The herbalist's method was known as the doctrine of signatures. According to this pseudo-scientific belief, plants suggest their healing properties by the location in which they grow, or by their shape. Thus the willow, which grows in marshy places, would provide the cure for rheumatism, which was worst in damp conditions; a preparation of willow bark was the herbalist's cure, and went on to became a central ingredient of aspirin. The doctrine of signatures explained why arnica - good for cuts and bruises - grows on rocky ground, and why dock leaves are the handily placed remedy for stings from nearby nettles.
These dubious yet apparently efficacious connections governed herbal medicine until the establishment of botanical testing grounds such as the Chelsea Physic Garden, founded by the Society of Apothecaries in 1673.
A herb is any plant with leaves or foliage that has a useful application - as flavouring, dye, cosmetic or medicine. Down the years, our knowledge of herbal remedies has been lost as industrialisation allowed mass-produced alternatives. Dozens of modern-day drugs have been synthesised from nature; foxgloves have been the source of successful heart drugs and the yew tree has provided anti-cancer treatments. Indeed, the word drug comes from droge, the German word for dried, describing the way most herbs were conserved for use.
Some old concoctions survive as pale imitations of their former selves. Marshmallows are the spongy descendants of a preparation of marsh mallow root, sweetened with egg white and sugar and given to children for sore throats. The soft drink dandelion and burdock was once taken to cleanse the blood.
Even common or garden herbs found on supermarket shelves have hidden properties. Thymol, the active ingredient of thyme, is found in vapour rubs and mouthwashes. In a previous life, rosemary was a household disinfectant, parsley was mashed into the scalp to repel head lice, sage served as an early anti-perspirant, and a concoction of oregano leaves was supposed to ease those "given over much to sighing". What a relief.
Herb collections to visit:Chelsea Physic Garden, London. A walled garden full of unusual and ancient plants on the banks of the Thames. Tel: 020 7352 5646; www.chelseaphysicgarden.co.uk.National Herb Centre, Warmington, Warwickshire. Gardens, plant shop, nature trail and cafe. Tel: 01295 690033; www.herbcentre.co.uk. Acorn Bank, near Penrith, Cumbria. Largest collection of culinary and medicinal plants in the north. Tel: 017683 61893; www.nationaltrust.org.uk.Illustration: 16th-century woodcut (Corbis)