MODERN ICONS: Pagan roots

When is a tree not a tree? When it's a 20ft phallus swathed in velvet

Once a year, millions of people in the Western world unwittingly participate in a powerful totemic ritual. They buy a Christmas tree. Institutionalised by the Victorians, this symbol of rebirth and immortality has been with us for centuries. Garl anded, venerated and ceremonially burned, the pine tree was sacred to the Romans and the Norsemen. To most households today, however, the Christmas tree is merely a seasonal decoration - its deeper meaning is lost.

With her Christmas tree at the Tate Gallery, Cathy de Monchaux seeks to re-open our eyes. In doing so she has had to subvert the tree's traditional festive appearance and reach the public by playing the media at their own game. "For the popular press in this country, contemporary art is a bloodsport," the artist says. Her approach seems to have been effective. "Tate presents the invisible Christmas tree", mocked one recent headline.

There is only one light on De Monchaux's tree, and it doesn't work. Her decoration involves encasing the branches in a huge piece of blue velvet, canvas side out. We have only the artist's word that beneath the swathe there is a tree at all. De Monchaux challenges our deep-seated perceptions of the homely Christmas tree and in so doing re-awakens lost symbolic memories.

The rich blue of the velvet, visible only in a few exposed "slashes", suggests the traditional colour of the Virgin's robe in Renaissance iconography. The slashes themselves, disclosing a textural sensuality, become almost vulval. This is part of the artist's intention. The tree for De Monchaux is "a 20ft-high phallic object", its wrapping an illustration of Mary's sexual independence. "I was using a pagan symbol to provoke ideas about the virgin birth," she says. Extending this metaphor, the small bronze clasps in the shape of hands, which pinch together the tree's velvet wrapping, emphasise the idea of angelic intercession. On another level, the wrapping might be taken to refer to the wrapping of Christ himself - in swaddling bands at birth ; in a shroud after his crucifixion.

In this powerful, sensual installation, De Monchaux serves to remind us that our own more conventional trees, their decorations and the gifts which surround them, are no mere festive gewgaws, but reminders of Christ's sacrifice and God's gift of his son.