laterals farther down; stipules 7-18 mm. long, with connate margins when young 

 to form tubular delicately fibrous blunt sheaths, soon tearing and deciduous; 

 peduncles slightly thickened upward, to 3 cm. long; spikes subcapitate or cylindric, 

 of 1 to 3 contiguous whorls of 2 flowers each; perianth blades flabellate, brown- 

 ish, 0.6-1 mm. long; fruits obliquely suborbicular, laterally compressed, 2-2.5 mm. 

 in diameter; dorsal keel with a thin undulate to dentate wing; lateral keels obscure; 

 beak erect, broad at base, 0.2-0.4 mm. long; exocarp fulvous or olive-brown; 

 embryo with apex pointing toward the basal end or slightly above; winter-buds 

 sessile in the axils or on short branches. 



In fresh (mostly calcareous) or brackish water of ponds, irrigation ditches 

 and slow or swift streams throughout most of Okla., Tex., N.M. and Ariz., mature 

 fruit from May to Oct.; throughout Can. and the U.S. to Mex. and the W.I. 



Var. macellus Fern. Similar to var. foliosus but smaller and more bushy- 

 branched; leaves bright-green, to 7 cm. long and 1.5 mm. wide; nerves 1 to 3; 

 midrib without adjacent lacunae or with a single row on each side below the 

 middle; fruits green, obliquely obovoid, 1.8-2.3 mm. long, the body longer than 

 broad; beak slender, 0.3-0.8 mm. long; winter-buds terminating elongate branches. 

 Same habitats as var. foliosus. This poorly-marked variety that differs only in 

 size is apparently rare in our region. 



6. Potamogeton pu&illus L. Fig. 36. 



Plants often with winter-bud at base; rhizome absent; stem usually much- 

 branched, slender, terete or slightly compressed, usually with a pair of small 

 translucent glands at the nodes; branches (late in the season) often terminated by 

 winter-buds; stele of the one-bundled-type or oblong-type; endodermis of 0-cells; 

 interlacunar bundles absent; subepidermal bundles present; pseudohypodermis 

 absent; leaves all submersed, linear to linear-setaceous, entire, light-green, to 7 

 cm. long and 3 mm. wide, acute to obtuse at apex; nerves 3 to 5, the lateral 

 nerves obscure in narrow extremes, joining the midrib one-half to 2 leaf widths 

 below the tip; midrib usually not bordered by lacunae but they are sometimes evi- 

 dent on the young uppermost leaves; stipules scarious-membranaceous, 6-17 mm. 

 long, clasping the stem and with margins united at base to above the middle, 

 this union tearing with age; peduncles axillary, filiform, 1.5-8 cm. long; spikes 

 cylindrical, with 3 to 5 separate few-flowered whorls, 6—12 mm. long; flowers 

 with perianth round-flabelliform and with slender claw, 1.2-2 mm. long; anthers 

 0.5-0.8 mm. long; fruits obliquely obovoid, 1.9-2.8 mm. long, 1-1.8 mm. wide; 

 dorsal keel obscure, very low and broad; lateral keels absent; beak facial, promi- 

 nent, erect or slightly recurving, 0.2-0.6 mm. long; exocarp olive-green, smooth; 

 endocarp loop solid; apex of seed pointing slightly above the basal end or between 

 the base and the middle of the opposite side. 



In neutral or slightly alkaline or slightly brackish water of ponds and rivers, 

 often forming large masses, in Okla. (Beaver Co.), throughout Tex., in N.M. 

 (Colfax, Rio Arriba and Sandoval cos.) and Ariz. (Apache, Coconino and Santa 

 Cruz COS.), mature fruit from May to Oct.; throughout much of U.S. and Can., 

 s. to e. Mex.; Euras. 



7. Potamogeton clystocarpus Fern. Fig. 37. 



Stem much-branched, slender, terete or slightly compressed, usually with a 

 pair of small translucent glands at the nodes; stele of the one-bundled-type; 

 endodermis of 0-cells; interlacunar bundles absent; subepidermal bundles present; 

 pseudohypodermis absent; leaves all submersed, linear, entire, light-green, trans- 

 lucent to subopaque, to 9 cm. long and 3 mm. wide, the acute apex often with a 

 sharp mucro; nerves 3 (5), often obscure, laterals joining the midrib near the 

 apex or disappearing in the apical area; midrib bordered on each side by one or 

 two rows of lacunae; stipules hyaline to subherbaceous, 0.5—1 mm. long, usually 



105 



