friesii Ruprecht, flat-stalked pondweed
Potamogeton obtusifolius Mert and Koch, blunt-leaved pondweed
These two pondweeds have long, narrow exclusively underwater leaves and short flower stalks. Blunt-leaved pondweed is seldom seen and is included on the state rare plant list. Because of their similarity, great care must be taken when identifying these and other narrow leaved pondweeds.
Leaf: Submersed, alternate, linear, stalkless. Flat-stalked: leaves to 8 cm long, 3 mm wide have 3-5 prominent veins and tips with a tiny point. White sheaths (stipules) to 18 mm long are tubular at the base, free of the leaf base, and shred into persistent fibers. Blunt-leaved: green to reddish, translucent leaves to 10 cm long, 4 mm wide, have 3 veins and rounded or sometimes pointed tips. Translucent sheaths (stipules) to 3 cm long are open to the base, free of the leaf base, and shred into fibers.
Stem: Branching. Pairs of bump-like glands occur at the leaf bases. Flat-stalked: Slender, flattened, 1.5 m long, 0.5-1 mm wide with sphere-shaped glands. Blunt-leaved: cylindrical to flattened, to 2 m long, slightly zigzag, with irregularly-shaped glands.
Flower: Flat-stalked: in 2 to 4 whorls widely spaced on 1-2 cm long emergent spikes; flattened stalks to 5 cm long, curve at the base. Blunt-leaved: small, densely packed on spikes 1-2 cm long; straight, short stalks to 2 cm.
Fruit: Achene. Flat-stalked: 2-3 mm long, slightly ridged, curved or straight beak 0.6 mm long. Blunt-leaved: 3-4 mm long, round or slightly ridged, straight beak to 0.7 mm long.
Root: Fibrous. Flat-stalked: sometimes with rhizomes.
Propagation: Seeds; dense leafy winter buds.
Importance of plant: Seeds and vegetation provide food and cover for aquatic animals. Blunt-leaved is on the Washington State Natural Heritage Program's rare plant list.
Distribution: Temperate northern hemisphere.
Habitat: Flat-stalked: fresh to brackish water, to 3 m deep. Blunt-leaved: shallow lakes and ponds.
May be confused with: Each other and with other pondweeds with narrow underwater leaves, and water star grass (Heteranthera dubia). Microscopic examination of the glands at the leaf bases, sheath characteristics, and achene shape can distinguish them. They may hybridize with other pondweeds, forming plants with intermediate characteristics.
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