On December 20th, 2015 we learned about

Mistletoe’s muddled history puts us kissing under a parasitic plant

Is anything more romantic than a parasite? Perhaps, although you’d be hard pressed to mention any other parasite that, by tradition, demands kissing from people standing below it. While the details of ‘a kiss below the mistletoe’ have changed over the years, they’re divorced enough from the ecology of the plant itself to leave room for plenty of speculation as to why we celebrate Christmas with trees in our homes and tree-killers over our doors.

From killing to kissing

The history of mistletoe in Europe predates any clear, written records. Norse mythology singles it out as the only thing capable of wounding Thor’s grandson, Baldur. An arrow made of mistletoe kills the god, and so we might now hang the plant in remembrance. While kissing might be part of that observance, it’s likely more connected to the role mistletoe played in fertility rituals of the ancient druids, possibly tied to the gooey, semen-like appearance of the plant’s berries. This makes the expectation of kisses a little more logical, and by the 1800s, rules were actually spelled out explaining that each kiss required the removal of one berry from the plant, ceasing when all the berries were picked.

Sugar sucker

Mistletoe’s white berries are actually an important bridge to the actual plant itself, without any kissing involved. Species like Viscum album grow sticky, white berries along their stalks. The stickiness is key to the plant’s life-cycle, as they help the seeds avoid falling to the ground. Instead, they’ll stick to birds who arrive to snack on the berries, only to be wiped off on other trees. While most seeds need to reach new, fertile, ground to grow new plants, mistletoe is a parasite that instead grows out of the trunk and branches of a living tree, and so it is instead ‘planted’ without dirt.

As a parasite, mistletoe steals sugar and nutrients from its host tree. It will sometimes sprout just enough stalks to expose its berries, and only some species grow leaves that actually perform photosynthesis. Even in those cases, the photosynthesis isn’t effective enough to fully sustain the parasite, and so the mistletoe still leeches calories away from its host at the cost of that plant’s growth or even life. The less efficient varieties often have a more yellow coloration as they depend even less on using their own chlorophyll for photosynthesis. All this evolved from earlier sandalwood plants, which used similar concepts, but only from the tree’s buried roots instead of the upper boughs.

So enjoy a kiss this Christmas, but just remember the trees, and Norse god, that suffered for each smooch.

Source: Mistletoe: The Evolution of a Christmas Tradition by Rob Dunn, Smithsonian.com

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