434 NAIADACE.fi. (PONDWEED FAMILY.) 



$ 1. Stipules united with the sheathing base of the leaf, scarious: leaiu all imrnened 

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



1. P. pectiliatus, 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. Robbinsii, Oakes. Stem sparingly branched, rigid, very leafy , 

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

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

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

 York, H. 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. Kipe fruit not seen. 



4 2. Stipules of the immersed (alternate) leaves adherent, as in 1, those of the floating; 

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

 and seed cochleate. 



3. P. liybridiis, 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 (^"-f" 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, Tuckerman : a slender 

 form.) Shallow pools ; common, especially southward. Var. spicAxus, 

 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 immersed: stems branching. 



4. P. Ttickerimtiii, 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 fr 

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



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

 linear, rather acute, 3 - 5-nerved ; spikes 4 - ^flowered, lax, often interrupted, long- 

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

 rather common northward. (Eu.) 



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

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

 pedunded; fruit distinctly crested or sinuate-toothed on the back. (P. gramineus, 

 Michx.) Ponds and streams; common, especially sou, h ward. -Leaves l'-3' 

 long, "-1" wide. 



