'It's obviously another of those quaint old Oxford traditions," remarked one delighted tourist to another as they joined the entourage who, armed with willow rods, were watching the Rev Hugh Lee chalk a cross on a boundary stone in Broad Street.

"We are really getting our money's worth today. First May morning and the Morris dancers, now this. I wonder what they are doing?" the other replied.

We were actually beating the bounds of the Parish of St Michael at the North Gate in Cornmarket Street. This annual perambulation round the streets of Oxford is a custom that's been performed on Ascension Day since 1429.

Ascension day is one of the Church's Rogation days, when the church asks for God's blessing on the crops, but as fields of wheat no longer border the parish boundaries, God's blessing is bestowed upon shops, colleges, works, visitors and students. Even bikes and recycling bins get a mention now as the priest leads the congregation through the twisting back alleys of the city and through shops such as Marks & Spencer, Miss Selfridge, Boots, Wagamama and the Covered Market, only stopping when one of the 28 boundary markers is reached and marked. Once the stone is marked with chalk, the congregation step forward wielding their branches crying "Mark! Mark! Mark!" before heading on.

The event concludes at Lincoln College, where bread, cheese, doughnuts and ivy beer were laid on for all those who participated.

While I was aware of the annual Mint Julep ceremony at New College, which owes its origins to a 19th-century American planter from Wynah in South Carolina, the Boar's Head ceremony and the Needle and Thread Dinner at The Queen's College, All Souls' Mallard Feast, Lincoln's ivy beer ritual on Ascension Day was new to me.

Having been brought up to avoid ivy berries and leaves (Hedera helix L), as they are said to contain a toxic poisoning, I was concerned. Did this beer really contain ivy? If so why?

I needn't have worried. The flavouring is ground ivy (Glechoma hederacea), a perennial with kidney-shaped leaves with scalloped edges and creeping stems that sprawl over the surface of the soil. Ground ivy can be found in most gardens and is usually considered a weed. It tends to crowd out other plants. Its history goes way back as it was used by the Anglo-Saxons to clarify their beer before hops were introduced into the country.

Its common name alehoff was derived from the old English word meaning ale ivy. It's other common name turnhoof was derived from the word tun, meaning tipple. Ground ivy was used to clear and flavour beer until the reign of Henry VIII. It was also added to beer taken on sea voyages so that it would keep longer.

The beer offered at our Ascension Day lunch at Lincoln was a lively bitter brewed by the White Horse Brewery in Stanford in the Vale, to which an infusion of freshly picked ground ivy leaves were added by the college butler the night before it was served. The ivy leaf infusion was barely discernable; the distinctive flavour of the White Horse Bitter dominated. Indeed, given that we'd spent more than two hours walking through Oxford's labyrinth of back streets, it proved quite refreshing, and highly quaffable. But why is ivy beer served on this special day? The story goes something like this . . . Many hundreds of years ago a mob of youths from the town were stoning a group of students outside Lincoln College. The students rushed to the college gate calling for help and the porter immediately let in all the Lincoln students, but refused sanctuary to a Brasenose man who was in the crowd. As a consequence the Brasenose man was stoned to death. Ever since that day, Lincoln, which abuts Brasenose, has brewed this special beer annually and offered it as penance to its neighbouring college. Why Ascension Day was chosen for this ritual is something I have yet to discover.

What I have discovered. though, thanks to Mike Brown's excellent book Oxon Brews (Brewery History Society, £12.50), published a couple of years ago, is that many of the Oxford colleges once employed their own brewer. Lincoln built its first brewhouse in 1667 at the cost of £300. This survived until 1826, when a new brewhouse was built in the New Quadrangle for twice that sum, but was demolished in 1889. Apparently Brewer's 1870 Dictionary of Phrase and Fable states that Brasenose derived its name from the old Flemish word Brasenhuis which means brewhouse. Today's publication of this excellent reference book makes no such claim.

While researching Oxon Brews, Mike Brown laboriously trawled through old inventories, maps and receipts and came up with 14 other colleges that once had their own brewer and brewhouse. They include The Queen's College, Balliol, Christ Church and All Souls, which had its own brewer right up to 1927.

Now, with the demise of Morrell's, not even the city of Oxford has its own brewery, (unless you take into account The Old Bog Brewery, linked to the Mason's Arms in Headington, which brews a barrel or two a week), yet in the 14th century there were 115 brewers in the city, many of whom settled in Brewers Street.

The county of Oxford is not without its fair share of fascinating breweries, however. Perhaps the most popular is the family-owned Hook Norton Brewery, which dates back 150 years. Then there's Wychwood at Witney, known for its popular Hobgoblin brew, which also brews Brakespear and Duchy Originals Organic Ale, but that has been recently taken over by Marston's. Other Oxfordshire breweries include Burford Brewery, Witney; Appleford Brewery; The Cotswold Brewing Company, Foscot; Lovibonds, Henley; and Loddon Brewery, Dunsden, near Henley.