BERLIN — Mistletoe. The word conjures winter holidays, office parties, stolen kisses, romance. But appreciation of the plant is no modern thing. Kissing under the mistletoe has been happening for at least 200 years. And some 2,000 years ago, the druids in what is now Britain venerated the plant when it grew on an oak. When they found it, they dressed in white, harvested it with a golden sickle, and sacrificed two white bulls. Or so says that great Roman, Pliny the Elder, in his “Natural History.” The druids — Pliny continues — believed that mistletoe could make barren animals fecund, and that it was an antidote to all poisons.
In the spirit of the season, I thought I’d investigate the plant for myself. So I bought a sprig from the florist down the street. I would’ve liked to pick it personally — I can see great balls of it hanging in the trees. But because mistletoes are spread by birds, they tend to be on thin branches way up high, out of easy reach of a wingless creature like me.
My sprig is from the plant the Romans called viscum: the common European mistletoe. But there are many other species — more than 1,300 in all. Despite the associations with winter, many of them grow in the tropics. There are even mistletoes that grow on mangrove trees. For that’s the point: all mistletoes are parasites, and grow on other plants, stealing water, minerals and other nutrients from their hosts. Some mistletoes even grow on other mistletoes. (To a botanist, “mistletoe” refers to a way of life rather than to one particular family of plants. It’s a habit that’s evolved independently several times.)
I said they were parasites. But that’s only half right. Most mistletoes are “hemiparasites.” This means they don’t rely on their hosts for all their needs: Instead, they harvest the sun’s energy to make some sugars for themselves. Nonetheless, if you’re a tree, you don’t want to be festooned with them. Two or three shouldn’t be a problem, but dozens of mistletoes can lead to water stress, insect infestation or even death.
The tree’s point of view is only part of the story, though. For many other life-forms, mistletoes are an important resource. Some birds collect the leaves to make a lining for their nests. Others nest in mistletoe clumps. And plenty of animals drink mistletoe nectar, or eat the leaves or the berries.
The berries. These are a key part of the plant’s allure. In Europe and North America, mistletoes make berries in winter. They thus provide food for birds when most other plants don’t. The winter fruits were, perhaps, part of why the druids thought the plant had magical powers. And presumably, those berries explain why mistletoe became a traditional decoration at this time of year, and how it came to be used in a kissing ritual.
My sprig has lots of berries. I pick one. It’s pleasantly firm, and looks like a small white grape. I’m not going to eat it, though. Even today mistletoe extracts are reputed to have medicinal properties and are sometimes prescribed as cancer therapies. But the berries of some species are said to be poisonous to people, and I’m not keen to find out if it’s true. I do, however, want to examine this berry more closely.
I squeeze it between my fingers. Ooh. The skin bursts open and the flesh inside shoots out. The skin is thick, while the flesh is jellylike and contains a seed. The whole thing smells faintly of — what? Something delicate. Like a pear. In fact, it smells delicious.
But watch this: The flesh is sticky, and forms strings and ribbons between my thumb and forefinger. For the mistletoe, this viscous goop — and by the way, viscous comes to English from viscum — is crucial. The stickiness means that, after eating the berries, birds often regurgitate the seeds and then wipe their bills on twigs — leading to the seeds’ getting glued to the tree, where they can germinate and begin the cycle anew. Less delicately, the stickiness persists through the gut, making it hard to defecate mistletoe seeds with any grace. To deal with this, birds like the silky flycatcher, Phainopepla nitens, that are mistletoe specialists, have evolved a “waggle dance” — also known as butt-wiping behavior — where they clean up by rubbing themselves against twigs. Indeed, the association with bird droppings may even be the origin of the word: Some evidence suggests that “mistletoe” comes from Anglo-Saxon for “dung on a twig.”
Eventually, I hold my hand under the faucet and rinse the goop away. I put the sprig in water. As I gaze at it, my mind fills with white-clad druids and golden sickles, magic and romance.
Olivia Judson is an evolutionary biologist and writer.