pusillus L. (Synonym: Potamogeton berchtoldii), small
Potamogeton foliosus Raf., leafy pondweed
Small and leafy pondweed grow in similar habitats and look alike. They also resemble other aquatic plants with thin leaves and delicate stems. These pondweeds have long, narrow leaves and, except for an occasional flower spike that briefly rises above the water, they remain underwater for their entire lives. Because the narrow leaved pondweeds look so much alike, close attention must be paid to minute details to distinguish between them.
Leaf: Submersed, alternate, stalkless. Small pondweed: linear leaves, 2-7 cm long, 0.5-2 mm wide, have pointed to rounded tips and 3 veins. Membranous tubular or open sheaths (stipules) are 1-3 cm long, free of the leaf base, and usually disintegrate before the leaves. Leafy pondweed: linear leaves, 2-10 cm long, 1-2.5 mm wide, have pointed tips and 1-5 veins. Sheaths (stipules) are free of the leaf-base with the lower portion forming a tube that eventually ruptures as new branches emerge.
Stem: Slender and profusely branched. Small pondweed: often with small paired yellowish glands at leaf base,. Leafy pondweed: slightly flattened, paired glands lacking.
Flower: Small pondweed: in 1-4 whorls on spikes measuring 3-15 mm long; spikes not always above the water; on stalks to 5 cm long, often curved at the base. Leafy pondweed: in 2-4 whorls on an initially crowded spike (1 cm) that elongates as the season progresses; stalk 1-3 cm long.
Fruit: Achenes. Small pondweed: 1.5-3 mm long, rounded back, straight beak to 0.5 mm long. Leafy pondweed: To 2 mm long, with distinctively wavy ridged achene, beak 0.5 mm long.
Root: Small pondweed: Fibrous, from base of plant; often non-rhizomatous. Leafy pondweed: fibrous, emerging from threadlike rhizomes.
Propagation: Seeds and winter buds form at lateral branch tips and near leaf bases. Leafy pondweed also has rhizomes.
Importance of plant: Seeds and vegetation provide cover and food for aquatic animals.
Distribution: Small pondweed: widespread in northern hemisphere. Leafy pondweed: widespread in North and Central America.
Habitat: Small pondweed: wide tolerance of habitats, including brackish conditions. Leafy pondweed: marshes and shallow standing water.
May be confused with: Each other and other narrow leaved submersed plants. Careful examination of the glands at the leaf bases, leaf characteristics, and fruits is required to distinguish them. The wavy keel of the achene of leafy pondweed is particularly distinctive.
Photographs: Closeup of Potamogetan pusillus
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