At holiday parties, watch out for sprigs of mistletoe hanging overhead. According to a tradition that started in England in the early 1600s, if you're standing under a bunch of the thick-leaved plant, anyone is allowed to kiss you.
In ancient Scandinavia, people believed that mistletoe had special powers. They thought it would keep witches away, heal diseases, make peace between enemies, and ensure that a bride and groom would have plenty of children.
The druids, ancient Celtic priests, also prized mistletoe. In a yearly ceremony on the first day of winter, a druid priest climbed high in an oak tree to cut mistletoe with a golden knife, while the other priests sang and danced around the tree.
Even more fascinating than legends about mistletoe, though, is what biologists and other modern scientists have learned about it.
Imagine you are walking in a forest in the western US. About 40 feet away, a bird lands on the branch of an evergreen tree. It jiggles a three-foot-diameter clump of mistletoe high in the tree's branches. Before you can figure out what's going on, the mistletoe shoots out its ripe seeds. Some hit you at nearly 50 miles per hour.
That's how some species of mistletoe spread their seeds. One researcher put ripe mistletoe berries into a paper bag and shook it. The little explosions sounded just like popping popcorn.
Other types of mistletoe spread their seeds through birds and animals. Here's one way it happens: The mistletoe has sticky seeds that birds eat. Some of the seeds stick to their beaks. They wipe them off onto a branch, and the seeds stick there. Birds also may deposit some seeds on branches through their droppings. The word mistletoe may come from the Old English word "mistiltan," meaning dung (mistil) on a twig (tan).
After a seed is deposited on the branch, the sticky gum surrounding the seed hardens, which glues the seed to the tree. Then the seed pushes wedge-shaped tissue into the bark, roots itself in the branch, and grows into a new mistletoe plant.
Mistletoe is nicknamed a "vampire plant" because it is a parasite. That means it sucks water and nutrients from its host tree or shrub, instead of from the soil. Some mistletoe species, such as Australia's 30-foot-high "Christmas bush," appear to be rooted in the soil. But they are underground "vampires." They attach themselves to the roots of grasses and other plants.
This parasitic habit does slow the growth and shorten the life span of the host tree or shrub. This is especially true of the leafless dwarf mistletoe that grows in western North America. Some people who sell trees think of the plant as a pest. So one biologist studied mistletoe to find a way to get rid of it. But after studying it for 50 years, he changed his mind about it being a pest: He discovered it is a much-needed part of the forest ecosystem.
Todd Esque, who works for the US Geological Survey, has also done research on mistletoe. He says there's no doubt that there's a link between the presence of mistletoe in a forest and the presence of birds.
Mistletoe provides food and/or nesting sites for owls, hawks, robins, bluebirds, thrushes, wrens, chipping sparrows, mourning doves, red crossbills, pygmy nuthatches, Western tanagers, and pine siskins, to name a few.
The phainopepla, a silky flycatcher, could not survive in some parts of the dry Southwest, where it lives, without mistletoe. It eats the berries.
Butterflies, honeybees, and a wide range of mammals – from squirrels to elk – eat the mistletoe leaves and berries, too. Mistletoe helps some animals survive the winter, when other foods are scarce. (Don't try eating the berries yourself, though. They are considered poisonous to people.)
Even when mistletoe causes more dead trees in the forest, those trees provide the perfect habitat for cavity-nesting birds.
The kind of mistletoe you might see at a holiday party is called American mistletoe. It grows wild in the eastern US, from New Jersey to Florida. It's similar to European mistletoe, which played such an important role in European folklore.
But these two species make up a tiny fraction of the world's 1,300 species of mistletoe. Many of them look dramatically different from our mistletoe. A rare species found in Ecuador and Peru sprouts red and yellow tube-shaped flowers almost a foot long. Another South American species grows silky blond "hair."
A desert variety called "quintral" lives inside cactuses. Each winter its red flowers burst through the cactuses' skin. Wouldn't that make a good Christmas decoration to stand near if you didn't want to be kissed?