Image copyright Stephen Greenberg, NLM Image caption A clone of the Hippocrates Tree was planted at the National Library of Medicine this month

Legend has it that Hippocrates, the ancient Greek "father" of medicine, taught his students under a tree on the island of Kos.

More than 25 centuries later, experts in the US have produced the first DNA barcode of the Oriental plane that is believed to be its descendant.

The original tree died centuries ago but the Greeks believe one of its descendents grows in the same place.

"In terms of symbolism this is huge," said team member Amy Driskell.

Dr Driskell manages the Smithsonian's Laboratories of Analytical Biology, which carried out the barcoding.

Hippocrates invented the idea that people with the same disease exhibit similar symptoms which produced similar outcomes.

His book, Prognosis, was the first to compare cases in an organised study and remains the basis of the theory of modern medical diagnosis.

Cuttings from this 500-year-old tree, a member of the Oriental plane tree species, have been presented as gifts to major medical institutions all over the world.

One was planted at the National Library of Medicine near Washington DC (part of the National Institutes of Health - NIH), when the building opened in 1962 - and the DNA barcode was created from this tree.

Barcodes are fragments of DNA that are unique to individual species and serve as their genetic fingerprint. More than 200,000 have been collected as part of the DNA Barcode of Life Project which aims to create a database of barcodes from every species on Earth.

Image caption The DNA barcode was produced at the Smithsonian's Laboratories of Analytical Biology

Each barcode comes from a specific specimen that is stored at a museum or similar permanent repository that provides access to research. The raw DNA data is also publicly available.

"The idea of a barcode is that you can go back and find the organism that it came from. So the original can be consulted a hundred years from now," says Dr Driskell.

The Hippocrates Tree at the National Library of Medicine has become the source of the first barcode for the Oriental plane tree species.

"I'm sure that Hippocrates would have been fascinated by the DNA Barcode Project and I think he would have been very excited about how DNA comparison and other modern methods are being used to better understand and ultimately treat human disease," said Dr David Lipman, director of the National Library of Medicine's National Center for Biotechnology Information.

But it very nearly didn't happen. In 1990 NIH chief landscape architect Lynn Mueller noticed the tree's health was declining and by 2003 it was almost dead. He began a desperate quest to find ways to clone the tree and save one of the few tangible links to Hippocrates in the US.

Nurseries around the country were given cuttings, but all failed to take. Eventually he contacted the Archangel Ancient Tree Archive in Michigan where experts managed to produce several clones.

"A new growth clipping is taken from the plant and the end is submerged in different rooting hormones to encourage new cell growth," said Mr Mueller.

"They're put into a special soil which is sterilised - and we have our new trees."

Last year the National Library of Medicine tree was pronounced dead and felled. Last week (April 25), one of its clones was dedicated and planted in the same spot.

"I've been worried for 25 years, no question. And I got more worried as the nurseries and propagators I contacted were failing," says Mr Mueller. "It was only the Archangel Ancient Tree Archive with their special techniques - they were the ones to save the tree for us."

Image copyright National Library of Medicine Image caption The Hippocrates Tree at the National Library of Medicine provided the DNA for the barcode

The DNA used to produce the barcode was extracted from the dead tree's wood, just under the bark. Genetic material extracted from a dead plant can sometimes be harder to find or inferior to DNA taken from a living species.

"It's probably not the best DNA in the world - it might be hard to make a genome from - but it was perfectly fine for what we needed to do," says Dr Driskell.

"This is all about the study of life," said Dr Constantine Stratakis, scientific director of intramural research at the National Institute of Child Health and Human Development.

"Most of the patients treated here have rare diseases and the discovery of what causes disease comes from studying DNA. The way DNA works is the same, whether you are a plant, a small organism, a human being or any other animal."

But there are some questions DNA can't answer. Did Hippocrates really teach students under a tree, and if he did, is the Hippocrates Tree at the National Library of Medicine really its descendent?

"I can't think of a way that exists right now that DNA could prove this one way or another," said Dr Driskell.

"The DNA tells me nothing more than that this tree belongs to this particular species (the Oriental plane tree)."

And so the legend of the Hippocrates Tree lives on. Science can't prove it existed, but it can't dispel the myth either.