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 plant is fresh and brackish ditches, streams, and ponds. The stem is slender, branched, densely branched, forming a thick flat mass. The root is a thick tuber. The leaves are long, linear, made up of 2 interrupted tubes, with a long narrow point, the upper leaves bristle-like, 1-3 ribbed, the lower flattened, 5-nerved. The stipules form a sheath with a free tip. The flowers are in an interrupted whorl, few, and distant. The stalks are long and slender, not thickened above. The fruit is large, swollen, broadly half-inversely ovate, the inner edge convex, rounded on the back, without ridges. The nut has 2 strong lateral ridges, and is not keeled, but has a short beak. The plant flowers in July and August, and is a herbaceous perennial.
This species is regarded as a variety of the last, found in Shetland.
Potamogeton flabellatus, Bab. ( = P. interrnptus, Kit.). - The habitatof this species is streams, ponds, and ditches, chiefly near the sea. The stem is, when floating, long, branched, wavy, fanlike (hence flabellatus). The rhizome is spreading from a tuber. The lower leaves are broadly linear, flat, with an abrupt apiculus, or long narrow point, 3-5-veined, the veins transverse, distant from the margin. The upper leaves are narrow, acute, 1-3-veined, the lateral veins marginal. The flowers are in interrupted spikes. The fruit is broadly half-inversely ovoid, the inner edge more or less straight, the lateral ribs not well-marked, and not ribbed on the back when fresh, but rounded. There is a prominent keel on the nut. The plant flowers in July and August, and is a herbaceous perennial.
This plant is found in lakes and ditches. The stem is slender (hence filiformis), branched below, simple above, with short branches. The leaves are like those of P. pectinatus, but longer. They are linear, bristlelike, I-nerved, the veins transverse, channelled, alternate. The flowers are in a much-interrupted spike, in dense whorls, with very slender stalks, longer than the leaves. The drupelets are hardly keeled, inversely ovate, rough, rounded dorsally when dry. The nut is round on the back, with a very short, terminal beak. The plant flowers from July to September, and is a herbaceous perennial.
The habitat of this plant is fresh and brackish waters, pools, ditches, ponds, and streams, stagnant water. The plant is floating or submerged, and has the pondweed habit. The stems are thread-like or bristle-like, with spreading branches. The leaves are opposite or in whorls, slender.
The flowers are without stalks, in the axils. The achenes are curved, with a smooth keel, stalkless. There are 1 or 2 stamens in the male flowers, which are long-stalked. The females consist of 1 free or shortly-stalked carpel, in groups of four or less, with a cup-like spathe, which may be entire. The plant flowers from May to August, and is a herbaceous annual.