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By Emily Schwartzberg and Mary Ryu

Every December, couples kiss under the mistletoe to celebrate their love during the holidays. Have you ever wondered how the tradition started? We found out the history and story of why people kiss under the festive plant.

What is a mistletoe?

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According to Ryan Guillou, a Curator at the San Francisco Botanical Gardens, a mistletoe is a hemiparasitic plant that originated primarily from the British isle in Northern Europe.

So, what does hemiparasitic mean? Guillou explained that hemiparasitic plants, like mistletoe, derive some or most of its energy source from other plants like a parasite, while still engaging in photosynthesis.

Although commonly found in Europe, the myriad of mistletoe species spread geographically and can now be found in Central Valley, California. If you happen to forage through the trees, you may find green balls up to 2 feet wide forming on the branches -- and that's the mistletoe!

One or two mistletoe on a tree branch may not be an issue, but when trees become infested, the invasive plant often drains the host, snapping and breaking the tree branches with it's weight. Destroyer of trees and other plants, the mistletoe survives, germinating berry seeds to birds' delight and human harm. Poisonous for humans, these seeds would be picked up by birds and often land firmly on tree branches with their sticky nature. The relocation of the seeds to new tree branches is pivotal to the life cycle and survival of the mistletoe plant.

While the plant may not seem the most romantic, mistletoe has become the epitome of love and affection during the holiday season.

What does the mistletoe symbolize?

The Greeks fawned over mistletoe for its healing powers. The Roman naturalist Pliny the Elder viewed the mistletoe as a treatment for poison, epilepsy and more. The Druids regarded the mistletoe as a symbol of vivacity, awed at how the plant blossomed in the coldest of winters. How do these beliefs correlate to romance?

According to The Smithsonian, the romantic symbolization of the mistletoe comes from ancient Norse mythology, which is a body of myths of the North Germanic peoples. In the tale, Baldur, the grandson of a Norse god, woke up convinced that each and every plant and animal on earth wanted to kill him. Fearing for Baldur's life, his mother Frigg, the goddess of love, pleaded with all the plants and animals of the world to promise not to harm her son. Except, she forgot to secure her son's life with one living being -- the mistletoe. Stabbed by the god Loki with an arrow made from mistletoe, Baldur's fear became reality.

Today, we hang mistletoe over our doors and kiss beneath the plant as a reminder of what Baldur's mother forgot. For all those looking for a happy ending, some optimists believe Baldur was resurrected from the dead. And to those romanticized minds, the mistletoe was deemed a symbol of love by Frigg, vowing to kiss all those who wandered beneath it.

Why do people kiss under mistletoe during Christmas?

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Fast-forward a few centuries and the history of kissing under the mistletoe continued to thrive. By the 18th century, it became ubiquitous with Christmas cheer. The mistletoe tradition blossomed, first among English servants and eventually expanding to the middle class. The basis of the mistletoe tradition was that men were allowed to kiss any woman seen underneath mistletoe -- and refusal to accept was considered bad luck.

Some are not entirely confident in the story that lies within Norse mythology, and believe that the sticky seeds that cling to the tree branches are symbolic of a kiss, never falling.

Whether the tale of Baldur appeals to your romantic side or the scientist within you falls for the story manifested in nature, a kiss under the mistletoe is filled with tradition and rich history.