Aquatic and Wetland Vascular Plants of the Northern Great Plains
55. Potamogetonaceae, the Pondweed Family
1. Potamogeton L. -- Pondweed19. Potamogeton zosteriformis Fern. -- Flatstem pondweed
Stems strongly flattened, freely branched, to 1 m long. Leaves all submersed, linear, 5-15(20) cm long, 3-5 mm wide, many (15-35)-nerved, acute to cuspidate at the tip, sessile and slightly narrowed at the base; stipules free, usually whitish, fibrous and shredding with age,1-4 cm long. Spikes densely flowered, usually uncrowded in fruit, cylindric, 1-2.5 cm long; peduncles compressed, 1.5-10 cm long. Fruits dark green to brown, obliquely elliptic-ovoid, 4-4.5 mm long, the dorsal keel sharp and somewhat undulate or dentate, the lateral ones obscure. Jul--Aug. Shallow to deep water of lakes, ponds and marshes; locally common at scattered locations, throughout the region; (Que. and N.B. to B.C., s to VA, IN, IA, NE, MT, ID and n CA).
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