pectinatus L., sago pondweed
Potamogeton filiformis Pers., slender-leaved pondweed
Potamogeton vaginatus Turcz., sheathing pondweed
These three pondweeds have long, narrow underwater leaves and no floating leaves. They have membranous sheaths (stipules) partially fused to the leaf bases. They have membranous sheaths (stipules) fused to leaf bases for at least 2/3 the length of the stipule. Often the leaves spread out to produce a bush-like appearance. Some authors place all three species in the genus Stuckenia.
Leaf: Alternate, thin, submersed with stipules fused to leaf base for most of their length Sago: 2-15 cm long, to 1 mm wide leaves have pointed tips and 1 (sometimes 3) veins. Sheath (stipules) 2-5 cm long. Slender: brownish leaves, to 12 cm long, 0.5-1.5 mm wide, have pointed tips and 1 vein. Sheath (stipules) to 3 cm are tubular when young, later splitting. Sheathing: often dark-brown leaves, to 10 cm long, 1-2 mm wide, have rounded tips and 1-3 veins. Sheaths (stipules) on main stem leaves are twice as wide as the stem, margins free.
Stem: Sago and slender: thread-like, branched. Sheathing: stout below, slender above, can have 2-3 branches emerging at each node.
Flower: Sago: 2-7 whorls in spikes 1-3 cm long, usually float horizontally just beneath the water. Stalk 3-25 cm. Slender: 2-5 whorls on open 2.5 cm spikes; the lowest whorl sometimes remote from the others. Stalk to 15 cm. Sheathing: 5-9 evenly spaced whorls on long-stalks to 12 cm.
Fruit: Achenes. Sago: 3-5 mm long, plump, back rounded, reddish-brown when ripe, beak to 0.7 mm long. Slender: 2-3 mm long, broad oval, back rounded, tiny beak (0.3 mm). Sheathing: 3-4 mm long, broad oval, rounded or slightly ridged, no beak.
Root: Fibrous, from plant base and rhizomes. Sago and slender produce tubers from rhizome tips.
Propagation: Seeds and rhizomes. Sago and slender: also from tubers.
Importance of plant: All provide food and cover for many aquatic animals. Seeds, tubers, and vegetation of sago pondweed are especially important waterfowl food.
Distribution: Many locations throughout the world.
Habitat: Sago: tolerates a wide range of conditions including brackish, alkaline, or nutrient- rich water. Slender: often in shallows of hard water lakes. Sheathing: cold, often deep water.
May be confused with: These species are so similar that some taxonomists consider them to be one species. Careful microscopic examination of the sheaths, flowers, and fruits is required to distinguish them. Also widgeongrass (Ruppia maritima) has similar leaves, but the sheath is completely fused to the leaf, and the fruits are differently shaped.
Line Drawings: Potamogeton pectinatus
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