434 NAIADACE^E. (PONDWEED FAMILY.) 



•v 



§ 1. Stipules united with the sheathing base of the leaf, scarious: leaves all immersed 

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



1. P. pC'Ct!SB;~BS55S, L. Stems thread-like, many times forked; leaves 

 bristle-form, l-nerved (2' -4' long); spikes interrupted, long-peduncled; 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. KoSjfeiSlSai, Oakes. Stem sparingly branched, rigid, very leafy , 

 leaves linear, flat, abruptly pointed, many-nerved, scrrulate-ciliate, approximate (3'- 

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

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

 York, II. J. Clark. Ohio, Dr. Canfleld. — 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. Pipe fruit not seen. 



§ 2. Stipules of the immerseel (alternate) leaves adherent, as in § 1, those of (he floating 

 leaves free from the petiole or nearly so : stigma becoming somewhat lateral : fruit 

 and seed cochlt ale. 



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

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

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

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

 cles ; fruit small (V'-§" 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. sctaceus, Pursh. P. Spirillus, Tuckcrman: a slender 

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

 Engelm., is a form with longer spikes [$'-¥ 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 immerseel : stems branching. 



4. P. Ts8Cltei'l!M£t8li, Bobbins, in herb. Slender and very delicate ; 

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

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

 flowered, long-peduncled ; fruit thick, obscurely 3-carinate when dry, the narrow 

 dorsal keel smooth and even; 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 -j- 

 Eobbins. Tewksbury, Mass., and in the Alleghany Mountains, Tuckerman. 



5. P. ssiSSaSIus, L. Stem slender, obscurely compressed ; leaves narrowly 

 linear, rather acute, 3-5-nerved; spikes 4 - 8-flowcred, lax, often interrupted, long- 

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

 rather common northward. (Eu.) 



6. P. paiiciil6riis, Pursh. Stem very slender and thread-like, but fla*- 

 tisb; leaves narrowly linear, acutish, 3-nerved; spikes few- (4-6-) flowered, short- 

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

 Michx.) —Ponds and streams; common, especially southward. —Leaves t'-3' 

 long, |"-1" wide. 



