Harriet Swain joins the fungi foragers who risk their taste buds and kidneys in pursuit of treasure

Pah. Nick Jardine spits out the nibble of pink and white mushroom violently onto the forest floor. It is as he thought - a fungus with the kick of a handful of chilli peppers, hot enough "to make your lips bleed".

Harvesting knife at the ready, the gleam of the enthusiast in his eyes, Jardine, professor of the history and philosophy of science at Cambridge University, is leading colleagues and students in their annual fungus hunt through Thetford Forest. Clasping their trugs, the group fans out through the trees in search of the rare, the edible and the dangerous, rushing back individually from time to time to bring Jardine their finds for inspection. This one he pronounces "a rare killer, a very nasty thing indeed", that one "edible but tastes like chickens' kneecaps", that "edible and delicious" although very similar to another with a pinkish base that is extremely poisonous.

Earlier, in his cosy room at Cambridge, grabbing a coat to wear over his tweed jacket, Jardine discussed the possibility of poisoning his students.

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Not that he doubts his own powers of identification: he has been a keen mycologist, or fungi specialist, since the age of 12 and he knows the names of more than 2,000 types of fungi. His fear is that students will attempt to identify fungi themselves, assuming that those that look alike are similarly harmless or harmful, which, he stresses, is definitely not the case.

Jardine starts his hunt by telling the cautionary tale of Michael Dettelbach, a former member of the department, who, inspired by the spirit of adventure that had driven Alexander von Humboldt, the subject of his research, spent one hunt tasting as he went. In the event, Dettelbach survived the experience. But Jardine recalls: "I spent a sleepless night - and so did he." The fear is not about death; it's much worse than that. "If you go rock climbing and fall off, that's that," says Jardine. "But if you get ill by eating a mushroom, you end up with no proper kidneys or no liver and then you become a burden to your family."

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There are three main attractions of mycology, Jardine says. One is that fungi can be delicious, especially if cooked by his wife, Marina Frasca-Spada, a fellow lecturer and a dab hand at serving the mushrooms with butter and parsley or deep-frying them in breadcrumbs. The other is the element of a treasure hunt - the desire to find rare specimens, preferably before anyone else. Finally, there is the attraction of danger, the machismo involved in dicing with slow and agonising death by poisoning.

There is also, of course, the power bestowed by being the only one to know which fungi delivers which result - delicious or deathly.

It is therefore ironic that when the Cambridge fungi hunt started it was as part of an attempt to avoid some of the macho competitiveness of academic life. It was one element of a seminar, "The cabinet of natural history", that started in 1988 as an informal series of meetings organised and chaired by research students. The aim was to recognise the importance of what was then in Cambridge the neglected field of natural history, and to give students a fair chance to lead and to contribute to discussions that were not dominated by the research agendas of senior staff.

Michael Bravo, a lecturer in the history and geography of science and an early secretary of the cabinet, says that while the rest of the seminar has become more formal over the years, the mushroom hunt retains the spirit of the first meetings, offering participants a chance to think about their subject in a relaxed way while having fun.

"As someone who does fieldwork myself, I always say walking and conversation is a tremendous way to do scholarship," he says. He suggests that when people walk around museums, the presence of interesting objects around them prompts them to have different conversations. "The mushroom hunt is about that too."

For Bravo, it encapsulates the spirit of discovery and the Romanticism that inspired many of the 19th-century scientists being studied in the department. Some of these early ecologists were exploring the same part of the country as we are today, although the environment is not quite as it was then. The hunt is taking place next to a major US airbase and the rural peace is continually shattered by fighter planes skimming over the trees to practise landing. In the past, a couple of Jardine's fungi trips have ended with him being escorted away by armed members of the US military. So the intrepid foragers face dangers from more than fungi.

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It nevertheless provides a refreshing contrast with their usual desk-bound existence. Francis Reid, the chairman of the cabinet, says most of the work done by research students in the history and philosophy of science department involves them poring over books, sifting through archives and objects. "Museums are full of things people have collected," he says. "We are trying to see why people collected things, how they went about it."

While for many of those on the hunt collecting is part of their job - a few are working on the collection of Darwin's letters, for example - what they are searching for are words and ideas. This, then, is their chance to get an insight into what drives the enthusiasm, bordering on obsession, of a collector of objects. What is more, these are objects that they can see, touch, smell and taste - and that provoke a particularly physical response.

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But the hunt isn't only about collecting, it is also about classifying. This is an aspect that has particular resonance for Christina McLeish, a PhD student who is looking at the way many of the labels given to scientific concepts become obsolete. When it is so difficult to detect differences between things as tangible as fungi, and when minute differences - in colour or smell, or environment - can put them into an entirely different category or, indeed, can mean the difference between death and dinner, what hope is there for reliable classification of something as intangible as a neutron?

Taxonomy is one of Jardine's passions. When the hunt is over and he has distributed the finds from the edible trug, always being very cautious, he will sort through the much fuller trug of inedible and potentially poisonous fungi, recording and mapping them for future reference, with the help of his many tomes on mycology. It takes hours, he says, but imparts order to the rather haphazard collection and adds to his expertise. This he has developed not through academic qualifications but through years of fungi hunts, first as the disciple of more experienced mycologists, then by himself.

But part of the thrill of the fungi hunt, as with academic research, is that expertise is not enough. You also need luck, and it was McLeish who found only the previous week probably the rarest fungus Jardine has seen - the Squamanita paradoxa, a parasitic toadstool that bursts out of its host fungus. This hunt has not turned up anything quite as exciting, but thanks to the warm autumn followed by heavy rain it has still been an exceptionally successful expedition. Highlights for sauteing are huge parasol fungi, cepes and crab mushrooms, which smell and taste like seafood. Those for Jardine's logbook include the toadstool that according to the biblical scholar John Allegro inspired a secret mushroom cult that he claimed was the true origin of Christianity. This, explains Jardine, was a theory developed after Allegro went completely mad.

Thankfully, no one has been shot by a US soldier, beheaded by a low-flying aircraft or damaged any internal organs by unwise ingesting, although there's still time. "Anyone want half an Eccles cake?" asks Jardine as the tired foragers troop back to the car. "I'll just wipe the Death Cap off my hands."

Mushrooms in broth and spices -  a 14th-century English recipe

Funges. Take funges and pare hem clene, and dyce hem; take leke and shrede hym small, and do hym to seeth, in gode broth. Colour it with safroun, and do therinne powdour fort.

Taken from Curye on Inglysch: Middle English recipes , edited by Constance B. Hieatt and Sharon Butler, published for The Early English Text Society by Oxford University Press, ?15.

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(Peel and dice some mushrooms, finely chop a leek and simmer in stock. Add a pinch of saffron and a mixture of ground spices, including pepper and cloves.)