Potamogeton. NAIADACE.E. 195 



Stem stout, flexuous above : leaves lanceolate, obtuse, cucullate at the 

 apex, rounded and sessile at base : fruit large, semicircular, 

 beaked. 8. P. PR^LONGUS. 



Stem slender, straight : leaves lanceolate, acuminate, clasping : sti- 

 pules small : fruit nearly orbicular. 9. P. PERFOLIATUS. 

 Leaves all submerged, sessile, narrowly linear or setaceous. 

 Stipules free from the narrow base of the leaf. 

 Spike linear-cylindric. 



Stem flattened : leaves grass-like, 3-nerved, clasping : stipules 



conspicuous : peduncle longer than the spike : fruit large. 10. P. ZOSTER^FOLIUS. 

 Spike capitate (or interrupted in n. 13) : stems very slender. 



Leaves very narrow, 1 or 2 inches long, 1 - 3-uerved, narrowed at 



base : peduncles very short : fruit small, undulate-carinate. 11. P. PAUCIFLORUS. 

 Leaves larger, 2 or 3 inches long by a line wide or less, 3-5- 



nerved. 12. P. NIAGARENSIS. 



Leaves narrowly linear, 1 or 2 inches long, sessile, 1 - 5-nerved : 



peduncles long, slender : fruit more obscurely keeled. 13. P. PUSILLUS. 



Stipules united with the sheathing base of the leaf : spikes interrupted. 

 Stem filiform : leaves very narrowly linear or setaceous, 2 to 6 



inches long, 1- 5-nerved : peduncles long : fruit large, acute. 14. P. PECTIN ATUS. 

 Similar : leaves setaceous : peduncles usually short : fruit small, 



tipped with the broad stigma. 15. P. MARINITS. 



Stem stouter : leaves many, distichous, linear-lanceolate, shorter, 

 many-nerved : peduncles short, stout : fruit large, keeled, 

 acutely beaked. 16. P. RoBBiNSll. 



* Floating leaves more or less coriaceous, with a dilated petioled blade (the upper- 

 most approximate and often opposite}, different in form from the thinner sub- 

 merged ones ; stipules free : spikes cylindrical, mostly dense, not interrupted. 



t- The submerged leaves reduced to narrowly grass-like or filiform sessile 



phyllodia. 



1. P. natans, Linn. Stem rather stout, simple or sparingly branched : floating 

 leaves thick, ovate-elliptic to lanceolate, acutish, slightly cordate at base, 21-29- 

 nerved, usually 2 or 3 inches long, mostly shorter than the petiole ; stipules long 

 and conspicuous, acute or acuminate ; upper submerged leaves with a small lanceo- 

 late blade, the lower (only formed early or late in the season) reduced to very nar- 

 row thickish elongated phyllodia : peduncle stout, bearing an emersed spike 1 J or 

 2 inches long : fruit turgid, obliquely obovate, acute, 2 lines long : nutlet with a 

 small deep pit on each side: embryo nearly circular/ Reichenb. Icon. Fl. Germ, 

 vii. 26, t. 50. 



In ponds and ditches ; Plumas County (Mrs. Austin) and northward to Washington Territory 

 (Lyall, Hall, Howell) and Sitka, in Northern Nevada and Utah ( Watson), and common eastward ; 

 also European and Asiatic. Fruit maturing in August and September ; in deeper or flowing 

 water the plant becomes more slender and often wholly submerged. 



2. P. Claytoni, Tuckerman. Stem compressed, from creeping rootstocks : float- 

 ing leaves narrowly oblong to elliptic, 11 17-nerved, obtuse or acutish, attenuate 

 below into a flattened petiole usually shorter than the (1 to 2| inches long) blade ; 

 stipules sheathing, soon deciduous, an inch long or less ; submerged leaves very thin, 

 linear (2 to 5 inches long by 1 to 2| lines wide), 5-nerved, with a close cellular re- 

 ticulation between the middle nerves : spikes | to 1 inch long, on short stout 

 peduncles: fruit obovate, 3-keeled, slightly apiculate, 1J lines long or less : nutlet 

 slightly depressed on the sides : embryo spirally incurved, forming about l coils. 



Amer. Journ. Sci. xlv. 38, and 2 ser. vi. 227. 



In still or flowing water ; Yosemite Valley (Bolander) ; Columbia Valley (Hall, Howell) ; com- 

 mon in the Atlantic States. 



t- -H- Submerged leaves lanceolate, rarely oval or linear. 



3. P. rufescens, Schracl. Floating leaves (often wanting) rather thin, 11 - 17- 

 nerved, narrowly oblong-elliptic or oblanceolate, 2 to 4 inches long, acutish, attenu- 



