NAIADACE^E. (PONDWEED FAMILY.) 485 



npper stipuh s very long, acute ; peduncle about the thickness of the stem ; spikes 1' -2* 

 long ; sides of the turgid nutlet with a small deep impression in the middle; embryo 

 coiled into an incomplete elliptical ring. — Var. pbolixtjs, Koch. More slen- 

 der; upper leaves lanceolate, frequently submersed, acute; stem (6° -12°), 

 submersed leaves (sometimes 21') and stipules very long. — Common in ponds 

 and ditches : the var. in deep flowing water. Aug. (Eu.) 



2. P. Oakesi&lius, Robbins, n. sp. Stem more slender, much branched; 

 floating leaves smaller (1'- 1^' long), ovate- or oblong-elliptical, obtuse, fewer- 

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

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

 duncles much thicker than the stem ; fruit smaller and more acute ; sides of the tur- 

 gid 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, not rare in E. Massachusetts. Aug. 



3. P. Clayt6nii, Tuckerman. Stems compressed, often simple from the 

 creeping rootstocks ; floating leaves chiefly opposite (1'- l£' 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 approximate, 

 2-ranked, linear (2' -5' long, and l"-2.i" wide), 5-nerved, the lateral nerves 

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

 reticulated; stipules very obtuse; spikes numerous, about the length of the thick- 

 ened peduncle ; fruit round-obovate, flattish, 3-keeled when dry ; nutlet distinctly 

 impressed on the sides ; curvature of the embryo transversely oval. (P. Nattallii, 

 Chamisso?) — Still or flowing water, and even in small streams: common. — 

 Perfectly distinct, and peculiar in the reticulation of the centre of the delicate 

 grass-like submersed leaves. July, Aug. 



-t- ■*- Characters of the preceding section, but all the parts small, slender and delicate 

 as in the next, or as in No. 20 (and like it propagated by autumn buds), but with 

 coriaceous floating leaves. 



4. P. Vaseyi, Robbins, n. sp. Very delicate ; stem almost capillary ; 

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

 petioles, w r ith 5 nerves deeply impressed beneath, cross-veins distinct ; sub- 

 mersed leaves filiform-linear, very attenuate (l'-2' long, J"-£" wide) and 

 acute ; stipules not adnatc, scarious ; spikes all emersed and similar, few, inter- 

 rupted-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 incurved, its apex transverse to the fruit. — Illinois, near Ring wood, 

 McHenry Co., Dr. G. Vasey. Apparently also in Quinsigamond Lake, Mass. 

 -t- -t- -t- Stems slender or filiform and much branched: floating haves sometimes 



wanting : stipules adnate to the base of the leaf: spikes of two kinds ; one emersed, 

 cylindrical and many-flowered, on a club-shaped peduncle ; the other submersed, 

 globular and few-flowered : fruit flat, cochleate, with thin or scarcely any flesh, 

 and a thin nutlet : embryo spiral. 



5. P. Spirillus, Tuckerman. Floating leaves varying from oval to 

 lance-oblong and lanceolate (the largest 10" long and 4" wide), usually obtuse, 



