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.
This has not been found in a fossil condition. It is a native of the Warm Temperate Zone, found in Europe, North Africa, and Temperate Asia. In England it is found in Somerset, Wilts, Dorset, Sussex, the whole of the Thames province, as well as in Anglia, throughout the Severn and Trent provinces, in Montgomery in Wales, but not in Mid Lancs. It is found in the Mersey district, throughout the Humber district, and in Durham. It is naturalized in a few parts of Scotland, as in Dumfries, and occurs in Ireland.
Like its congener the Water Cress, the Great Yellow Cress is an aquatic plant, half hydrophyte, half a land plant, being amphibious, as the Latin specific name indicates. It is fond of damp watery places, and very often grows luxuriantly and tall in rivers and canals, or in lakes. It has been united with Horse Radish, a plant which, likewise, though terrestrial enough in our gardens, where it is difficult to eradicate it, is found more or less wild by water.
The habit of this plant is much more like that of Water Cress than any other plant which grows in water, but it is more rigid, more erect, and being taller it makes a greater show above water than Water Cress. It has almost entire broadly lance-shaped, sometimes coarsely-toothed, dark-green foliage, and being deeply rooted by means of long stringy roots, like a Water Dropwort, it spreads out from the banks for some distance in deep water, forming a fringe along a canal bank, or in shallow streams filling the channel entirely.
It may be recognized by its Nasturtium habit, combined with the yellow flowers, the petals twice as long as the calyx, the flower-stalks spreading or turned down; the pouch is egg-shaped, with a stigma with a pin-head, and the seeds are small, the silicules being ellipsoid and swollen, the pods being shorter than the flower-stalks, and there is no vein on the pouch.
This plant grows to a length of 4 ft. It is in flower from June to September. It is a herbaceous perennial, and reproduced by seeds or by division. The structure of the flower is similar to that of N. sylvestre, in which at the base of the flower between every 2 stamens there is a green, fleshy honey-gland. In this there are 6 nectaries in a ring at the base. The anthers of the 4 longer stamens are nearly on a level with the stigma, the other 2 are deeper, and all are turned towards the centre. The anthers spread out when the flower is open, and open towards the stigma. Or they may make a half-turn and so avoid possible self-pollination. Visitors insert the head between the stigma and stamens, and each side of the head is dusted; while the insect remains in the same flower the same side touches the stigma, but if it visits others the opposite side may touch the stigma, and cross-pollination will follow, while if the same insect inserts its head into the same flower several times it may cause self-pollination. In wet weather the anthers of the long stamens touch the stigma and the plant is self-pollinated. It is visited by Hymenoptera (Tenthre-dinidse, Tenthredo), Diptera (Empidae, Empis, Syrphidae, Rhingia, Syritta, Eristalis).
Great Yellow Water Cress is dispersed by the plant's own agency. The seeds are small, and are dispersed after the tension of the oblong pods, when dry, has caused the pod to open lengthwise, and scatter the seeds to a distance.
It is a hydrophyte and aquatic, rooted in the alluvium of the river-or lake-bed in the reed swamp.
There are apparently no fungal or insect pests that infest this plant. The second name amphibia (Latin) alludes to its amphibious habit. This plant is called Bellragges, Water Charlock, Laver.
Essential Specific Characters:25. Radicula amphibia, Druce. - Stem erect, tall, leaves pinnatifid, entire or dentate, flowers yellow, petals twice as long as calyx, pod straight, ovoid, shorter than pedicels.