Of all the customs that have lingered on in Britain from ages past, the idea of kissing under the mistletoe at Christmas must be one of the most enduring. No one can see a bunch hanging from an apple tree in summer, or even hear the name mentioned in passing, without immediately thinking of that distinctive winter activity. Those of a more fertile imagination find themselves dwelling on notions of druids with golden sickles in some unspecified ancient religious ceremony conducted beyond the mists of time in a culturally significant grove of oaks. But why, in a modern society that disregards so many of our ancestral jollifications, should this one have universally persisted among old and young? I wonder.

Mistletoe is a very curious plant, at least in Britain. It’s only capable of functioning as a hemi-parasite, which is to say that it needs to be attached to the bark of certain trees, from which it derives some of its nutrients, but makes the rest for itself using the power of the sun and the chlorophyll in its leaves. It can’t live on its own in the soil because, effectively, it has no roots. These are replaced by mysterious, and outwardly invisible, tissues called haustoria, which work their way through the bark of the host tree and help themselves to a modicum of the sugary fluids in the vessels that transport chemical compounds up and down the tree.

No doubt, this has a slightly debilitating effect on the tree, but nothing too devastating, and both parties seem to be able to muddle along together in a rather British way. Mistletoe, significantly, is evergreen, and can therefore do a lot of the work for itself, although it is, in human terms, a selfish plant, as it helps itself to good things from its host while apparently providing nothing in return.

Mistletoe in midwinter is a striking sight. Typically, it’s attached to a deciduous tree, so that it only becomes obvious in the dead of the year. Part of its attraction to us must be that it is full of life and vigour when everything else, including its host, appears to have shut up shop for the duration. Not only is it evergreen, keeping its biological engine quietly tootling along irrespective of the season, but it bears its fruit in the shortest and darkest months.

Hanging from a tree, it looks as if it has been tied up there, like a primeval hanging basket. This bunch-like appearance is due to its habit of always forming pairs of buds at the shoot tip, like lilac, so that it’s constantly branching out rather than merely getting longer like most woody material. And there’s something simultaneously elegant and unearthly about that combination of curving, tough, palely green leaves mingled with strikingly pearl-like berries that searches its way into some ancient corner of our collective imagination.

For some reason, presumably connected with climate, mistletoe is only found in certain parts of England and Wales. Its heartland is Herefordshire, Gloucestershire and the adjacent counties, where any dog-walker will encounter it constantly on a winter’s day, hanging from a surprising range of trees. Apple orchards are, of course, the best-known habitat, apparently ideal for mistletoe because it prefers to grow in thinly scattered woodland with access to plenty of light. I’ve seen it on many other trees, including some unexpected ones. Older perry pear trees often support big bunches, which survive simply because the tree is tall and the mistletoe is out of reach.

Poplars of all kinds are classic hosts: if you take the train from Bristol Temple Meads to Cheltenham Spa and further on towards Worcester or Birmingham, you can see poplars in the Severn Vale festooned with the stuff.

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Among the less common hosts, I’ve sometimes seen mistletoe on lime trees in the countryside, reasonably enough, as, like poplar, the bark and timber are soft, but most surprisingly on Robinia, a much harder wood. And whereas apples, pears, hawthorn and so forth are all quite closely related, poplars, limes and Robinia are not, so we may infer that mistletoe is promiscuous in its domestic arrangements.

But what about oak, I hear a strangled cry, the locus classicus of mistletoe? If druids were symbolically harvesting it in groves of oak, where can we go to see this ancient and evocative scene? Well, I don’t know, as I’ve never seen it on an oak. Cue a flood of contradictory correspondence, which I will be delighted to read.

Mistletoe spreads between these various host trees by birds-often by thrushes and their friends and relations, such as blackbirds, taking the berries. I imagine these are young and naïve birds, as the desired berry turns out to be glutinous beyond belief, and the apprentice bird is then reduced to desperately trying to smear off the gunge onto a conveniently rough young branch.

If the match is a good one, the seed, stuck fast by this natural bird-lime, stealthily begins to penetrate the host tissue with its aforementioned haustoria, resulting initially in a swelling on the twig, then, in a year or two, in the gradual development of the familiar green bunch. Many of us have tried to emulate this process over the years on apple trees in the garden, usually without success. This is typically because the mistletoe sold in shops is often imported or too dried-out to be effective. Try taking a few berries from a living bunch in about March, when the seeds are fully ripe, and you may have more luck. I wish you every success cleaning your fingers afterwards.

Beyond these shores, the startling news is that there are other species, indeed genera, of mistletoe. Ours is Viscum album, which has two sub-species on the Continent, one dependent on pines and another on firs. There are five other species in Europe as a whole. But consider this: there are at least 84 species of mistletoe in Australia. They hang, in my limited experience, in vast brown bunches from eucalyptus trees, looking for all the world like dead branches of the relevant host. And they are much more determined than ours, as they will, in time, kill the gum tree altogether.

So, our own mistletoe, whose bunches at Christmas trigger conflicting reactions in the minds of those approaching them, is botanically just a small twig of a great family tree of worldwide mistletoes. Ours seems to be the only one endowed with romantic associations, hence the big annual wholesale market of cut bunches at Tenbury Wells in Worcestershire, ready to fan out across the country to parts where it’s not found in the wild and where puckering up needs a little encouragement. And please remember, for each kiss, a berry must be removed.

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