beak linear, erect, to 1 mm. long; loop solid; apex of seed pointing a little above 

 the basal end. P. americanus Cham. & Schlecht. 



In streams and lakes throughout Okla. and Tex. to N. M. (Colfax, Sandoval 

 and San Miguel cos.) and Ariz. (Coconino, Gila, Maricopa, Navajo and Yavapai 

 COS.), mature fruits in late spring and summer; in much of the U. S., Can. and 

 n. Mex. 



13. Potamogeton Ulinoensis Morong. Fig. 43. 



Rhizome buff, spotted or suffused with red; stem simple or branched, terete, 1-5 

 mm. in diameter; stele with the prototype, triotype or oblong-type pattern; endo- 

 dermis of U-cells; interlacunar bundles in the outer interlacunar circle, sometimes 

 a few in the next to the outer circle; subepidermal bundles present or absent; 

 pseudohypodermis absent or of 1 cell thick; submersed leaves thin, elliptic to 

 lanceolate, often somewhat arcuate; blades to 2 dm. long and 45 mm. wide, sessile 

 or tapering into a petiole to 4 cm. long, acute and usually somewhat mucronate at 

 apex; nerves 7 to 19; lacunae of 2 to 5 rows along midrib and larger nerves; 

 margin entire or with fugacious 1 -celled translucent denticles; floating leaves 

 (often absent) more or less coriaceous, transition to submersed leaves usually 

 gradual; blades elliptic to ovate-elliptic or oblong-elliptic, to 19 cm. long and 65 

 mm. wide, obtuse-mucronate at apex, cuneate or rounded at base; petioles 2-9 cm. 

 long, shorter than the blade; nerves 13 to 29; lacunae of 2 or 3 rows of cells 

 along midrib, sometimes obscure; stipules persistent, divergent and conspicuous, 

 obtuse, those of the submersed leaves 1-8 cm. long and 3-12 mm. wide at base, 

 prominently 2-keeled, with 15 to 35 finer nerves; those of the floating leaves 

 broader; peduncles as thick as or thicker than the stem, 4-30 cm. long; spikes in 

 anthesis compact, of 8 to 15 whorls of flowers, at maturity cylindric and crowded, 

 2.5-7 cm. long, 8-10 mm. thick; flowers sessile or on pedicels to 0.5 mm. long; 

 perianth orbicular to oval, 1.3-3.2 mm. wide; anthers 0.6-2 mm. long; fruits 

 obovate to orbicular or ovate, 2.5-3.6 mm. long (excluding beak), 2.1-3 mm. 

 wide, the sides flat; keels prominent and acute, the dorsal strongly developed 

 above and below, the laterals less strongly developed but often each with a pro- 

 jecting knob at the base; beak facial, short, erect or curved toward the back; 

 exocarp gray-green to olive-green or brownish, sometimes reddish; endocarp with 

 keels low but prominent or with dorsal keel thin and very weak; beak deltoid and 

 weak, about 0.5 mm. long; loop solid; apex of seed pointing at the middle of the 

 opposite side or between middle and base. P. lucens L., P. angustifolius Bercht. 

 & Presl. 



In quiet or flowering water of ponds, canals and rivers in s.-cen. Tex., especially 

 on the Edwards Plateau and in the Guadalupe Mts., w. to N. M. (Eddy Co.) and 

 Ariz. (Coconino Co.), fruiting by early May; throughout much of U.S. and Can. 



A variable species due, in part, to habitat. Hybrids may occur between this 

 species and P. nodosus, especially where the two are found together. 



14. Potamogeton gramineus L. Fig. 44. 



Plant from a mass of rhizomes; stems slender, occasionally fistulose, 2-15 dm. 

 long; submersed leaves abundant, typically sessile (occasionally petioled), linear 

 to lanceolate or oblanceolate, 3-12 cm. long, 1-15 mm. wide, acute and often 

 with a short-attenuate tip, the stipules persistent; floating leaves on slender petioles, 

 the blades ovate to elliptic, 1.5-7 cm. long, 1-3 cm. broad, usually shorter than 

 petioles; stipules lanceolate, somewhat keeled, persistent, 5-30 mm. long; pe- 

 duncles stout, 2-10 cm. long; spikes compact, 1-4 cm. long when mature; nutlets 

 obovate, 1.5-3 mm. long, obscurely keeled, the beak somewhat recurved. 



Ponds, lakes, marshes and sluggish streams in N.M. (San Juan and Sandoval 

 cos.) and Ariz. (Coconino and Maricopa cos.), May-Sept.; Greenl. to Alas., s. to 

 Pa., N. Y., 111., la., N.M., Ariz, and Calif.; Euras. 



114 



