This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
P.gramineus, L.). - The habitat of this plant is ponds, ditches, pools, and lakes. The stem is much-branched below, slender, round in section, green or red. The leaves are not clasping, nearly flat. The submerged leaves are limp, stalkless, lance-shaped, narrow both ends, wavy, with small teeth, and a small blunt point. The floating leaves are more or less leathery, elliptic or oblong, stalked, or absent. The stipules are broadly lance-shaped, blunt, with 2 stout prominent ribs. The lower are linear to lance-shaped, and without the ribs, equally veined, the upper opposite, widely spreading. The flower-stalk is thickened upwards. The fruit is blunt dorsally, and, when dry, 3-ribbed, small, with a short beak. The plant is 2-4 ft. in height, flowering in June and July, and is a herbaceous perennial.
Potamogeton falcatus, Fryer = P. gramineus X P. nitens). - The habitat of this plant is fen ditches.
The stem is branched from near the base. The submerged leaves are stalkless, elliptic, strap-shaped, not symmetrical, rounded at the base, the upper leaves clasping the stems, the margin entire, wavy, the floating leaves stalked, elliptic, with a short, blunt point, leathery. The stipules are herbaceous, persistent, the upper broad. The fruit is small, ovoid, rhomboidal, with an acute keel, and distinct lateral ridges. The flower-stalk is thicker than the stem, hardly swollen upwards, shorter than the leaves. The plant is floating. It flowers in June and July, and is a herbaceous perennial.
The habitat of this plant is rivers, shallow streams, and fen ditches. The stem is very slender, with few branches, fragile. The submerged leaves are stalkless, straight, linear, or oblong to lance-shaped, entire, acute, not blunt-pointed, with a chain-like network near the midrib. The floating leaves are more or less leathery, elliptic to lance-shaped, shortly-stalked. The stipules are awllike, the uppermost broader with 2 dorsal raised ribs. The leaves are dark-green, blunt, netted, or with chain-like veins all over. The flowers are in a very short, ovoid spike. The flower-stalks are short, not thickened upwards, slender, equal, from the upper floating leaves, which are opposite. The drupelets are rounded, 3-toothed ventrally, nearly straight dorsally, with a ventral beak. The flowers bloom between July and September, and the plant is a herbaceous perennial.
Potamogeton miens, Weber = ?P.graminifolius x P. perfoliatus. - The habitat of this plant is pools and lakes. The plant is much-branched below, the stem somewhat wavy. The submerged leaves are alternate, lance-shaped, rounded below, half-clasping, wavy at the border, transparent, bent-back, shining. The floating leaves are leathery, elliptic, stalked. The upper leaves are not always leathery. The stipules are equally veined, the upper ones large, persistent, nearly half as long as the flower-stalk. The flower-stalk is swollen upwards. The fruit is keeled when dry, with 3 ridges on the back. The spike is short. The plant is in flower in July and August, and is a herbaceous perennial.
Potamogeton lonchiies, Tuckerm. ( = P. salicifolius = P. graminifolius, Fr.). - The habitat of this species is rivers. The stem is stout with few branches, long. The submerged leaves are straight, narrowly strap-shaped, stalkless, narrowed both ends, entire, with a few parallel veins next the midrib, 7-9-ribbed, alternate, very long, translucent. The floating leaves, when present, are more or less leathery, oblong to lance-shaped, elliptic, long-stalked, opposite. The stipules are long, blunt, not winged. The flowers are in a short spike. The flower-stalk is thickened upward, long. The fruit is obovoid, or obliquely inversely ovate, 3-keeled, with a short beak. The plant is floating, flowering from June to September, and is a herbaceous perennial.