NAIADACE^E. (PONDWEED FAMILY.) 63 



a very broad short petiole; submerged leaves as large as the floating ones, sessile 

 or nearly so, narrowly oblong-lanceolate, obtuse or acute : spikes rather slender, 

 on stout often elongated peduncles . fruit round-obovate, acutely margined, beaked 

 by the rather long style: nutlet pitted on each side. In Colorado and Montana, 

 and common in the Atlantic States ; also collected sparingly in California. In 

 streams or ponds. 



3. P. lonchites, Tuckerman. Stem rather slender, branching: floating 

 leaves thickish, 1 1 to 23-nerved, long elliptical to oblong-lanceolate, acute or 

 acutish, rather abruptly narrowed into a petiole usually longer than the blade ; 

 submerged leaves thinner and longer, mostly linear-lanceolate, more attenuate at 

 base, the lower sessile : spikes on stout peduncles : fruit obliquely obovate, cari- 

 nate, acute: nutlet somewhat 3-keeled, the sides scarcely impressed. Am. Jour. 

 Sci. ii. vi. 226. From Mexico to the Atlantic States; also in the Pacific 

 States. Usually in streams. 



4. P. amplifolillS, Tuckerman. Stems often stout, simple : floating leaves 

 (sometimes wanting) 30 to 5Q-nerved, elliptic to oblong-lanceolate, acute, 

 mostly rounded or slightly cordate at base, on stout petioles about equalling the 

 b'ade; submerged leaves often very large, mostl y falcate and somewhat undulate, 

 acute, attenuate to a usually short petiole : spike thick, on a very stout peduncle : 

 fruit large, 3-keeled, with a broad stout beak : sides of the nutlet not pitted. 

 Am. Jour. Sci. loc. cit. 225. From New Mexico to the Atlantic States ; 

 also in California and Oregon. In ponds and streams. 



5. P. gramineus, L. Stems very slender, branching : floating leaves 

 rather thin, 9 to 15-nerved, small, oblong-elliptic, acutish, rounded or cuneate at 

 base, on slender petioles mostly equalling or exceeding the blade ; submerged leaves 

 linear-lanceolate, variable in length, more commonly short, acute or acuminate, 

 narrowed at base : spikes rather loose, on stout often elongated peduncles : fruit 

 round-obovate, acute, scarcely keeled. From the Yellowstone eastward; also in 

 Nevada and California. In still or flowing water. 



# * Leaves all submerged and uniform, thin and dilated (lanceolate to oval), nu- 

 merous, mostly sessile : spikes dense, on stout peduncles. 



6. P. lucens, L. Stem stout, branching : leaves usually large (2 to 6 inches 

 long), oblong-lanceolate or oblanceolate, abruptly acute or acuminate, often undu- 

 late-serrate, narrowed at base to a short petiole or subsessile; stipules large: 

 peduncles often elongated : fruit acute, slightly keeled. From New Mexico 

 to California ; also eastward to Florida and New England. 



7. P. perfoliatUS, L. Stem more slender, flexuous, branching: leaves 

 broadly cordate to cordate-lanceolate, \ to l inches long, obtuse or acute, clasp- 

 ing at base ; stipules small : spikes somewhat compound, on mostly short pe- 

 duncles : fruit obtusely keeled, beaked by the short slender style. 



Var. (?) lanceolatus, Robbins. Leaves longer (2 to 4 inches or more), 

 and more lanceolate, acuminate, undulate: peduncles thickened upward: fruit 

 nearly orbicular. Gray's Manual, 488. 



* * * Leaves all submerged and uniform, narrowly linear or setaceous, sessile. 

 H- Stipules free from the narrow base of the leaf. 



8. P. pusillus, L. Stem filiform : leaves 1 or 2 inches long, rarely a line 

 wide, often nearly setaceous, 1 to 5-nerved, biglandular at base : spikes capi- 

 tate, or elongated, or interrupted, on slender flattened peduncles. 



