Submersed Plants

 
Species: Potamogeton robbinsii Oakes, fern-leaf pondweed
Family: Potamogetonaceae

Fern-leaf pondweed is a stiff, robust plant with underwater leaves only. It is usually easily recognized because its dark green, closely spaced leaves are arranged in a rigid, flattened spray, giving it a palm frond or fern-like appearance. Fern-leaf pondweed is usually a low-growing plant and only approaches the water surface when flowering. The flowering stalks have more widely spaced leaves that are less fan-like in appearance. Like other pondweeds, fern-leaf pondweed has membranous appendages (stipules) at the leaf bases. Stipules form a short sheath around the stem and are partially fused to the leaf blades causing the leaves to seem jointed or bent at the base if they are pulled from the stem.

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Leaf: Fern-leaf pondweed has all submersed leaves. The olive green to brown stiff leaves are linear, minutely toothed, and are attached about halfway along the stipular sheaths, which are then attached to the stem. Leaves measure up to 12 cm long and 6 mm wide and have many parallel veins. Leaves from a rigid flattened spray that resembles the leaf arrangement of a palm frond or a swordfern. The white sheaths (stipules) are less than 3 cm long with the lower 10-15 mm fused to the blade; the sheath tip shreds into fibers. 

Stem: Fern-leaf pondweed has stout stems up to 3 m long that often creep along the bottom and will root at the lower nodes. 

Flower: Small flowers with 4 petal-like lobes occur on spikes near the water surface. The up to 2 cm long spikes occur on flat, stiff stalks with widely spaced leaves.

Fruit: The achenes are up to 4 mm long, are keeled, and have a curved beak to 1 mm long.

Root: Fibrous roots emerge from slender rhizomes and the lower stem.

Propagation: Seeds, rhizomes, winter buds. 

Importance of plant: Seeds and vegetation provide food and cover for aquatic animals and waterfowl.

Distribution: Northern North America.

Habitat: Shallow to deep water, usually with low alkalinity.

May be confused with: The dark color and palm-frond-like appearance of fern-leaf pondweed are distinctive. It may hybridize with other pondweeds to form plants with intermediate characteristics. 

Photographs: Closeup of  Potamogeton robbinsii leaves and stems

Line Drawings: None available 


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