484 XAIADACKJE. (PONDWEED FAMILY.) 



1. Stipules united with the sheathing base of the leaf, scarions : lemn all linineiied 

 and similar, alternate, grass-like: stigma terminal: seed //cx,L\ d-an n<l. 



1. P. pectmatus, L. Stems thread-like, many times forked; leans 

 brittle-form, \-ncrvcd (2' -4' long); spikes interrupted, long-pcdunclul ; nutlets 

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

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



2. P. Robtlinsii, Oakes. Stem sparingly branched, rigid, very leafy , 

 leaves linear, fiat, abruptly pointed, many-nerved, semdate-ciliate, approximate (3'- 

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

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

 York, H. J. Clark. Ohio, Dr. Canjield. A very remarkable species. Steins 

 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 somewhat 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 ("-" 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. SPICA.TUS, 

 Engehn., is a form with longer spikes (;}'-' long), W. Illinois and southward. 



$3. Stipules all entirely free from the petiole or leaf: haves alternate: stigma tertni- 



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

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



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

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

 point, nearly terete, nerveltss, 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. pllSillllS, L. Stem slender, obscurely compressed ; leaves narroirly 

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

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

 rather common northward. (Eu.) 



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

 tish ; leaves narrowly linear, acutish, 3-nervcd ; spikes few- (4-0-) flowered, short' 

 pedunclfd; fruit distinctly crested or sinuate-toothed on the back. (P. gramincus, 

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

 long, $"-1" wide. 



