Oxford's first example of Italian Renaissance building is Canterbury Quad in St John's College. It was built and paid for by the great churchman, William Laud, Archbishop of Canterbury, as a gift to his old college to celebrate his own rise to power as Chancellor of the University.

The richly ornamented building came out way over budget. When Laud formally received the account-book on March 26 1636, the cost proved to be £5,553 - more like £5m in present-day currency.

More lavish expenditure was to come. Five months later, Laud used his new quadrangle as the setting for a grand entertainment of King Charles I and his wife Henrietta Maria, whose fine statues dominate the site. The bill for the banquet and play that followed came to a further £2,666 - nearly half the cost of the building itself. Food in vast quantities was laid in for the visiting Court: deer, oxen, sheep, rabbits, capons, ducks, pheasants, swans and quails. Wine flowed freely and sweetmeats included confectionery fantastically modelled from paste and marzipan to represent university bigwigs: the doctors, professors, canons, masters and so on. A wit reported that the ladies water'd (bout the mouth) to see and taste so sweet a Universitie.' Big money met avant garde style in the design of the quadrangle itself. Breaking with mediaeval building tradition, its columns and arcades drew on the classical styles of Greece and Rome, and there is much ornamentation, too, influenced by motifs from Antwerp where the exuberant Peter Paul Rubens held artistic sway.

Though commissioned by a churchman, the quad is a humanist manifesto, celebrating learning and the liberal arts in allegorical busts of geometry, music, arithmetic and the like.

Here too are some remarkable images of the Green Man in his Renaissance incarnation. A mysterious leafy mask or a human head disgorging foliage, the Green Man is the subject on an ongoing survey in these pages, as part of the Faces of Oxfordshire initiative in this year's Oxfordshire 2007 celebrations.

In exploring the many examples that can be seen around the county we have discovered many mediaeval church carvings where the strange, often disquieting images seem to have had some religious or magical significance.

Things changed during the Reformation. The new Protestant orthodoxy was mistrustful of mysteries and graven images alike, calling instead for the Gospel to be preached in plain English from the pulpit. The congregation was there to listen rather than look, and Green Men were now rarely carved into doorways, roof bosses, corbels or choir stalls.

Remarkably, the resourceful refugees managed to survive the upheaval. Banished from church architecture, Green Men set out for the secular world and ensconced themselves in private homes and gardens. From Jacobean times foliate faces started to peer out from domestic panelling, chimney pieces, picture frames, garden fountains and ornamental iron screens. Unlike the disgorging heads so commonly represented in Gothic churches, the 17th-century effigies are generally leafy masks or faces turning into foliage.

In Canterbury Quad (pictured below) one example gazes out from a keystone in an arch of the western arcade. His is a handsome head, with classic good looks not greatly disturbed by the vegetation spurting from his flesh. More astonishing are the phantasmagoric leaf creatures repeated around the quad's double pedestals. Here foliate faces morph into extravagant draperies, scrolls, shells and even octopus tentacles that might have been imagined for some wildly luxurious court masque.

In fact, many Green Men of the Renaissance are notably theatrical - resembling two-dimensional actors' masks more than fully modelled faces.

Sir Christopher Wren placed some such high up on Tom Tower, which he built for Christ Church in 1682.

I do not believe that too much can be read into the Green Men of the era. Perhaps they were associated in some general way with the vitality and fruitfulness of Nature. Commercial prosperity may have been signalled too. More than anything, though, they speak of an imaginative need to reconnect with Greece and Rome. Foliate heads featured in Roman temples of Bacchus, and the Green Man of the 17th century perhaps signalled a building's classical pedigree, alongside woodland nymphs, satyrs and horns of plenty.

If the Green Man re-enters church in the 17th century, it is rarely as part of the fabric of the building, but chiefly on wall memorials of wealthy families. I have seen many such examples in Oxfordshire churches; the Pleydells' memorial tablet at Sparsholt church, and the Cookes', at St Mary's in Bampton, are just two of them.

The more intense symbolic power of the mediaeval church carvings may have been lost at this time, but one can still discern some significance in the way Green Men are placed. In domestic settings foliate heads appear frequently at gates and doorways, suggesting some continued role as an auspicious or protective device.

Eighteenth-century examples can be seen at Blenheim Palace, the baroque masterpiece built (1705-22) for the 1st Duke of Marlborough to designs by John Vanbrugh. Given by Queen Anne to the hero for his military services, Blenheim is known for its sumptuous décor, landscaped park and associations with Sir Winston Churchill, grandson of the 8th Duke.

With so much else to occupy the eye, it is hardly surprising that Blenheim's Green Men have been largely overlooked by writers on the Palace.

They are there nonetheless. Enter Blenheim by the East Gate and look up above the carved lions. You will see grotesque heads adorning the pinnacles, and at least one of them, looking into the courtyard, is a Green Man with oak leaves streaming from his eyes.

Approach the main entrance and you see two superb Green Man door knockers. Step into the entrance hall and look up again; the gilded lanterns overhead are decorated with fierce little leaf masks, sticking out their tongues. Are these merely ornamental?

I believe that in the age of elegance, when the Green Man certainly becomes less a magician and more of a poseur, a lingering symbolism survived.

Even at the home of the all-conquering hero of Blenheim, it seems, superstition required that foliate heads be present at the threshold to ensure the family's prosperity and keep evil spirits away.

Tim Healey will be talking about secular carvings in church architecture in Stones that Speak, a three-part series of broadcasts on BBC Radio 3, beginning Monday, August 6.