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 ditches. The stem is slender, flattened. The leaves are linear, the lowest blunt, the upper with long narrow points, with 1 pair of lateral veins running nearly to the apex. The stipules are long, with a long tapering point. The flower-stalks are two to three times as long as the rather dense spike. The fruit is oblong-elliptic, obscurely keeled, the inner edge nearly straight. The plant flowers in July, and is a herbaceous perennial.
The habitat of this plant is rivers, ponds, and ditches. The stem is slender, not usually flattened, cylindrical, much-branched. The leaves are all submerged, half-clasping, narrow, linear, usually 3-veined (or 5-7), with no intermediate veins, more or less acute, the lateral veins half-way between the midrib and margin. The stipules are small, and acute. The spikes are short, loose, shorter than the stalk, with 6-10 flowers. The sepals are roundish to kidney-shaped. The stalks are hardly flattened, slender. The drupelets are small, obliquely ovoid, swollen, bluntly keeled, with a stout, more or less terminal beak. The plant flowers in July and August, and is a herbaceous perennial.
This species is found in streams and rivers and is very rare. The stem is very slender, with few branches. There may be no floating leaves, more or less leather)', lance-shaped, long-stalked. The submerged leaves are linear, narrowed at both ends, very long (2 ft.), stalkless, entire, without an apiculus, few and veined, with numerous parallel veins near the midrib. The leaves are longer than in P. lanceolatus, green when dry, and do not possess chain-like reticulations. The stipules are very long, blunt, not winged. The flower-stalks are slender, not club-shaped. The fruit is acutely keeled. The plant flowers in July and August, and is a herbaceous perennial.
Potamogeton sturrockii, A. Benn. - This has been regarded as a variety or sub-species of P. pusillus. The stem is slender, rather flattened. The leaves are thin, blunt, 3-5-nerved, bright-green, membranous. The flowers are in a short spike, with a slender stalk. The drupelets are small, with a short beak.
The habitat of this species is muddy ponds and ditches. The stem is more or less round in section, hairlike (hence trichoides), thickened below the joints, and repeatedly branched, or forked and spreading, the branches tufted. The leaves are half-clasping, bristle-like, spreading, I-veined, rigid, dark-green, with a fine point. The stipules are slender, acute. The flowers are in a short loose spike (3-6), curved, slender, on long, not thickened stalks, longer than the spikes, with large stipules. The sepals are rounded, stalked. The fruit is solitary, transversely kidney shaped, obliquely ovoid, flattened, with an obscure keel, with a straight inner edge, with a tooth near the base, warted on the back, with a tubercle at the base when fresh, and with a short beak. The plant is in flower in July and August, and is a herbaceous perennial.