Potamogeton praelongus

Rare but occurs almost everywhere in Finland.

Perennial aquatic herb.

0.5–2.5 m long.

The rhizome is creeping. The stem is flaccid and geniculate near the summit.

The leaf arrangement is spiral. The leaves are sheathless. There is a stipule at the joint of the stem and a leaf. The stipules are 3–4 cm long and persistent. All leaves are submerged. The leaves are sessile. The lamina is 15–20 cm long, 2–3 cm wide, dark-green, broad and often slightly amplexicaul at the base. The apex is obtuse and cucullate. The margin is entire and undulate.

The inflorescence is a pedunculate spike. The peduncle is 8–20 cm long, thinner than the stem and evenly thick. The spike is typically 4–5 cm long. The flowers are bisexual. The perianth is four-leaved and calyx-like.

The fruit is drupe-like and 4.5–5.5 mm long.

Flowers in late summer (VII–VIII).

Eutrophic and clear-watered lakes, slow flowing rivers and brackish water bays with low salt concentration, in deep water, often on a sand-humus bottom.