How many aspirin can you find in an English physic garden?

Many modern medicines are derived from plants. Clare Sansom takes a turn round a garden specialising in healing herbs.

The healing power of chemicals in plants has been known for millennia. But when people nowadays think of plants in medicine, they tend to think of alternative therapies.

In fact, more than half the world's 25 best-selling conventional pharmaceuticals come from the natural world, and a large proportion of these are derived from plants. The Chelsea Physic Garden, a unique garden in the heart of London, is celebrating the millennium with an exhibition illustrating the plants that will still be at the heart of 21st-century medicine.

The Chelsea Physic Garden was founded in 1673 by the Society of Apothecaries of London. At that time plants were the main source of medicines. The garden was one of many throughout Europe. In time, most became simply "botanic gardens" but the one in Chelsea, which still has a strong reputation for research into medicinal plants, has kept its original name. It was in effect re-founded in the early 18th century by Sir Hans Sloane, a rich and influential physician who owned the manor of Chelsea. The curator still pays his heirs, the earls of Cadogan, a nominal rent of Pounds 5 a year. The garden was opened to the public only in 1983, after it had been established as an independent charity with its own board of trustees.

The Pharmaceutical Garden was planted at Chelsea last spring. It groups about 30 different species of plants into medical specialities. The garden's curator, Sue Minter, has deliberately used the scientific names for these disciplines, for example, "oncology" rather than "cancer". This emphasises the fact that extracts from these plants are used conventionally, in modern, technological medicine. Other featured disciplines include cardiology, dermatology, ophthalmology, neurology and rheumatology.

Most of the plants will not be at their best until the summer. Between July 9 and September 3 the garden will host a photographic exhibition alongside the Pharmaceutical Garden. Sue Snell, photographer-in-residence at Chelsea, is taking portraits of people who have been successfully treated with drugs derived from these plants.

The idea for the Pharmaceutical Garden came from an exhibition of plants in medicine at the Royal Horticultural Society's autumn show in 1996. Ms Minter explains: "We had not realised how little people knew about the plant origins of medicines or how interested they would be. We often overheard visitors saying things like: 'So nifedipine comes from this plant I take that'."

In the 18th century, infusions of willow bark were used to treat acute rheumatic fever. The active constituent of this bark is salicin, which is converted in the body into salicylic acid. Although salicylic acid is a painkiller, it irritates the stomach. The chemical company Bayer developed a safer alternative, acetyl salicylic acid. This product was launched in 1899 and is still, as aspirin, a best-selling drug. This first successful "commercial" pharmaceutical is aspirin. Its name derives from Spiraea, the old name for the meadowsweet, from which salicylic acid was first obtained.

The plants displayed under "oncology" in the Pharmaceutical Garden include the rosy (or Madagascar) periwinkle, Catharanthus roseus (pictured below) and the Pacific yew, Taxus brevifola. Plants in the periwinkle family contain a group of chemicals known as the vinca alkaloids, from the plant name in Latin, Vinca. The potent drug vinblastine is now used to treat childhood leukaemias. This and similar drugs target rapidly dividing cells such as cancer cells. They kill cells as they divide by preventing the formation of structures known as "spindles". The spindle is formed only during cell division, when it pulls one set of chromosomes to each end of the cell.

However, Vinca proves that folk wisdom is not always all-knowing. The rosy periwinkle was originally a folk remedy for diabetes. This attracted the attention of scientists who discovered the potential of the vinca alkaloids while proving that the plant is no use to diabetics.

The development of taxol is one of the most dramatic success stories of modern cancer research. This effective treatment for breast cancer is derived from the bark of the Pacific yew. But one average-size, 60-year-old yew tree produces only enough taxol to treat a single breast cancer patient. With 15,000 deaths every year from this disease in the United Kingdom alone, the demand for the drug would very quickly destroy the world's stocks of this relatively rare tree. Furthermore, taxol has a complex structure and is very difficult to synthesise. In this case, the solution was found in the much more common English yew. Its leaves contain a related compound, baccatin III, which is now used in the laboratory as a "building block" for taxol synthesis.

The pharmaceutical company Glaxo established a collaboration with the Chelsea Physic Garden in 1988. Since then, Glaxo, now Glaxo Wellcome, has taken plant samples from the garden to test for a wide variety of physiological activities. They are not necessarily looking for plant extracts that will become drugs, but for new types of chemistry that they can use in drug development. As James Niedel, executive director for research and development at Glaxo Wellcome, stated: "The need to conserve rare species and not to imperil biodiversity is of particular concern."

Working with established botanical gardens, such as Chelsea, is one way of doing this. Jane Lewis of Glaxo Wellcome's compound diversity unit says: "It amazes me what diversity of plants they have in such a tiny area."

The Chelsea Physic Garden will be open to the public on Wednesday and Sunday afternoons between April and October.