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 wet places, ponds, pools, etc. The habit is prostrate, submerged, or floating. The plant is limp, small, straggly. The stem is frequently submerged, wavy, fairly thick. The floating or submerged leaves are twice or thrice pinnate. The lower leaves are hair-like, much divided, 3-cleft, linear, and some of the upper leaves rise above the surface. When growing on the mud not in water the leaves are all much broader, with wedge-shaped segments, 3-cleft. The small white or pinkish flowers are in small umbels of 2 or more unequal rays, with a leaf opposite each, on short flower-stalks. There are no bracts. There are 4-6 lance-shaped, 3-nerved bracteoles. The petals are in-bent. The fruit is more or less stalkless, elliptic to oblong. The styles are bent-back. The plant is 4-10 in. in height, and flowers in June and July, being a herbaceous perennial.
The habitat of this plant is watery places, ponds, ditches, peaty pools. The habit is erect. The rootstock is short, stout, with septa. The roots are fibrous and slender. The stem is hollow below, furrowed, round in section, thick, leafy. The leaves are large, triangular, the radical leaves twice or thrice pinnate, borne on stout leaf-stalks, those of the lower leaves long. The leaflets are linear to lance-shaped, acute, doubly coarsely toothed, and run down the stem, being oblique. The flowers are small, white, in large, flat umbels, with no general involucre, or one or 1-2 slender bracts, the partial involucre consisting of numerous short, slender, awl-like bracteoles. The calyx-teeth are ovate. The fruit is broader than long. The slender styles are bent-back. The plant is 2-4 ft. high, flowering in July and August, and is a herbaceous perennial.
The habitat of this plant is watery places, ditches, rivers, river-sides. The habit is erect. The root-stock is short, and bears stolons. The stem is erect, stout, hollow, angular, grooved, and branched above. The leaves are large, pinnate, with 9-13 large, stalkless, distant leaflets, which are linear to oblong, acute, lance-shaped, equally toothed. The submerged leaves are much-divided. The flowers are small, white, the outer larger, in large, flat, terminal umbels, the rays numerous. The flowerheads, at first erect, turn sideways. The involucral bracts are lance-shaped. The bracteoles are leafy and large. The style is slender. The fruit is broadly ovoid, with prominent ridges. The plant is 3-6 ft. high, flowering in July and August, and is a herbaceous perennial.
The habitat of this plant is wet places, ditches, etc. The habit is erect. The rootstock is creeping-, with stolons, bearing- leaves at the joints. The stem is leafy, round, finely furrowed, erect. The leaves are pinnate, large, with the leaflets in the radical leaves stalkless, regularly toothed, ovate to oblong, those of the upper leaves irregularly coarsely toothed, lance-shaped, few, smaller. The flowers are small, with few lateral umbels, with a leaf opposite. The rays are few and unequal. The bracts are numerous, lance-shaped, entire or irregularly lobed. The fruit is not so long as in the last, with fewer vittae, and conical disk-lobes. The plant is 1-3 ft. high, flowering in July and August, and is a herbaceous perennial.
The habitat of this plant is ponds and wet ditches, pools in peaty places. The plant is erect in habit, or floating and ascending. The stem is erect, spindle-shaped below, thick, stoloniferous, the flowering shoot dying down each year, the plant being continued by offsets. The leaves are twice or thrice pinnate, finely divided, the segments much divided, the submerged leaves hairy, with spreading, hair-like segments. The aerial leaves have broad, small, blunt lobes, ovate, cut, spreading, the segments numerous, acute, pale-green. The flowers are white, in lateral umbels, opposite the leaves, with 7-10 rays, and numerous bracteoles. The outer florets are sometimes irregular. The fruit is round in section, ovate, narrow to oblong, twice as long as the styles, which are slender and wavy. The ultimate flower-stalk is not thickened above. The plant is 1-4 ft. high, flowering from July to September, and is a herbaceous perennial.