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 margins of lakes, ponds, etc., stagnant water, slow streams, ditches. The habit is erect. The stems are erect, simple, or branched below. The rootstock is submerged, stout, and creeping. There are long, limp, submerged, transparent leaves, which are thin and broad, without hard tips. The leaves are close, in whorls, 6-12, linear, acute, with a hard point. The flowers are borne in the axils of the leaves, and are green, apetalous, stalkless. The anthers are red. The fruit is a drupe, green, smooth, and very small. The plant is 6-24 in. in height, flowering in June and Jul), and is a herbaceous perennial.
Whorled Milfoil (Myrivphyllum verticillatum, L.). The habitat of this species is ponds, ditches. The habit is aquatic. The rootstock is creeping. The stems are floating, leafy. The leaves are in whorls, 5 in a whorl, divided nearly to the base, with awl-like segments, distant, hair-like, collapsing when moved from the water. The spike is long, erect in bud, the flowers white, in axillary whorls. The bracts are in whorls of five, divided nearly to the base, or pectinate, longer than the flowers. There are no flowers in the upper axils. The anthers are linear. The fruit is nearly round, green, with carpels round on the back. The plant is floating, or submerged, flowering in July and August, and is a herbaceous perennial.
The habitat of this plant is ponds and ditches, streams, etc. The habit is aquatic. The plant has the same habit as the next, but is more slender. The leaves are in whorls of 3, submerged. The flowers are borne on a leafless spike, curved at the tip in bud, then erect, the sterile flowers alternate, 6, and the fertile flowers are about 3 in axillary whorls below. The bracts of the female flowers are pectinate, in whorls of 3 or 4, longer than the flowers. Those of the male flowers are alternate or opposite, entire or serrate, shorter than the flowers. The flowers are not so numerous as in the next species. The plant is submerged. It flowers from June to August, and is a herbaceous perennial.
The habitat of this plant is ponds and ditches. The habit is as in the Whorled Milfoil. The leaves are four in a whorl. The flowers are borne in a spike, erect in bud, and the bracts are small, entire, whorled, in fours, shorter than the flowers. The lower bracts may be pectinate. The fruit is hardly rounded. The plant is submerged. It flowers from June to August, and is a herbaceous perennial.
The habitat of this plant is ponds, ditches, sluggish streams. The habit is aquatic. The stem is round in section, little branched, submerged. The leaves are inversely ovate to spoon-shaped, the floating ones forming a rosette, 3-nerved, or rarely wanting, narrowed at the base, with submerged, linear leaves. The flowers are in the axils, male and female in opposite axils. The bracts are white, straight, and soon fall, being bent inwards. The fruit is nearly stalkless, longer than broad, with parallel lobes, bluntly keeled on the back, convex on the sides, united below for half their length. There is a hollow furrow which is not extended to the base. The anther-stalks are very slender. The plant is 3-12 in. in height, flowering between April and October, and is a herbaceous perennial.