14. Submersed leaves lanceolate to oblong; floating leaves mostly with fewer than 



30 nerves (15) 



15(14). Floating leaf blades usually cordate, rarely rounded at base, with 21 to 

 29 (sometimes more) veins; submersed leaves tapering rather 

 abruptly to a sessile base or short petiole to 1.5 cm. long; mature 

 fruit light-brown to olive-green, 3-3.5 mm. long 10. P. pulcher. 



15. Floating leaf blades cuneate or rounded at base, with 9 to 21 veins; sub- 



mersed leaves tapering gradually to a petiole 2-13 cm. long; mature 

 fruit usually reddish, 3.5-4 mm. long 12. P. nodosus. 



1. Potamogeton latifolius (Robbins) Morong. Western pondweed. Fig. 31. 

 Rhizome creeping, rooting freely at the nodes; stem whitish, simple below, 



repeatedly branched above; stele of the one-bundled-type or oblong-type; endo- 

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

 subepidermal bundles absent; pseudohypodermis absent or partly 1 cell thick; 

 leaves all submersed, linear, entire, green to bronze, rather opaque, to 7 cm. long 

 and 7 mm. wide, the apex obtuse to rounded or shortly apiculate to acutish on 

 the upper leaves; nerves 3 to 5, with strong crossveins making a rectangular pat- 

 tern; stipules prominent, 8-12 mm. long, adnate to the base of the leaf to form a 

 broad sheath, hyaline along the margin, the free portion 1-4 mm. long; peduncles 

 2-25 cm. long; spikes with 4 to 6 whorls, contiguous when young but soon be- 

 coming moniliform; basal internodes 5-12 mm. long, the upper shorter, in fruit 

 2-4 cm. long; flowers sessile; perianth semiorbicular, slightly wider than long, to 

 5.2 mm. wide; anthers about 1.8 mm. long; fruits obliquely obovate, the sides 

 convex but somewhat compressed, 3-4 mm. long, 2-4 mm. wide; dorsal keel 

 obscure, lateral keels rounded; beak facial, slightly recurved, about 1 mm. long; 

 exocarp olive-green to fulvous; endocarp loop solid or with a spongy area; apex 

 of seed pointing above the basal end. 



In quiet or flowing fresh or brackish water, in s.w. Tex. (Cameron, Pecos, 

 Reeves and Val Verde cos.) and Ariz. (Mohave Co.), flowers and mature fruit 

 from May to Sept.; rare in s.w. U.S. 



2. Potamogeton filifonnis Pers. Fig. 32. 



Slender much-branched wholly submersed plant of brackish waters, with hori- 

 zontal stolons bearing white tubers 1-2 cm. long; stipules adnate to leaf and 

 sheathing the stem, the sheaths 0.4-2.2 cm. long, connate below, the tips free, 

 scarious, 1-5 mm. long; leaves setaceous, to 12 cm. long, 0.2—0.5 mm. wide, blunt; 

 peduncles filiform, flexuous, to 1 dm. long; spike moniliform, 1.5-5 cm. long, 

 with 2 to 5 whorls, the upper whorls 3-12 mm. apart, the lower ones 0.7-2.5 cm. 

 apart; connectives 0.5-1 mm. long; styles almost wanting; nutlets sessile, 2-2.7 

 mm. long, 1.5-2 mm. wide, rounded on back, the beak short, wartlike, nearly 

 central. 



Ponds, slow streams and ditches in N.M. (Rio Arriba Co.) and Ariz. (Pinal 

 Co.), Apr.-Sept.; Greenl. to Alas., s. to Pa., Mich., N.M., Ariz, and Calif.; Euras., 

 Afr. and Austral. 



3. Potamogeton pectinatus L. Sago pondweed. Fig. 33. 



Rhizome creeping, much-branched, 1-1.5 mm. in diameter, bearing terminal 

 tuberous bulblets; stem terete or slightly compressed, about 1 mm. in diameter, 

 mostly simple near base but abundantly branched near summit; stele with the 

 oblong pattern or one-bundled in slender branches; endodermis of U-cells; inter- 

 lacunar bundles present; subepidermal bundles present or absent; pseudohypodermis 

 1 or 2 ceils thick; leaves all submersed, filiform to narrowly linear, entire, to 15 

 cm. long and 1 mm. wide, occasionally wider on robust forms, the apex tapering 

 to a narrowly acute point (sometimes obtuse on young seedlings); nerves 1 to 3, 



100 



