A NEW stained-glass window was dedicated earlier this year in the lovely little Romanesque Iffley Parish Church, outside Oxford.

Iffley already boasts one of John Piper's happiest windows: a gathering under the Tree of Life of the animals associated with the nativity. Scrolls come from their mouths with inscriptions based on the Elizabethan wall-paintings at Shulbrede Priory, which evidently caught Piper's fancy as they had that of Beatrix Potter, who wove them into The Tailor of Gloucester.

The font at Iffley stands immediately below Piper's south window; and now Roger Wagner's north window has come to occupy its other side. For all that Wagner's Tree of Life carries the Crucified, there is yet a poignant note of joy; for the tree itself is in riotous spring blossom, and the River of Life, which winds down the green hill from the base of the Tree towards the font, is teeming with fish.

Both Piper and Wagner come to stained glass as painters. Piper worked in the closest collaboration with stained-glass artists, especially Patrick Reyntiens, who interpreted his ideas on paper into glass. To be able to convey luminescent colour was an intoxicating magic to the Middle Ages, and still is today, when we have all the skills of film. To watch a good stained-glass window changing through the moods of the seasons and the sun is one of the great rewards of regular church attendance.

One of the technical difficulties lies in the necessity for support. With only small spans of glass possible in the earlier Middle Ages, the leads had to be closely meshed. Nevertheless, if you see a bar running across a face, for instance, in a Romanesque window, it will be mending an ancient break. Nowadays, the joint can be healed invisibly with silane.

Our 12th-century ancestors used their leads, whenever possible, to outline figures and drapery, and other compositional elements. Curiously enough, as skill in making wider expanses of glass grew, the aesthetic effect did not necessarily grow likewise. No one has ever improved upon the windows of Chartres. Now, of course, most things are possible.

Here at Iffley, the deep, fretted embrasures and small, simple rounded windows are ideal for jewel-like glass in the 12th-century tradition; and Wagner has responded fittingly. His window was already provided with a ferramenta of horizontal bars, which he has absorbed into his composition. Unlike Piper, Wagner has mastered for himself the whole art of stained glass, at the able and generous hands of his friend Tom Denny.

This Christus hangs among the upper branches of the flowering Tree. The blossom around him is of pink and white against a sky of changing blues, and rooted in a Golgotha of darker blue. The foreground is of that "green hill far away" which, for me, will evoke the choirboy voice with which my grandmother still sang in her 80s. She sat back in her wing-back chair by the open french window on to that gentle summer evening, joining in that hymn on the radio. Her knitting had slipped down in the chair; for her lap was occupied by the tame rabbit and the cat curled up together, while the spaniel slept on her slippers. Well, the rabbit and his browsing mate are in this window, feeding among what look to me like fritillaries. From the foot of the Tree springs a glinting River of Life, winding down the green hill, and teeming with dancing fish. Under the shade of the Tree cluster seven black-faced sheep and a lamb. There is much here, as there in much in the Piper window opposite, to delight a child, and much to give pause to the rest of us.

It was only after Wagner had conceived his design that he visited San Clemente in Rome, and saw the great mosaic of the 12th century where the Christ hangs on a Tree of Life covered with white doves, the scrolls of its flowering branches filling the apse. He did not know that the equation of the Rood with the Tree of Life had been made before by Jacobus de Voragine (c.1230-98), who spelt it out in The Golden Legend, as famously illustrated by the wall-paintings by Piero della Francesca in San Francesco in Arezzo.

That the Tree of Life that opens and closes the Christian Bible, the Tree of the Garden of Eden and of the Heavenly Jerusalem should also have been identified in the Middle Ages with the Tree of the Cross takes explaining. This the Dominican Jacobus set out to do, following what was clearly established legend. Those of us who have studied the frescoes painted by Piero in the years 1452-64 around the choir of San Francesco in Arezzo are familiar with the story.

When Adam realised that he was going to die, he sent his son Seth back to the Garden of Eden, seeking a message from God. The angel guarding the closed gate gave Seth a seed from the Tree of Life with the promise that when it bore fruit he would be made whole. Seth returned to find his father dead. He planted the seed in his mouth, or, in some versions, a seedling on his tomb. Hence the skull beneath the cross in some crucifixes, and the name Golgotha. The tree grew mightily, and was cut down by Solomon to form a bridge over the moat to his palace. The Queen of Sheba recognised the trunk for what it was, and told Solomon, who, horrified, took down the bridge and had the trunk buried. Then, of course, it was dug up in time to be used for the cross of Christ; buried again; and finally rediscovered by the Empress Helena. Fragments of the True Cross tend to stem from Constantinople.

It is appropriate that this legend should have been widely disseminated by the friars. The thrust of their great success in the 13th century lay in their skills as preachers, and stories like this were a compelling power in their sermons. The Franciscan Johannis Metensis, preaching in Paris in the 1270s, took meditation on the Tree of Life a stage further. Franciscans preached extempore (some of them were illiterate anyway). Easily memorised visual diagrams were invaluable to them. In Johannis Metensis's Speculum Theologiae, as illustrated in the mag- nificent illuminated fragment of the psalter of Robert de Lisle (BL Arundel MS83 11), of the very early 14th century, there are a number of such memorable diagrams.

It opens with the sphere of the Franciscan Archbishop of Canterbury, John Peckham, and contains nine further folios of diagrams, most of them originating from Johannis Metensis's book. F.125 verso shows the Tree of Life. The whole is framed by an array of prophets, led by St Peter and St Paul. A heart-rending painting of the Crucified hangs from a slender green tree, with, at the crown, the Pelican in her Piety in a nest of oak leaves. The leaves of the Tree itself are of the vine, but the fruits are apples, each labelled with a virtue.

So the apple and the vine, referring both to the Tree of Life and to the disastrous Tree of the Knowledge of Good and Evil, bear the suffering Saviour in this medieval version. Some of us can remember when the atom was split, and the final consequences of meddling with the serpent's tree were fully revealed.

Such is the ancestry of Wagner's conception. For him, the Tree of Life is laden with apple blossom, the whole agony of its burden almost hidden by the annual promise of new life; "for they do these things in the green leaf. . ."

This artist has already proved himself a great painter, a fine engraver, and a poet. I hope that there will be more stained glass from his mind and hand. Meanwhile, Iffley is the place to seek if you are looking for the best that we have done and can do today.