1911 Encyclopædia Britannica/Pondweed

​PONDWEED, a popular name for Potamogeton natans, a cosmopolitan aquatic plant found in ponds, lakes and ditches, with broad, more or less oblong-ovate, olive-green, floating leaves. The name is also applied to other species of Potamogeton, one of the characteristic genera of lakes, ponds and streams all over the world, but more abundant in temperate regions. It is the principal genus of the natural order of Monocotyledous Potamogetonaceae, and contains plants with slender branched stems, and submerged and translucent, or floating and opaque, alternate or opposite leaves, often with membranous united stipules. The small flowers are borne above the water in

(After Wossidlo. From Strasburger's Lehrbuch der Botanik.)
Potamogeton natans.

1, Apex of flowering shoot. 3, Flower viewed from the side.
2, Flower viewed from above. 4, Diagram of flower.

axillary or terminal spikes; they have four stamens, which bear at the back four small herbaceous petal-like structures, and four free carpels, which ripen to form four small green fleshy fruits, each containing one seed within a hard inner coat; the seed contains a large hooked embryo. An allied genus Zannichellia (named after Zanichelli, a Venetian botanist), occurring in fresh and brackish ditches and pools in Britain, and also widely distributed in temperate and tropical regions, is 'known as' horned pondweed, from the curved fruit.