Nasturtium officinalis is a member of the enormous cabbage eamily - Cruciferae - named for the cross shaped arrangement of their flower petals, The second part of its Latin name 'officinalis' affirms its reputation as a medicinal plant (officinalis appears in the Latin name of many medicinal or 'official' plant species)
A British native, it not only adds a peppery zing to salads, it is high in iron, phosphates, iodine, sulphur and folic acid. It contains vitamin C and has more calcium, weight for weight, than whole milk. It is also one of the richest sources of dietary fibre and is high in Vitamin A. If you are watching your weight, then is offers only 14 calories per 100g (Bartram, 1995). A great sandwich is watercress, with finely chopped red chilli and freshly grated lemon peel or lemon butter (thanks to Sophie Grigson for this idea) - I love it - and if you are not watching the calories, some Cheshire or Lancashire cheese complements this splendidly.
It is not used so much in herbal medicine any more, but if used as a 'herb' it does have the reputation for stimulating the pancreas, being good for lung complaints and the seeds are said to cleansing for the urinary system complaints. It also is thought to be part of a remedy for restoring hair to bald scalp when applied externally - although more than this, I wouldn't know, so don't try this at home!!
Watercress although it grows wild is best bought, rather than gathered from streams - the water it grows in needs to be clean and free from parasites.
References
Barker, J. (2001) The medicinal flora of Britain and Northwestern Europe, Winter Press.
Bartram, T. (1995) Encyclopeadia of herbal medicine, Grace Publishers.
Catherine Schofield Herbalist Cheshire Herbs and Herbal Medicine