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.
The habitat of this species is ponds, lakes, rivers, and streams. The stem is stout, round in section, slightly branched, regularly forked above. The leaves are transparent, heart-shaped to ovate or lance-shaped, clasping, not hooded, the tip flat, blunt, or more or less acute, entire, 5-9-nerved The upper leaves are opposite, translucent. The stipules are small, more or less acute, not persistent. The flowers are in a dense spike. The flower-stalks are thick, equal, terminal, and in the forks short, stout, not thickened upwards. The perianth-segments have a long claw. The fruit is rounded on the back when fresh, keeled when dry. The plant is in flower between June and September, and is a herbaceous perennial.
The habitat of this species is ponds, ditches, streams, etc. The stem is flattened, slender, repeatedly forked. The leaves are in 2 rows, transparent, crisped at the margin, half-clasping, oblong, linear, blunt, stalkless, wavy, coarsely toothed, 3-nerved, close, spreading, bent-back. The stipules are small, blunt, and do not persist. The flowers are few, 6-8, in a loose small spike. The flower-stalks are curved, tapered upwards, stout or slender, equal, long. The fruit is keeled on the back when dry, with a long beak as long as the drupelets, which are obliquely ovoid, flattened. The plant is in flower in July and August, being a herbaceous perennial.
The habitat of this species is ponds, shallow streams, ditches. The stem is brittle, slender, repeatedly forked. The leaves are all submerged, opposite, the margins rolled in in bud. They are in 2 rows, elliptic, lance-shaped or ovate to heart-shaped, transparent, clasping, with a few teeth, 3-5 nerved, close, bent-back, acute, keeled, and the nodes of those that are more or less opposite are confluent with an overlapping base. There are no stipules except on the upper leaves. The spike is loose, short, shortly-stalked, with few flowers, 3-6, bent-back ultimately. The flower-stalks are as long as the leaves, bent-back at length. The sepals are triangular. The drupelets are flattened, with a thin pericarp, more or less round, with a sharp keel, and a short beak which is bent-back. The plant flowers between July and September, and is a herbaceous perennial.
The habitat of this species is rivers, lakes, streams. The stem is flattened, winged, broad, with long internodes. The leaves are all submerged, alternate, linear, ligulate, with a narrow point, abrupt, with 3-5 primary and numerous, close, parallel, intermediate veins, half-clasping. The stipules are free, large, with a narrow point. The flowers are numerous in a cylindrical spike, 10-15. The sepals are transversely oval. The flower-stalks are longer than the spike. The drupelets are flattened, inversely ovate, 3-keeled, the inner edge rounded, with convex faces. The style is terminal, forming a beak in fruit. The plant flowers in July and August, and is a herbaceous perennial.
Potamogeton acuti/bltus, Link. - The habitat of this species is lakes and ditches, marsh or fen ditches. The stem is flattened. The leaves are all submerged, alternate, linear to ligulate, long-pointed, with 3 principal and many close parallel, slender, intermediate veins. The stipules are lance-shaped, acute. The flowers are in a very short spike, which is ovate, as long as the stalks. The sepals are rhomboidal. The stalk is very short. The drupelets are convex, the inner edge straight, with a tooth near the base, inversely ovate, flattened, keeled. The styles are hooked, and continue the inner edge of the fruit, being ventral, and bent-back. The plant flowers from June to August, and is a herbaceous perennial.
Potamogeton obtusifolius, Mert. &. Koch ( = P. gram incus, Sm.).- The habitat of this plant is pools, ditches, ponds, and it is a rare species. The stem is slender, flattened, with rounded edges, much-branched. The leaves are stalkless, lance-shaped, linear, more or less acute, or blunt, 3-nerved, dark-green, opaque, with no veins between the 3 main ones. The stipules are very blunt. The flowers are in a small ovoid spike, dense, continuous, as long as the stalk. The sepals are rhomboidal, shorter than the leaves, terminal or in forks. The drupelets are flattened, broadly obliquely ovoid, keeled. The plant flowers in July and August, and is a herbaceous perennial.
Potamogeton friesii, Rupr. ( = P. mucronatus, Schrad.). - The habitat of this species is ditches, canals, etc. The stem is flattened. The leaves are grouped, linear, with an apiculus, 5-7-veined, the lateral veins close, and nearer the margin than the midrib, with no intermediate veins. The flowers are in an interrupted, short, loose spike, not so long as the flower-stalk. The latter is flattened, club-shaped. The sepals are transversely oval. The fruit is obliquely ovate and bluntly keeled, with a short beak. The plant flowers in June and July, and is a herbaceous perennial.