tot Sint Jans, Geertgen: The Tree of Jesse (c1490)

You can (if you're family-minded) make a family tree " a diagram of mothers and fathers, and their children, and their spouses, and their children, and so on, down the generations. Down is the operative word. The odd thing about these trees is that they grow downwards. They spread out like roots. They hang like mobiles.

We may call them trees, and talk about families in burgeoning vegetable terms (seeds, branches, scions etc). But in these diagrams, the metaphor of descent overrides the metaphor of growth. The ancestors sit above their descendants. Things are handed down from generation to generation. What is earlier, is higher. What comes after, comes below. Our reading and writing habits are no doubt responsible for this particular space-time equation, and beyond that gravity itself.

But some family trees can't go in a downward direction. Higher may mean earlier, but it means superior too. The genealogy of Jesus Christ is a case in point. It has to culminate with Jesus at the top of the tree.

Jesus is the Son of God, by the Virgin Mary, and no better pedigree could be required. But his human ancestry has also been important in Christian belief. 'And there shall come forth a rod out of the stem of Jesse, and a branch shall grow out of his roots: and the spirit of the Lord shall rest upon him': so says the Old Testament book of Isaiah, and the words were taken as a prophecy. The messiah is a linear descendant of the Jewish patriarch Jesse.

To prove it, the Christian gospel-writers provided long genealogical name lists. 'And Jesse begat David the king, and David the king begat Solomon... And Solomon begat Roboam; and Roboam begat Abiah; and Abiah begat Asa; and Asa begat Josaphat; and Josaphat begat Joram; and Joram begat...' till you eventually get to Jesus, via his human stepfather, Joseph.

In stained-glass windows, in illuminated manuscripts, this line of descent is often pictured as a tree " a proper rising tree, the Jesse Tree. The images show old Jesse lying at the bottom, with a stem growing from under his cloak, vaguely from the genital area, and rising up and ramifying. Figures representing the ancestors of Jesus are set among its branches. Mary and Jesus sit at the top of the tree. These images are basically diagrams. Everything is clearly laid out.

Not so in The Tree of Jesse by Geertgen tot Sint Jans. At first sight, the scene is almost unreadable. You have an upright image, full of people, but you can hardly grasp what's pictured. The people seem to be arranged in a tower; most of them are in the top half of the picture; there doesn't seem to be anything holding them up, nor do their postures suggest people who are hovering. You just have a suspended mass of richly costumed bodies, with glimpses of buildings peeping out among them. You may suspect that the picture doesn't really work at all. It's as if several different images were mixed up together.

Actually, the scene is perfectly realistic. It shows solid bodies occupying three-dimensional space, and it keeps strictly to its realist rules. The content may be symbolic, but nothing physically impossible, nothing miraculous, nothing visionary is depicted here. When you look more patiently, you can sort out what the picture shows and see that it makes sense. So why is it so baffling at first sight? Why, every time you look at it, is it baffling again?

Jesse lies asleep in a garden courtyard. Two prophets stand on the ground next to him, plus a kneeling woman (the picture's commissioner). A tree trunk grows up vertically from inside Jesse's cloaked loins, and expands into something roughly the size of a big apple tree. Perched and clambering around its branches, like a gang of tree-climbing children, there are a dozen fancily dressed figures. These are the ancestors of Jesus (or a representative sample). They are arranged in an S-shaped queue, which leads from Jesse up to Mary and Jesus and two angels at the top.

That may be clear. But the problem in this Jesse Tree is with the tree itself " the tree that's supposed to sustain this rising human formation. You can't see the tree for the folk. The ancestors are not nicely spaced out around its branches. They sit together in such a heavy throng, they obscure the structure that holds them up. What you see is a gathering in the air, solid bodies that are unsupported though apparently seated.

Faced with this bizarrerie, you try to make sense of it. You try to find something that will underpin these figures. The most plausible supports available are the bits of building, the wall and steps that appear behind them. And in fact this connection partly works. There are points where the figures could almost be sitting or standing on the stone ledges in the background. But then at other points the connection fails. Things become more confusing.

So what the image offers is a choice of disorientations. You can see the group of figures as sedately and impossibly defying gravity. Or you can try to make them sit on the stone ledges behind them, generating an unstable spatial slippage between near and far. What you can never do is keep your attention fixed on what the image really does depict " a perfectly well-oriented scene, a bunch of people sitting in a tree.

Yet what makes Geertgen's image so delightful, to our eyes at least, is precisely that we can't quite get a grip on its dense, multicoloured, weightless, jumpy confusion of bodies and space. The scene is lost in decorative effect. It has a festive spirit " and that may not be wholly unintended. The birth of Jesus is its subject, after all. And some people believe that the 'stem of Jesse', hung with figures, and with angels on the top, is the origin of our Christmas tree.

THE ARTIST

Very little is known about Geertgen tot Sint Jans (roughly 1460- 90). Early Netherlandish " or, as they used to say, Dutch Primitive; his name means Little Gerard of the Confraternity of St John. No likeness exists. (Was he little?) No stories are told. His life is presumed short. He is his art, but his art is mostly guesswork. It's a small body of work, always with Christian subjects. It consists of one documented altarpiece, with the rest, including The Jesse Tree, attributed on grounds of style " rich and delicate patchwork colour, gentle doll-like figures, sweet piety. In the National Gallery, London, there's a Geertgen Nativity where the baby shines out of the darkness like a light bulb.