The annals of medical anomalies bulge with stories from far-flung places where the idea of a reliable source is a chap sitting on a gate in a goatskin fleece who waves to passersby, even if there are none. And so to the Urals, where medics are reported to have removed a tiny fir tree from a man's lung, after he complained of chest pains. Before doctors opened him up, they were convinced he had lung cancer. Now, they're convinced he inhaled a seed, which sprouted inside him.

Surgeon Vladimir Kamashev at Izhevsk hospital was about to remove a large part of 28-year-old Artyom Sidorkin's lung, when he took a closer look, according to reports. He was stunned to see a 5cm-long spruce inside, the Russian news agency Pravda says.

A spokeswoman for the Royal Botanic Gardens in Kew, London, is flummoxed. "A seed might be able to germinate in the damp, dark conditions of a lung, but it's still bizarre," she says.

The gruesome photo released with the story claims to show the spruce jutting from a clump of Sidorkin's lung tissue. The plant looks firm and healthy, with bright green needles. It's as if it had been grown in the best soil with plenty of sunlight. It lacks roots in the way fresh clippings do.

Lungs are good at getting rid of unexpected visitors. They are lined with mucus that traps everything from mould spores to flies. This is pushed out of the lungs by tiny hairs called cilia. You end up coughing it out, or swallowing it.

"The closest I've heard to this are balls of mould that grow in patients who have abnormalities in their lungs," says Simon Johnson, a reader in respiratory medicine at Nottingham University. "They can get up to a few centimetres, but you would know because you would be coughing up blood."