4$4 NAIAD ACE^E. (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. pectilliitllS, L. Stems thread-like, many times forked; leaves 

 bristle-form, \-nervcd (2' -4' long); spikes interrupted, long-pcdunclcd ; nutlets 

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

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



2. P. Robin aisii, 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. Rabbins. White Plains, New 

 York, 77. .7. 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. 



$ 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. liybridlBS, Michx. Slender (6'- 12' long), branching; immersed 

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

 or lanceolate to oval (j'-l' 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. 

 divcrsifolius, Barton. P. setaceus, Pursh. P. Spirillus, Tiickerman: a slender 

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

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



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

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

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



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

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

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

 flowered, long-pcduncled ; fruit thick, obscurely 3-carimite when dry, the narrow 

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

 4 'ham., which is monogynous, and is rough with small tubercles on the obtusely 

 crested kcd, &<.) Clear ponds, White Mountains, New Hampshire, Oakes fr 

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



5. P. pus'.Slais, L. Stem slender, obscurely compressed ; leaves neamwfy 

 linear, ratln.r acute, 3-5-nen:ed; spikes 4-8-flowered, lax, often interrupted, long- 

 jiedniicli'd : fruit cnstliss. (P. compressus, Smith.) Ponds and clear pools; 

 rather common northward. (Eu.) 



6. P. lait< JfJoi us, Pursh. Stem very slender and thread-like, but flav 

 tish ; litres nitri-oirti/ limar, aciitish, 3-nerved ; spikes feto- (4-6-) flowered, short- 



i nit didinetlij crested or sinuate-toothed on the back. (P. gram metis, 

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

 lung, J"-l" wide. 



