Mistletoe makes its annual appearance each December as millions of Americans and Europeans hang a sprig of it in their doorways during the holiday season. According to custom, if a woman is caught standing under the mistletoe, a man may kiss her.

So what is it about this little plant that gives it its power to make people pucker up?

For centuries, mistletoe has been considered a plant that increases life and fertility. Norse legends tell the tale of Balder, son of the goddess Frigga.

As the legend goes, Balder was killed by an evil spirit with an arrow made of mistletoe. Saddened by her son’s death, Frigga wept tears of white berries, which brought Balder back to life. Frigga was so overjoyed that she blessed the plant and promised a kiss to all who passed beneath it.

Mistletoe traditions have evolved over time. In ancient times, visitors would kiss the hand of a host under the mistletoe when they arrived.

Since then, traditions have grown a bit more personal. Today, any couple caught standing underneath the mistletoe should prepare to pucker up!

So what, exactly, is mistletoe? The far-from-romantic answer is that it’s a parasitic plant, which means it depends on another plant for survival.

Mistletoe can only grow if its seeds are carried to a “host” tree by birds that have eaten mistletoe berries. Typically, a bird will squeeze a mistletoe berry in its beak, squishing out a sticky, coated seed.

The bird eats the fruit and cleans the sticky coating, called “viscin,” off its beak by wiping it against a nearby branch. As the viscin hardens, the seed becomes firmly attached to the host tree.

The mistletoe then invades the host, “stealing” nutrients and water from it. In fact, the scientific name for American mistletoe (Phoradendron) is Greek for “thief of the tree.”

More fun facts about mistletoe:

• Birds can eat mistletoe berries, but they’re highly toxic to humans.
• Approximately 20 species of mistletoe can be found on the endangered species list.
• Celtic druids believed that mistletoe contained the spirit of the tree in which it grew; this was the only part of the tree that stayed green all winter.