There's a crystal tree, hung with pricey wine glasses, and a Harrods tree, an intergalactic monstrosity of pulsing purple neon; there's a caviare tree, a silver sturgeon standing in a pearly oyster shell, and a bat tree, a metallic cartoon loaded with clothes from Versace, Dolce & Gabbana, and Gianfranco Ferre. There's a snowman and a video-bubble tree, a tree made of silk cushions and another crammed with fake diamonds. They look good as smart decorations in the marble foyer of a bank; they wouldn't look out of place in the first-class lounge at an airport. But if Coutts were in Seattle, the windows would have been smashed. The exhibit carries only the faintest echo of anything to do with trees, and merrily trashes the essential beauty of the custom - that little touch of forest we introduce to our festive living-rooms.
Like many of our most ancient traditions, Christmas trees are neither ancient nor British; and certainly there is nothing Palestinian or biblical about our taste for Nordic spruce.
This has roots in Roman and pagan customs (the Druids celebrated the winter solstice with mistletoe; the Saxons preferred holly and ivy) but the illuminated conifer was a German idea, perhaps even a revelation of Martin Luther's. One night, the story goes, he was staring at the trees, thinking idle thoughts about religious upheavals, when he was struck by the transcendent grandeur of the fir tree before him, quiet, solemn and haloed by the glittering night sky. He installed one in his home, with candles to symbolise the stars.
The idea didn't catch on here until 1840, when Queen Victoria and her German husband, Prince Albert of Saxe-Coburg, were pictured at Windsor castle with their Prussian tree. So the tradition of Christmas trees, though it seems part of our folklore, is in fact only 160 years old - a baby, as traditions go.
Intriguingly, it was taken up as a symbol of avarice almost as quickly as it had been assimilated as an emblem of holiness. An 1856 engraving in the Illustrated London News shows people lunging and scrambling for the trophies hanging from the tree's branches, wholly neglectful of the pleading angels hovering above. The modern market in Christmas trees is opaque. Talk to traders in Portobello or Columbia Road and they'll turn away, reluctant to talk in any great detail about the exact provenance of their trees. This isn't altogether surprising: a good many clearly marked Nordic trees in fact come from Surrey. Christmas trees don't have serial numbers, and burglary is common.
Every year, farmers look aghast at empty fields which have been plundered in the night. The botanical dictionaries list an alarming number of pests threatening to the Norway spruce (Picea abies) - the green spruce aphid, the red spider mite, the conifer spinning mite, spruce gall adelgids, the European spruce sawfly and spruce budworm - but one of the most dangerous is left till last: theft.
But if the market is unruly - the trees are trucked in from Denmark and Scotland and sold from the back of lorries in petrol stations and on roadsides - it is also very substantial. An estimated seven million trees will be sold in the next few weeks, which means that the market is worth, at a rough guess, more than pounds 150m.
We're getting pickier. We want more expensive varieties that do not shed their needles, and ecological considerations are fuelling a small boom in pot-grown trees, which don't need to to be tipped on to the bonfire in January.
"You have to be careful about potted trees, though," said Simon Haynes, of Clifton Nurseries in London's Maida Vale. "Not all the trees in pots have been pot-grown. They won't last." A tree that has been crudely shoehorned out of a field and into a pot will have lost its thirstiest and most efficient roots, the ones that curl around the rim of the drip area, the perimeter formed by the wide branches. Most potted trees have lost these roots; you can drown them in water, but you cannot make them drink.
If we think of the modern Christmas tree as Norwegian, that is probably because of the annual lighting-up ceremony in Trafalgar Square. Every year since 1947, the city of Oslo has donated a tall spruce to the City of Westminster, as a gesture of thanks for British sacrifice during the Second World War. It has become one of the fixed points in Britain's Christmas countdown, and on Thursday night the mayor of Oslo threw the switch on this year's tree. The tree itself looked just right - tall, dark, handsome and, well, spruce - but the ceremony couldn't help feeling a touch dog- eared. A crowd composed mainly of tourists clustered around to giggle at the number plates on the official cars (WE 1 on the Mayor of Westminster's Daimler; 1NWY on the Norwegian Volvo). A camera unit from a children's TV network, set up between the paws of the lions at the foot of Nelson's column, rustled up some Norwegian children, told them what to say and then interviewed them. "So that's it from Trafalgar Square," she said. "And coming up, Power Rangers in Space!"
The choir of St Martin in the Fields sang a few carols, with the choirmaster gamely trying to stir up some support from the crowd. "I could hear them," he said, indicating the boys in their chilly-looking red and white robes. "But I couldn't hear you." What with the traffic and the December wind, it felt like a desperate bid for unlikely rock-star glamour. I half expected him to tap out a few thuds on the microphone and shout, "HELLO OSLO!!!"
I put my hands over my ears and tried to feel what Martin Luther had felt, tried to sense the transcendent spirit that drifted through those dark leaves and glimmering lights. But it was hard. Buses and taxis groaned and blared up towards the National Gallery, bearing huge advertisements for the millennial Christmas offerings of the brother-and-sister duo, Will and Delia Smith.
Pigeon droppings lay deep and crisp and even on the cold concourse between the fountains. The leader of the brass band, perhaps not used to the high- power sound system strung from the lampposts, could be heard to mutter: "OK, let's do `Jingle Bells' next." The essential ingredients - the dark German forest, the night sky of northern Europe, the sense of humankind huddling for shelter in the cold night - were all missing.
And these days, carol medleys performed by brass ensembles sound like nothing so much as a shopping centre. One blast of "God Rest you Merry Gentlemen", and I was fighting to quell a panicky reflex to buy Badedas for Grandma. Still, it was better than trying to find a Christmas Tree on the Internet. So much has been written in recent months about e-shopping that I assumed, like any publicity-victim, that ordering a tree would be a piece of cake. Not a bit of it. I clicked and browsed, trawled and searched, but came up empty-handed. I found fake Roman busts, rattan trays, angora drapes and chrome-wire fruit baskets; I found zip-top polar fleeces and portable CD players; I almost found myself bidding for a 10-minute spin in a Hunting Jet Provost T Mk3 1966 XR679. But there wasn't a Christmas tree in sight, let alone one in site. My memory failed half a dozen times, and access was bluntly refused more than once. When I typed "Christmas tree", I was dismissed with a curt "No matches found". I'm sure this was my own fault, but when it comes to Christmas chopping, it seems our brave new technology is of little help.
Everyone knows that Internet euphoria has run a long way ahead of the dull, push-me-pull-you reality of electronic shopping. Probably, in some remote galaxy of the Web, there is a sparkling little outfit called Trees u Like or Trees R Us, with millionaire directors and a listing on Wall Street. I wish them luck, and they may need it. According to the Mintel Report on Christmas shopping habits, only 3 per cent of us intend to use the net for our gift buying this year (exactly the same percentage as last year - perhaps the market has peaked). Considering that 5 per cent of us intend to hop over to France for some cross-Channel shopping (even without duty free) this December, that is a very damp response indeed. And so far as Christmas trees are concerned, it looks as though this year we'll be scouting one out at a lay-by somewhere, as usual.