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Mistol is the Anglo-Saxon word for "dung." And tan means "twig." So its common name means "dung on a twig." The scientific name of the common American species, Phoradendron, is not much better. It means "thief of the tree" in Greek.

Urban forester Todd Watson spends most of his time trying to kill the plant. He says once it infects his region's elm and oak trees, there is little he can do to stop it.

"It's more difficult to control than an insect," says Watson, who teaches at Texas A&M University in College Station, Texas. "It's like those things in the movie Alien, it gets inside the tree."

Still, there are those who love it.

From a Sticky Seed

Job Kuijt of the University of Victoria in British Columbia has spent years studying and classifying the parasitic plant. So far close to 2,000 species of mistletoe have been identified and Kuijt believes there may be many more — some that may never be found in time.

"Many of the colorful ones in the tropics and ones that we haven't even found yet in South America are likely going extinct," he says. "Once those forests go, they go too."

Mistletoe doesn't stand a chance during deforestation, since the plants literally live off the trees. Different species of mistletoe infect their hosts in different ways. The kind most Americans identify as mistletoe, Phoradendron, is found from New Jersey to Florida and west through Texas.

This mistletoe's life begins within a seed, which is coated in a sticky film. Some varieties of a needle-leafed plant known as dwarf mistletoe have a spring mechanism inside its berries that shoot out seeds for distances up to 50 feet.

All other kinds of mistletoe, however, rely on the voracious appetites of birds to spread their seeds.

Although the mistletoe's berries are poisonous to people, squirrels, porcupines, chipmunks and several birds, including robins, bluebirds, mourning doves and grouse, devour the berries and then pass the seeds through their waste. They can also spread the plant's seeds by stepping in another's seed-filled feces and then flying or scurrying to another tree.

Once a mistletoe seed reaches a tree (and the plant is not that particular about what kind of tree it infects), it penetrates the tree's bark using a combination of chemicals and mechanical pushing. Then it begins sucking the tree's water and nutrients.

Plant That Won’t Die

Mistletoe is not a true parasite since the plant still sprouts some leaves to conduct photosynthesis, but it steals most of its sustenance from the host tree.

There is debate, however, over just how debilitating a mistletoe "infection" is for a tree. Robert Bennetts of the U.S. Geological Survey says the Forest Service has often cut down any trees rangers see infected by the plant.

But as he points out, the plants have been a part of forest ecosystems for thousands, if not millions of years.

"It's not as if these plants are sweeping through the forests killing trees," Bennetts says. "They only move about a foot a year. It takes a lot to kill a tree."

The mistletoe is equally hardy.

Just ripping mistletoe from a tree can help the plant from spreading but it won't kill it, since half the plant lives inside the tree. Watson says he has to either cut down entire branches infected by the plant or cover the plant in black plastic bags to suffocate its ability to perform photosynthesis.

Magical Berry

So how did such a ferocious parasite become a quaint part of holiday folklore? No one is quite sure, although it may have something to do with the fact that mistletoe's leaves, although dormant, stay green throughout winter.

In a forest of trees that lose their leaves in winter, such a splash of green can seem magical. Also the way it appears to grow in the air out of nowhere (since it grows from inside the tree) may have added to its mystical appeal.

The tradition of kissing under the mistletoe dates back to the ancient Celtic druids and also to the Romans. Both used tiny extracts of the mistletoe berry to spike their winter solstice festivals. They considered the berry a fertility herb, which could have led to the kissing habit.

Apparently the number of kisses given under a mistletoe sprig is limited by its berries. Every time a kiss is planted, a berry is plucked. When the berries run out, it's time to tear off another plant.

And don't worry, it will grow back.