12 Days of Christmas Science: Daisy chains of parasites meet mistletoe madness.

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The name mistletoe originally referred to the species Viscum album (European mistletoe, of the family Santalaceae in the order Santalales). A related species with red rather than white fruits, Viscum cruciatum, can be found in southern regions of the Iberian Peninsula as well as in Morocco and in southern Africa. Viscum album was introduced to Northern California in 1900.

The eastern mistletoe native to North America is called Phoradendron leucarpum and is a distinct genus of the family Santalaceae.

European mistletoe has smooth-edged, oval, evergreen leaves borne in pairs along the woody stem, and waxy, white berries that it bears in clusters of two to six. The eastern mistletoe of North America is similar, but has shorter, broader leaves and longer clusters of 10 or more berries.

European mistletoe (Viscum album) fruit, Wrocław, Poland. (CREDIT: Agnieszka Kwiecień)

All forms of mistletoe are obligate hemiparasitic plants that attach to their host tree or shrub by a structure called the haustorium, through which they extract water and nutrients from the host plant. Over the centuries, the term mistletoe has come to include many other species of parasitic plants with similar habits, found in other parts of the world.

As hemiparasites, mistletoes contain chlorophyll so they can actually make a lot of their food themselves. They mostly parasitize other organisms for water. Most tropical mistletoes are pollinated by birds, most temperate species by flies and wind. Fruit-eating birds distribute the seeds in their droppings or by wiping their beaks, to which the seeds often adhere, against the bark of a tree. 

European mistletoe. (CREDIT: Guillermo César Ruiz)

One particularly interesting type of mistletoe is the Dwarf mistletoe (genus Arceuthobium). It feasts mostly on coniferous trees. Rather than relying on animals or wind to disperse its seeds, the Dwarf mistletoe self-detonates in an explosion of liquid and sticky seeds. It employs hydrostatic pressure to shoot their seeds away from the parent plant at speeds of ~50 miles (80 km) per hour. 

After a mistletoe germinates, a modified root (haustorium) penetrates the bark of the host tree and forms a connection through which water and nutrients pass from host to parasite. Mistletoes are slow-growing but persistent; their natural death is determined by the death of the hosts. The only effective control measure is complete removal of the parasite from the host.

Mistletoes are like crazy parasites with no shame in their game. Mistletoe can sometimes be found hanging off other mistletoe while hanging off a tree. It’s like a daisy-chain of parasites. These mistletoes have become what’s called a hyperparasite: a parasite that parasitizes another parasite. 

Viscum album growing in Düsseldorf-Himmelgeist. (CREDIT: Frank Vincentz)

While mistletoe has been an important herb dating back to antiquity, its origins as mistletoe as we know it probably has its origins in Celtic culture in the 1st century A.D. when it was considered a symbol of virility and vivacity. The fact that it was able to blossom during the winter may have played a role in that interpretation.

The tradition of kissing under mistletoe is a relatively recent practice. Nobody is certain how the herb made the leap into Christian Christmas tradition. One theory suggests that it first became popular among servants in England during the 18th century before catching on with the middle classes.

Kissing under mistletoe was practiced in the early United States as well. Washington Irving referred to it in his collection of essays and stories, The Sketch Book of Geoffrey Crayon, Gent. (1820). According to Irving, each time a couple kissed under a mistletoe sprig, they removed one of the white berries. When all the berries were gone, so was the kissing magic.


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