glandless at base. 

 Spikes short-peduncled, axillary; leaves broader than 



the diameter of the stems . . . . .30. 

 Spikes long-peduncled, 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 depressed, sessile. 

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



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



Stigma nearly in line with the straightish ventral face of 

 the fruit ; leaves narrowly linear, with blunt or rounded 



tips 35. P. interior. 



Stigma capitate, tipping the definite style. 



Fruit not keeled 36. P. pectinulit*. 



Fruit prominently keeled 37. P. interrupts. 



z. Leaves 4-8 mm. wide, ciliate-serrulate 38. P. Robbiiixii. 



1. P. natans L. Stem simple or sparingly branched; floating leaves 2.5-10 

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

 point, 21-29-nerved, flexible 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, acute; 

 peduncle about the thickness of the stemj 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 Bobbins. 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. polygonifblius 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 L, N. S. and Nfd. 

 'Aug. (Greenl., Eurasia, Afr., Austr.) 



4. P. epihydrus Raf. Stems compressed, often simple from the creeping 

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

 11-27 -nerved, oblong, tapering into a short petiole, the lower gradually narrow- 

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

 mate, conspicuously 2-ranked (5-13 cm. long, 2-6 mm. wide}, 5-7 -nerved, the 

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

 coarsely cellular-reticulated; stipules vcnj nbtusc ; spikes numerous, about the 

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

 dry, 2.5-3.5 mm. long ; seed di^iitn-thj impressed mi (lie sides ; curvature of the 

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

 Still or flowing water. July-Sept. 



Var. cayuge"nsis (Wiegand) Benn. Stouter ; floating leaves 5-8 cm. lone:, 

 2-3.5 cm. wide, '2V-4l-nerved ; submersed ones less distichous, 1.2-2.2 dm. 

 Ion.;, o.f>-l cm. wide, H-lX-nerved ; fruit 3.5-4.5 mm. long. N. B. and Que. to 

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



5. P. alpinus Balbis. Stems mostly simple ; floating leaves (often wanting) 

 3.5-8 cm. long, rather thin. vecfye-O&faJM/eoiate, iirr<>n-<'<1 fn/ slmrt />////<-, 

 11-21-nerved ; submersed leaves almost sessile, lanceolate anil lance-oblong, 



