NAIADACE.E. (PONDWEED FAMILY.) 559 



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

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

 plant becomes more slender and often' wholly submersed (var. PROLI'XCS, 

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



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

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

 (17-23-) nerved; lowest submersed ones almost capillary (only -" 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 aud 

 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 (I -3 loug), 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-J" 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, Tucker m.) Still or flowing water, N. Brunswick to S. C., west 

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



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

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

 flowered ; propagated by autumn buds. 



4. P. Vaseyi, 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, &-" 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. Eug. ; 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 lini>nr, 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-flowered) ; fruit obliquely obovate (hardly 1" 

 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. 



