NAIAD A.CEJE. (PONDWEED FAMILY.) 559 



embryo coiled into an incomplete elliptical ring. Ponds and ditches, N. Sco- 

 tia to Va., westward across the continent. In deeper or flowing water the 

 plant becomes more slender and often wholly submersed (var. PROL^XUS, 

 Koch). Aug., Sept. (Eu., Asia.) 



2. P. Oakesianus, Bobbins. Stem more slender, much branched ; float- 

 ing leaves smaller (1 - 1%' long), ovate- or oblong-elliptical, obtuse, fewer- 

 (17-23-) nerved; lowest submersed ones almost capillary (only J--J" wide), 

 continuing through the flowering season; spikes shorter (f-1' long), on 

 peduncles much thicker than the stem; fruit smaller and more acute; sides of 

 the turgid nutlet not at all impressed ; curvature of the embryo nearly circular, 

 its apex directed to a point above its base. Ponds, and especially pools and 

 stagnant ditches, Mass, to N. J. ; also Anticosti. Aug. 



3. P. Pennsylvanicus, Cham. Stems compressed, often simple from 

 the creeping rootstocks ; floating leaves chiefly opposite (1 - 3^' long), 11-17- 

 nerved, oblong, tapering into a short petiole, the lower gradually narrowing 

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

 mate, 2-ranked, linear (2 - 5' long, and 1 - 2-" wide), 5 - 7-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-obovate, flattish, 3-keeled when dry ; nutlet 

 distinctly impressed on the sides ; curvature of the embryo transversely oval. 

 (P. Claytonii, Tuckerm.) Still or flowing water, N. Brunswick to S. C., west 

 to N. Ind. and Minn. July, Aug. 



H- - Like the preceding section, but all the parts small, slender and delicate, 

 only the fertile plants producing floating leaves; spikes very small and few- 

 flowered ; propagated by autumn buds. 



4. P. Vas6yi, Bobbins. Very delicate; stem almost capillary ; floating 

 leaves obovate (3-5" long) and about the length of their filiform petioles, 

 with 5 nerves deeply impressed beneath, cross-veins distinct ; submersed leaves 

 filiform-linear, very attenuate (1-2' long, J-J" wide) and acute; stipules 

 not adnate, scarious, long, acute ; spikes all emersed, few, interrupted-oblong, 

 3-5-flowered, on a thickish peduncle; fruit oblique, round-obovate (" long), 

 compressed, slightly sharp-margined, tipped with a distinct recurved style, the 

 sides impressed and face acute ; upper portion of the embryo circularly in- 

 curved, its apex transverse to the fruit. Canada and N. Eng. ; also 111. The 

 fruiting form, with floating leaves, rare ; the submerged form apparently much 

 more abundant. 



5. P. lateralis, Morong. Stem filiform, branching; floating leaves ellip- 

 tical (4 -6" long by 2" wide), with 5-7 nerves deeply impressed beneath, 

 tapering at base into a somewhat dilated petiole shorter than the blade ; sub- 

 mersed leaves linear, acute (1 -3' long by \-\" wide), 1 -3-nerved, the mid- 

 nerve with fine veins or cellular reticulations on each side, bi-glandular at 

 base ; stipules short ; peduncles with a very peculiar lateral appearance, widely 

 spreading at maturity, sometimes even recurved, often thicker than the stem ; 

 spikes often interrupted (2- 4-flowe red) ; fruit obliquely obovate (hardly V 

 long), the back much curved, with two fine grooves upon it ; embryo oval in 

 its curve, the apex nearly touching the base. Mass and Mich. ; rare. Unde- 

 veloped specimens resemble P. pusillus. 



