72 NAJADACEAE (PONDWEED FAMILY) 



flj. Plant without winter-buds. 

 Leaves bi-glandular at base. 

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



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

 Leaves glandless at base. 

 Spikes short-peduncled, axillary ; leaves broader than 



the diameter of the stems ..... 30. P. follows. 



Spikes long-peduncled, terminal ; leaves narrower than 



the diameter of the stems ..... 81. P. confervoides. 



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



rupted 8. 



0. 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.filiformis. 



Stigma nearly in line with the straightish ventral face of 

 the fruit ; leaves narrowly linear, with blunt or rounded 

 tips ........... 35. P. interior. 



Stigina capitate, tipping the definite style. 

 Fruit not keeled ......... 36. P. 



Fruit prominently keeled ....... 37. P. interruptus. 



ves 4-8 mm. wide, ciliate-serrulate 



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



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 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. polygonif&lius 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., Eurasia, A fr., 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}, 5-1 -nerved, the 

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

 coarsely cellular-reticulated ; stipules very obtuse ; spikes numerous, about the 

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

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

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

 Still or flowing water. July-Sept. 



Var. cayug6nsis (Wiegand) Benn. Stouter ; floating leaves 5-8 cm. long, 

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

 long, 0.5-1 cm. wide, 9-T3-nerved ; fruit 3.5-4.5 mm. long. N. B. and Que. to 

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



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

 3.5-8 cm. long, rather thin, wedge-oblanceolate, narrowed into a short petiole, 

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



