This holiday season, you might be hoping to catch someone under the mistletoe — or maybe that’s a prospect you’d like to avoid. Mistletoe’s association with kissing and Christmas in the Western world goes back to the 19th century, but it’s been linked to romance and fertility since ancient times.
“Mistletoe is actually an evergreen plant,” said Marcos A. Caraballo-Ortiz, a botany research associate at the Smithsonian’s National Museum of Natural History who studies the mistletoes of the Caribbean Islands. This evergreen status, combined with the fact that it keeps its fruits in the winter, turned mistletoe into a symbol of fertility and vitality.
To celebrate the holidays, Caraballo-Ortiz shared some more fun facts you might not know about the plant behind the tradition.
Mistletoes are parasites
You read that right — all mistletoe species are parasites. But it’s a little more complicated than the Hollywood depiction of parasitism. Mistletoes are specifically known as hemiparasites, a term for a plant that gets some or all of the nutrients it needs from another living plant, explained Caraballo-Ortiz. In a mistletoe’s case, it attaches to the branches of a woody tree or shrub and siphons water and food from the host.
But mistletoes aren’t incapable of going it on their own. “They can do some photosynthesis” in the early part of their life cycle when they first attach to their host tree, he explained. And mistletoes don’t commonly kill their host. Sometimes the host plant will experience stunted growth resulting from the uninvited guest. “I have seen trees that have branches with so many mistletoes on them, the branch can die,” Caraballo-Ortiz said. “But some of them you don’t notice on the host at all.”
They don’t grow out of the soil
Because of their parasitic nature, mistletoes don’t ever touch soil. “They don’t touch the ground,” Caraballo-Ortiz said. Instead, when a mistletoe seed drops onto a potential host plant, it “grabs” on and starts to germinate. “Their fruit is covered with a sticky substance called viscin,” explained Caraballo-Ortiz. “It’s like a fiber that allows the seed to attach on the branches of trees.” The seed uses its own photosynthetic powers to produce a hypocotyl, or stem, that pokes out and kicks off the mistletoe’s growth. It then forms a structure called a haustorium, which acts like a root by burrowing into the host branch and funneling water and nutrients from host to parasite.
Some mistletoes spread their seeds by exploding, while others depend on birds
So how do mistletoes manage to land their seeds on far-flung tree branches? Some species shoot seeds from their fruits by building up water pressure within their berries and exploding. “It’s really cool — they can fly really long distances,” said Caraballo-Ortiz, in some cases as far as 20 feet and at speeds up to 30 miles per hour.
But many mistletoes, including most of the tropical species Caraballo-Ortiz studies, get a big assist from birds. Many birds choose to chow down on mistletoe berries, which have the plant’s seeds tucked inside them. “Birds really like them, because they have special sugars and different colors and textures,” Caraballo-Ortiz said of the fruits. “And it often is the only fruit available in winter in many cases.” As the birds flit and fly from branch to branch, they deposit the seeds through their poop. The sticky viscin latches the seeds to the branch, leaving them primed to begin germinating and burrowing into a new host tree.
Some mistletoes eat other mistletoes
Most mistletoes are adapted to use trees as their parasitic host. But some mistletoes take it a step further and parasitize other mistletoes. It’s not uncommon for birds to disperse one mistletoe’s seeds while they feed on the fruits of another mistletoe. Because mistletoes are apt to latch on to any plant they can, some species have adapted to using these secondary mistletoes as a host. “The birds have been constantly throwing seeds over other mistletoes,” Caraballo-Ortiz explained, “so they’ve taken advantage of that.”
In these cases, you can find a mistletoe hanging off a mistletoe hanging off a tree, stacked in a sort of parasitic plant chain. These mistletoes have become what’s called a hyperparasite: a parasite that parasitizes another parasite. The hyperparasite pulls food from the first mistletoe, which in turn is siphoning nutrients from the tree.
Mistletoes grow almost everywhere on Earth
While mistletoes are associated with the holiday season and cold weather in America, there are over a thousand known mistletoe species growing all over the globe. “You can find them almost everywhere, except in extreme environments,” said Caraballo-Ortiz. “But even some of them are adapted to very cold places like Siberia or northern Canada.” These mistletoes have special adaptions that help them tolerate the cold, while other species are adapted to survive in bone-dry conditions. “As long as they have a host, they can find a way,” he said.
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