434 NAIADACE^. (PONDWEED FAMILY.) 



§ 1. Stipules united with the sheathing base of tlie leaf, scarious: leaz^es all immeised 

 and similar, alternate, grass-like : stigma terminal: seed hooked-curved. 



1. P. pectinatUS, L. Stems thread-like, many times forked; leaves 

 hristle-form, 1-nerved (2' -4' long); spikes interrupted, long-pedunclud ; nutlets 

 rounded-obovate. — Brackish water along the coast (P. marinum, L.) ; also not 

 rare in fresh water, especially along the Great Lakes and northward. (Eu.) 



2. P. Bobttinsii, Oakes. Stem sparingly branched, rigid, very leafy ; 

 leaves linear, flat, abruptly pointed, mani/-nerved, serrufate-ciVjute, .approximate (3'- 

 4' long, 3" -4" wide), recui-ved-spreading ; spikes oblong. — Ponds, not uncom- 

 mon in New England, detected in 1829 by Dr. Robbins. White Plains, New 

 York, iT'. J". Clark. Ohio, Dr. Canfield. — A very remarkable species. Stems 

 l°-3° long, entirely invested by the sheathing bases of the leaves and the elon- 

 gated and taper-pointed free portion of the stipules. Ripe fruit not seen. 



§ 2. Stipules of the immersed {alternate) leaves adherent, as in § 1, rfio.se of the foaling 

 leaves free from the petiole or nearly so : stigma becoming sonieu-hai lateral : fruit 

 and seed cochleate. 



3. P. liybl-idus, Michx. Slender (6' -12' long), branching; immersed 

 leaves narrowly linear or almost capillary ; the floating ones varying from linear 

 or lanceolate to oval {^'-1' long), 3-7-nerved, short petioled, rarely wanting; 

 spikes capitate, few-flowered, lateral, on very short somewhat club-shaped pedun- 

 cles; fruit small (J"- 1" long), orbiculate, flattened on the sides, keeled on the 

 back, the keel more or less toothed or crested; embryo spirally coiled. (P. 

 diversifolius, Barton. P. setaceus, Pursh. P. Spirillus, Tuckei-man : a slender 

 form.) — Shallow pools ; common, especially southward. — Var. spicItus, 

 Engelm., is a fonn with longer spikes (i'-^' long), W. Illinois and southward. 



§ 3. Stipules all entirely free from the petiole or leaf: leaves alternate: stigma termi- 

 nal : seed hooked-curved or nearly forming a ring. 

 * Leaves grassy-linear or thread-shaped, sessile, all immersed : stems branching. 



4. P. Ttirkemidni, Bobbins, in herb. Slender and very delicate; 

 stem terete, much branched ; leaves setaceous or capillary, tapering to a sharp 

 point, nearly terete, nerveless, pellucid (confei-va-like, about 2' long) ; spike few- 

 flowered, long-peduncled ; fruit thick, obscurely S-carinate when dry, the narrow 

 dorsal keel smooth and ei'en; style obsolete. (P. trichoides, ed. 1, &c., not of 

 Cham., which is monogynous, and is rough with small tubercles on the obtusely 

 crested keel, &c.) — Clear ponds, White Mountains, New Hampshire, Oakes Sf 

 Robbins. Tewksbury, Mass., and in the Alleghany Mountains, Tiickerman. 



5. P. pusillus, L. Stem slender, obscurely compressed ; leai'es narrowly 

 linear, rather acute, 3 - b^erved ; spikes 4 - 6-flowered, lax, often inteiTupted, long- 

 peduncled: fruit crestless. (P. compressus. Smith.) — Ponds and clear pools; 

 rather common northward. (Eu.) 



6. P. paucifldrus, Pursh. Stem very slender and thread-like, but flat- 

 tish ; leaves narrowly linear, acutish, 3-nerved ; spikes few- (4-6-) fiowei-ed, short- 

 peduncled; fruit distinctly crested or sinuate-toothed on the back. (P. gram incus, 

 Michx. ) — Ponds and streams ; common, especially southward. — Leaves 1 ' - 3' 

 long, J"-l" wide. 



