Plant Palette

Plant Palette

Mistletoe

Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture
jaschult@illinois.edu

We may associate mistletoe with kisses during the holiday season, but mistletoe is a bully in the plant world. OK, technically it is a parasite. But practically speaking it is like a schoolyard bully looking for a victim to steal lunch money from.

There is a lot more to mistletoe than I ever thought. Honestly, my experience with mistletoe is limited to the artificially-colored preserved sprigs that pop up in stores this time of year. I was shocked to find there are over 1300 different species of this plant parasite worldwide. Mistletoes are also related to the sandalwoods, a group of parasitic plants mostly known for their scent.

Mistletoe is an obligate parasite, meaning it cannot live without its host plant, but it is also considered a hemi-parasite ("half-parasite"). For mistletoe, this means it absorbs only water and materials dissolved in water carried through the host plant. Mistletoe carries out photosynthesis to produce its own food sources, just like any other green plant. The range of possible host plants depends on each mistletoe species. Some have very broad host ranges, others very narrow.

Several of mistletoes exist in the U.S. and are typically considered pests, as they may disfigure or kill its host plant over time. A particularly troublesome mistletoe is Dwarf Mistletoe (Arceuthobium spp.), which looks nothing like the mistletoe we see during the holidays, but is still a mistletoe. It is only about an inch tall and its leaves resemble flattened scales.

Unlike other mistletoes, the dwarf mistletoes are more parasitic on their host plant, robbing them not only of water, but of nutrients as well. If there are enough dwarf mistletoes on a given tree, they will slowly starve the tree to death. They parasitize conifers other than junipers or cypress, and each species is host-specific, meaning a given species can only grow on specific species of conifers.

The only connection mistletoe has to its host plant is through a complicated structure called a haustorium. The formation of the haustorium begins when a mistletoe seed lands on a branch. Each mistletoe berry contains one seed, and each seed may contain more than one embryo. The berry is also filled with a sticky glue-like substance that helps the seeds stick to birds that eat the mistletoe berries, and ultimately the branches where the seeds germinate.

Generally the first year of growth is dedicated to forming the haustorium. As the embryos germinate and grow, they grow root-like structures called holdfasts that penetrate the host's bark and fuse with the host tissue. The fused tissues grow rapidly, distorting the wood and forming a large lump, or gall called a haustorium. This is the interface where the mistletoe diverts water and solutes from the xylem vessels of the host plant for its own needs. Depending on location of the haustorium, the remainder of the host branch may die from lack of water.

The specific events, mechanisms, and hosts responsible for a mistletoe plant being able to grow vary widely among species. The individual mistletoes themselves come in an incredible assortment of shape, size and color. Most mistletoe hosts are woody plants, but cacti and even grasses have supported specific mistletoe species.

Many mistletoes are fairly inconspicuous, with small flowers and fruits at specific times of year, but many put on spectacular displays of large colorful flowers. Some depend on birds or other animals to disperse their seed, others have evolved berries that explode with enough force to spread seed far and wide. Scientists have even found specimens where one mistletoe plant growing on its woody host has two additional mistletoes stacked on top of each other, each parasitizing the mistletoe it was attached to!

This diversity has attracted attention from researchers. Dr. Dan Nickrent of SIU Carbondale used DNA fingerprinting to construct a pedigree of the enourmous group of plants labeled as mistletoes, hoping to find a common ancestor for their parasitic lifestyle. Instead, he found that among mistletoes, the parasitic lifestyle had evolved five independent times.

How in the world did a parasite like mistletoe become associated with kisses and the Christmas? There are lots of legends told about mistletoe, but the familiar association with Christmas has its roots in Europe. The ancient Druids used a golden sickle to cut mistletoe from their most-revered tree, the oak as part of ceremonies celebrating fertility that included human sacrifice. Looking to the Druids for a direct link to kissing your sweetie at Christmas seems a bit of a challenge.

A Norse myth is believed to be the origin of mistletoe's link to kissing. According to the myth, an arrow made from mistletoe killed Balder, the son of Frigga, the goddess of love and beauty. The other gods resurrected Balder, and Frigga's tears of joy formed the white berries seen on the common European mistletoe species, Viscum album. Legend says that the berries represent kisses bestowed by Frigga to people that meet under the mistletoe. Some say that a berry should be removed from the mistletoe for each kiss, and that the mistletoe loses its "power" once all the berries are removed.

The name mistletoe comes from second century Anglo-Saxon descriptions of the plants as "misteltan," derived from the word "mistel" meaning dung, and "tan" meaning twig. These early people associated the appearance of mistletoe with droppings from birds on tree branches. Not exactly the most romantic legend around, but they did think there was some magical process at hand that spontaneously generated the resulting mistletoe plants.

A French legend links mistletoe to Christmas through Christ's crucifixion, using the fact that mistletoe is poisonous. According to the legend, the original mistletoe plant was growing on the tree that was made into the cross on which Jesus was crucified. This made the mistletoe cursed, causing it to be forever poisonous and a parasite, never allowed to grow independently on the ground.

Mistletoe may be poisonous, but at various times it has also been considered an aphrodisiac. Medically, it can be an abortifacient, meaning it will cause miscarriage of a pregnancy. Some writers have suggested this is one reason mistletoe is linked with fertility, which in some cultures also meant uninhibited sexuality and promiscuity. In any case, ingestion of mistletoe is likely to cause severe cardiac, digestive, and neurological malfunction and death is likely. You'd be wise to search for aphrodisiacs elsewhere!

Interesingly, the mistletoe celebrated in European folklore is usually the species Viscum album. The mistletoe sold in the U.S. is an assortment of species from a totally different genus, Phoradendron.

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