Scientists and other nerds love a good cocktail party fact, and one of my favorites for the holidays is that mistletoe is actually a parasite. While mistletoe is green and can get its own sugar from photosynthesis, its roots are modified to attach and penetrate through the bark of a tree, sucking out water and mineral nutrients from the host.
Mistletoe is usually pollinated by insects or by birds, which eat the berries and then help spread the seeds to other trees. The seeds are large and extremely sticky, coated in a sugary molecule called viscin. The seeds can either get stuck to the birds' beaks, who then rub them against tree bark to get them off, or can be digested and pooped out onto trees, still sticky after their transit through the bird's digestive system. One possible etymology of the word "mistletoe" is based on this aspect of the plant's life cycle, coming from the german Mist--dung--and Tang--branch.
The holiday traditions around mistletoe have a long history, rooted in the mythology of Scandinavia and the practices of the ancient druids. According to the 1898 Dictionary of Phrase and Fable, the tradition of kissing under the mistletoe came from a story about the Norse god Loki, who made an arrow out of mistletoe that was used to kill the god Balder. After that:
the mistletoe was placed in future under the care of Friga, and was never again to be an instrument of evil till it touched the earth, the empire of Loki. It is always suspended from ceilings, and when persons of opposite sexes pass under it, they give each other the kiss of peace and love in the full assurance that the epiphyte is no longer an instrument of mischief.
So next time you pass under some mistletoe hung in a doorway for Christmas decoration, don't just look for someone to smooch--think of mischievous gods, plant parasites, and seeds stuck in bird poop.