western yellow pond-lily

Nuphar polysepala is a perennial, rhizomatous, aquatic herb with an extensive root system. The rhizomes are up to 5 metres (16 ft) long and 3–8 centimetres (1+1⁄4–3+1⁄4 in) wide, spongy, creeping, and branching. The leaves are submerged or floating. The ovate to oblong leaf is 10–45 cm (4–17+1⁄2 in) long and 7–30 cm (3–12 in) wide. The terete petiole is up to 2 m (6+1⁄2 ft) long. The flowers are solitary, pedunculate, bisexual, and 5–10 cm wide. They float on the water surface or extend beyond it. They have (6–)9(–12) green to bright yellow sepals, which occasionally have a red base. The 10–20 green to yellow, thick petals are hidden by the stamens. The fruit is ovoid to cylindric, strongly ribbed, 4–6(–9) cm long, and 3.5–6 cm wide. It bears seeds 4 millimetres (1⁄8 in) in length. The chromosome count is 2n = 34.

It is native to western North America. It is commonly found in shallow muddy ponds from northern Alaska and Yukon southward to central California and northern New Mexico, and can be recognized easily by its large floating leaves and bright yellow blossoms. It occurs in slowly flowing streams, ponds, and lakes in up to 2,4 m deep water.

The seeds are edible; they pop like popcorn, and can be steamed as a vegetable, dried and ground for flour, or can be cooked like oatmeal. Historically they have been a significant source of carbohydrates for the Klamath and Modoc peoples who inhabit the area near Oregon's Upper Klamath Lake. The leaves and rootstocks have traditionally been used for ulcerous skin conditions and swelling. The rootstock infusion is used as a traditional gargle for mouth, sore throats and douche for vaginal inflammation. One cup of boiling water is used for two tablespoons of chopped rhizome.

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