This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Common and widespread as this plant is, it is not found in any deposits in which seeds of recent plants are preserved. It is almost cosmopolitan, occurring in Europe, West Asia, North Africa, and it has been introduced into North America, and the colonies of the British Empire, even to the extent of choking some rivers in New Zealand. In the British Isles it is ubiquitous, growing in every vice-county of Great Britain, in Ireland, and the Channel Islands. In the North of England it grows at an altitude of 1000 ft.
Water Cress, as a rule, is a Hydrophyte growing submerged, with its roots alone fixed in the mud along the margin of its habitat. But it may also be found growing as a hygrophilous or moisture-loving plant, out of the water on damp mud, at the margin, gravitating, like many so-called truly aquatic plants, between a life on land and a life in the water. It is to be found not only in ditches, ponds, and pools, but also in rivers and lakes. But, as a rule, it is most luxuriant and at its best in shallow running water.
Water Cress is a large plant when allowed to grow rank, the stem being long but wavy and hollow, and it is seldom or never erect, but like all aquatic plants, when submerged grows lengthwise in, or in this case upon, the water, floating on account of its lightness. At the base it is creeping, and attains a semi-erect habit only at the upper extremity, where the flowering stems rise above the water. At other times it is dwarf, floating, or growing prostrate upon damp mud.
It may be recognized by its leaves, with lobes each side of a common stalk, egg-shaped, oblong, leaflets slightly toothed, and nearly heart-shaped at the base, the white flower often finally purple, with petals twice as long as the calyx, with round pods with swollen valves, beaded, the pod upturned, and the stigma small. The pods are longer than the flower-stalks, and the seeds are in two rows and flattened lengthwise.
Water Cress is often as much as 4 ft. long or high. The flowering stage lasts from May to July and August. It is a herbaceous perennial, increasing by seeds as well as by division.
Photo. L. R. J. Horn - Great Yellow Watercress (Radicula Amphibia, Druce)
This illustration shows well how the marginal reed swamp may encroach on the centre of a shallow pool and gradually fill it up.
Two green, fleshy honey-glands are hidden on the inner side of the base of each short stamen. The shorter anthers open towards the stigma, which overtops them. The taller stamens stand at first on a level with the stigma, but are afterwards overtopped by it, and open towards the shorter stamens. The bee visiting the flower touches the stigma and the pollen-covered faces of 3 of the anthers (1 short, 2 long).
Photo. Flatters & Garnett - Water Cress (Radicula Nasturtium Aquaticum, Rendle And Britten)
When the weather is adverse, self-pollination is effected by the longer anthers.
Water Cress is one of the numerous plants dispersed by its own agency. The seeds are dispersed by tension of the valves, the seeds being rounded and flattened lengthwise.
It is a Hydrophyte and aquatic, and when growing on land prefers a sand soil, or sandy loam, or river-valley alluvium, free from peat. It may grow in the reed swamp in the submerged or half-submerged leaf-association.
Water Cress is galled by Cecidomyia sisymbrii. No fungi infest it so far as is known, nor any insects. Water snails are fond of it, e.g. Limnaea, Planorbis, Succinia, etc.
The old generic name Nasturtium, given by Pliny, is derived from nasus, nose, tortus, twisted, in reference to the hot character of the plant.
The English names are Billers, Brooklime, Brown Cress, Carsons, Water Crashes, Water Cress, Eker, Water and Well Grass, Water Kerse, Rib, Teng-tongues, Well Grass, Well Kerse.
The plant was said to drive warts away if laid on them. The Greeks used it as a salad and as a medicine. Pliny says it was used for brain troubles. In England it was first cultivated in 1801 or 1808. An aromatic oil which it contains renders it nutritive. The mineral salts which the plant contains render it nutritive.
Water Cress requires running water, and, when cultivated, plants are bedded at intervals, in rows in the direction of the current. The beds should be kept free from mud and other plants, and occasionally thinned out.
Essential Specific Characters:24. Radicula Nasturtium aquaticum, Rendle and Britten. - Stem branched, erect, succulent, leaves bipinnate, lower larger, leaflets rounded, dentate, flowers white, twice as long as calyx, pod linear, curved.