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 pondweed is ponds, lakes, rivers, and streams, and the plant is common in deep water. The plant has the pondweed habit. The stem is stout and long. The plant is shining and glistening. The leaves are large, more or less stalkless, translucent, transparent. They are mainly submerged, oval, oblong, or lance-shaped, linear, blunt-pointed, wavy, with small serratures, many-nerved. The upper leaves are leathery, opposite, floating. The stipules are winged or keeled on the back, large, and long. The flowers are in a stout, cylindrical, dense spike. The flower-stalks vary in length, are swollen upwards, and are as long as the spikes. The fruit is rounded on the back, keeled when fresh, the drupelets being small and turgid, with a short, blunt beak. The plant is 3-6 ft. long, flowering from June to September, and is a herbaceous perennial.
The habitat of this plant is streams and canals, and the plant is very rare. The stem is long, much-branched. The leaves are membranous, transparent, stalk-less, more or less rounded, or oblong to lance-shaped, not hooded, blunt, blunt-pointed, more or less entire, wavy at the end, the uppermost more or less stalkless. The stipules are short, not, or scarcely, winged. The flowers are in a dense spike 1-2 in. long. The flower-stalks are equal, stout. The fruit is as in the last but smaller. The plant is in flower in July, and is a herbaceous perennial.
Potamogeton griffithii, A. Benn. ( = P. alpimts X P. pralongus). - The stem of this plant is branched, round in section. The submerged leaves are more or less clasping, strap-shaped, wavy, hooded, membranous, with a concave tip, II-nerved, with 4-5 fainter nerves near the midrib. The upper leaves are long-stalked, inversely lance-shaped, tapered into the leaf-stalk, and are 13-17-nerved, blunt, more or less leathery. The stipules are long, narrow, blunt. The flowers are in a dense spike. The flower-stalks are slender, not so long as the upper leaves. The young fruit is ovoid, the beak terminal from the ventral face. The plant is in flower in July and August, and is a herbaceous perennial.
Potamogeton zizii. Roth ( = P. angnstifolius, Presl). - The habitat of this plant is lakes and rivers. The stem is long, much-branched. The leaves are nearly all submerged, membranous, transparent, narrow to lance - shaped, oblong, blunt - pointed, wedge - shaped, narrowed below, wavy near the end, with small teeth bent back. The floating leaves are inversely ovate-oblong, more or less leathery. The stipules are long, broad, with a broad, double keel. Sometimes there are aerial leaves, short-stalked, leathery, oblong, acute both ends. The flowers are in a short spike. The stipules are very long, straight, swollen upwards, thicker than the stem, longer than the leaves, terminal. The drupelets are rounded, with 3 keels. The plant flowers in July and August, and is a herbaceous perennial.
Long-stalked Pondweed (Potamogeton pra-longus, Wulf - The habitat of this pondweed is lakes and deep rivers, ditches chiefly in the East. The stem is round in section, greenish - white, robust, growing in deep water. The leaves are transparent, half-clasping, linear to oblong, blunt, hooded at the end, entire, with a swollen midrib, with parallel veins, the upper leaves opposite. The veins are numerous and close. The stipules are not winged, blunt, large. The flowers are in a dense spike. The flower-stalks are not thickened upwards, stout, equal, very long. The fruit is strongly acutely keeled on the back when fresh, or winged when dry. The plant flowers from May to July, and is a herbaceous perennial.
Potamogeton salicifolius, A. Benn. ( = P. lithuani-cus, Gorski). - The habitat of this species is rivers. The plant is not much branched. The stem is slender, round in section. The leaves are translucent, submerged, half-clasping, lance-shaped, acute or more or less so, entire, 3-nerved, with numerous secondary nerves. The stipules are blunt. The flowers are in a short spike. The flower-stalks are not enlarged upwards. The drupelets, when dry, are flattened, rounded, blunt. The British plant does not fruit. It flowers in July, and is a herbaceous perennial.