NAIADACIL*:. (PONDWKKI) FAMILY.) 559 



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

 tia to Ya.., westward across the continent. In deejjer or flowing water the 

 plant becomes more slender and often wholly submersed (var. ruoLfxrs, 

 Kocli). — Ani;., Sept. (Kn., Asia.) 



"2.. P. Oakesi^nus, I\<»bbins. stem more slender, /«(/(•// hraur/ied ; float- 

 ing leaves smaller (l-H' loiiS)» ovate- or oljlong-ellijjtical, obtuse, fewer- 

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

 continuing through the flowering season; spikes shorter (J-T long), on 

 jicilnnclfs muck thicker than the stem; fruit smaller and more acute; sides of 

 the tuajid nutlet not at all impressed ; curvature of the eml)ryo nearly circular, 

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

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



3. P. Pennsylv^nicUS, Cham. /Stems compressed, often simple from 

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

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

 and passing into tlie submersed ones, which are very numerous and appn^xi- 

 mate, 2-ranked, linear (2-5' long, and 1 -2V' wide), 5-7-uerved, the lateral 

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

 cellular-reticulated ; stijiules veri/ obtuse; S]jikes numerous, about the length of 

 tlie thickened peduncle; fruit round-obovate, flattisli, 3-keeled when dry ; nutlet 

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

 (P. Claytouii, Tnckerm.) — 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, 

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

 flowered : propagated bi/ autumn buds. 



4. P. Vaseyi, Robbins. Very delicate ; stem almost ca\)'\\\iiry; 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, ^-.i" 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 aj)parently much 

 mor*^ abundant. 



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

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

 tapering at base into a .«?omewhat dilated petiole shorter than the blade ; sub- 

 mersed leaves linear, acute (1 -3' long l)y |-^" wide), 1 -.3-nerved. the mid- 

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

 base ; stipules short ; peduncles with n verg peculiar lateral appearance , widelv 

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

 spikes often interrupted (2-4-flowered) ; fruit obliquely obovate (hardly \" 

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

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

 veloped specimens resemble P. j)usillus. 



