Scientific name: Potamogeton filiformis Pers.
Common name: Slender-leaved Pondweed
Description
Habit: Aquatic perennials. Stems not compressed. Hybrids are rather frequent in this genus.
Leaves: Leaves mostly alternate, some opposite, submerged, subtending inflorescences, all with membranous sheath or stipules; like P. pectinatus, but with leaves always very narrow and thread-like, obtuse; submerged leaves linear, 0.3-1.2mm wide, obtuse to rounded at apex, 3-veined but 2 lateral veins very faint and submarginal; stipular leaf-sheath without free margin, but forming a closed tube around the stem.
Flowers: In spikes or heads, spikes widely interrupted, especially in fruit; usually borne above the water-surface; perianth of 4 free, greenish segments; carpels usually 4, free; stamens 4, anthers stalkless, attached to the base of the adjoining perianth-segment.
Fruits: Drupes or achenes, 1 or more present, 2-3.2mm, with sessile stigma, with a very short subapical beak, with thick pericarp, soft on outside but with bony inner layer.
Habitat: Lakes mostly in the Northern half.
Distribution: Very rare.
Native status: Native
Of conservation interest: No