434 NAIADACEjE. (PONDWEED FAMILY.) 



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. 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. Ripe fruit not seen. 



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 someichat lateral : fruit 

 and seed cochleate. 



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

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

 or lanceolate to oval ('-!' long), 3-7-nervcd, short-petioled, rarely wanting; 

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

 cles; fruit small ("- 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/Jwcfema?*: a slender 

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

 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. Tiickcrm&iii, Robbins, 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, OaJces $ 

 Rcbbins. Tewksbury, Mass., and in the Alleghany Mountains, Tuckermc. . 



5. P. pusillus, L. Stem slender, obscurely compressed ; leaves nzrroicty 

 linear, rather acute, 3-5-nerved; spikes t-&-flowered, lax, often interrupted, long- 

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

 rather common northward. (Eu.) 



6. P. paucifldrilS, Pursh. Stem very slender and thread-like, but flair 

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

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

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

 long, $"-1" wide. 



