Former tree-dweller Andy Letcher shares his passion for psychedelic fungi with Mandy Garner

Andy Letcher is scared of heights - not a great qualification for living 25m up a tree for three months. But that is just what Letcher, a lecturer and writer, did when he was part of the infamous protest against the building of the Newbury bypass in the mid-1990s. "I was a pretty useless road protester," he says, adding that he spent most of his time entertaining the other protesters, playing the mandolin and the English bagpipes. He was in one of about camps that, by the end of the protest, were all connected by aerial walkways. Protesters had to walk along a rope, attached by a harness, to pass supplies and information up the line. "It was a siege mentality," remembers Letcher, who says the treehouse network grew organically over time rather than being methodically planned, a little like his own career.

Letcher joined the protest after doing a PhD in ecology at Oxford University. Jaded by the competitive sink-or-swim mentality at Oxford and realising that ecology was more about politics than science, he decided he wanted to make a difference. Since his university days he had been interested in alternative ways of life. "I generally find alternative culture is fascinating for what it reveals about the mainstream... It is like its shadow side," he says.

A big part of this alternative culture at the time was magic mushrooms and, following eviction from his treehouse, Letcher drifted into an academic post that allowed him to marry his practical experience of psychedelic drugs with his intellectual interests. He says he suffered a crisis after being cautioned for his role in the bypass protest. He was playing in a festival band - which "wasn't really happening" - when he was rung up out of the blue by the future supervisor of his second PhD. He had funding for research into contemporary paganism and activist spiritualities. "It was three years off the dole, so I said 'yes'," Letcher says. "I would like to say it was a carefully thought-out career plan, but it was opportunistic."

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He had a series of temporary contracts teaching religious studies at institutions such as Oxford Brookes University, and in an attempt to get a permanent lecturing post he decided to write "a dry book on psychedelic spirituality".

His introduction on the history of magic mushrooms was so interesting, however, that Faber, his publisher, persuaded him to lose the long words and go for something more accessible. The result is Shroom: A Cultural History of the Magic Mushroom . It is a book that breaks with "the received orthodoxy" among mushroom enthusiasts: that they have been used since ancient times; that they inspired the building of Stonehenge and the Aztec pyramids; that Plato drank mushroom tea as part of ancient Greek rites; that medieval witches used them in moonlit rituals; and that patriarchal Christian culture and the Industrial Revolution combined to stamp out their use and heritage. Letcher traces how the magic mushroom was reborn in the modern era and became in the space of just 50 years one of the illegal drugs of choice - a quarter of a million people in the UK admitted to taking them in 2004.

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A lot of the book is dedicated to an investigation of key figures in recent magic mushroom history - particularly Gordon Wasson, the vice-president of a US bank with more than an average interest in fungi. Wasson, an adventurous type who longed for academic credibility, embarked on a series of arduous trips into the mountains of Mexico in the 1950s to back up his theory that the magic mushroom lay at the root of the human religious impulse. There he discovered Maria Sabina, the holy grail of mushroom theory - the last priestess of the ancient mushroom cult. His writings on her and his theory, now in an archive donated to Harvard University, have, according to Letcher, been instrumental in Westerners viewing magic mushrooms in a positive light and beginning to experiment with them.

Letcher sought to subject Wasson's theory to critical analysis for the first time and found it wanting. Although he expects his book to upset some mushroom enthusiasts, he thinks what he calls "crazy mythology" does no favours to the "genuine tradition" of people who take their psychedelic experiences seriously. "Their cause is not helped when people say mushrooms have been brought by aliens or some such crazy idea. I hope that my research can help them be less marginalised and taken more seriously."

Part of his interest in magic mushrooms and their growing use and commercialisation in the 20th century is in what they show about attitudes to modern life.

"In the UK, traditional religions have been rejected, but there is still a void that people are looking to fill. They are looking for enchantment.

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Science has stripped them of a sense of magic and meaning. Mushrooms may seem to answer that need," says Letcher, although he is keen to stress that many people are put off after a bad experience and many take them only once in their lifetime. He does not go into his own use of mushrooms, but quotes others who have seen angels - "a lapsed Catholic... he was quite pissed off at seeing them" - and aliens. "It is how these experiences make people feel they have a purpose that is interesting."

He says he is amazed by the people who admit to taking them. "It's a real cross section of people." Part of the reason is that they were sold legally until last year. "The fact that they were on sale gave them legitimacy," he says. But since they were classified as illegal, he says the British mushroom scene has "gone dead". Some former users are experimenting with other hallucinogenic plants.

Letcher says the rise in use of magic mushrooms has been mirrored by an increased interest in psychedelic drugs in academic circles in recent years. "It's probably because the Sixties generation is now running the show," he says. He contrasts them with the "bland high-street identity" of many students. Indeed, he seems increasingly disillusioned with academic life. He hopes to dedicate himself more to writing and says he couldn't have written his book if he had had a full-time academic job.

"Academics are now paid to be middle managers rather than to research and teach," he says. Perhaps they are in need of a bit of enchantment.

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Shroom: A Cultural History of the Magic Mushroom is published by Faber, Pounds 12.99.