Mistletoe: A Festive Parasite

Mistletoe: A Festive Parasite

I'm sure you have heard of mistletoe, that cute little sprig that hangs overhead to garner kisses during the holiday season.  But did you know that it's actually a parasite?

Mistletoe is a hemi-parasite, which means that it both draws sustenance from the host plant, and uses photosynthesis to feed itself.  (The chlorophyll it uses for photosynthesis is responsible for the plant's festive green color.)

Parasitic plants are somewhat rare, with about 4,100 species in existence according to Wikipedia.  (Out of about 350,000 total plant species in the world.)  Mistletoe parasitizes the stems and branches of other plants, particularly trees.

There are two main species of mistletoe, one which is native to the eastern United States, and one which is native to Great Britain and Europe.  Both varieties are harvested for use in holiday decorations, although these days plastic mistletoe is far more common than the real stuff.

Mistletoe is spread by its seeds, which it produces inside tasty, sticky berries.  Birds eat the berries and then pass the seeds through.  The seeds, which have a sticky coating, stick to the branch and help keep the seed fastened tightly until it can sprout and attach itself to its host. 

Because of this stickiness, mistletoe berry juice has been used to catch birds.  Hunters chew the berries into a paste, then pull them into a sort of taffy-like collection of glue-y strings, which are then wrapped around a tree branch.  When a bird lands on the branch it gets caught in the "bird lime," and can easily be plucked from the branch and killed.

The European mistletoe has a similarly close (but less fatal) relationship with the Mistle thrush.  The Mistle thrush is omnivorous, but it particularly likes mistletoe berries.  It will stake out a mistletoe tree, and defend it against other birds.  In exchange, the Mistle thrush's favorite species of mistletoe is spread almost exclusively by the Mistle thrush.

(The Mistle thrush habitually calls before a bout of bad weather.  This habit led to the Mistle thrush's original name, "Stormcock.")

Although mistletoe is damaging to its host tree, it rarely kills its host. Only a heavy infestation of mistletoe is enough to kill a healthy host tree.  And despite its somewhat unsavory eating habits, mistletoe turns out to be an extremely important species for the woodland ecology.  It attracts birds, provides shelter for birds and insects, attracts pollinating insects, and provides food and shelter for a wide range of animals.

Mistletoe is quite poisonous.  According to French legend, this is because mistletoe was growing on the tree that was cut down and used to crucify Jesus.  Because of its perceived role, it was doomed to roam the world as a homeless parasite, a sort of plant version of the Wandering Jew.

Many cultures have a whole host of myths around mistletoe, including the Druids, medieval Europeans, and Norse mythology. Today, we are familiar with mistletoe mainly for the holiday kissing tradition. 

Technically, each time a man kisses a woman beneath the mistletoe, he is to remove one of the berries from the sprig.  When all the berries are gone, the kissing has to stop.  However, keeping mistletoe hanging all year long is said to prevent house fires.  (Which is a great excuse, if guests point out that your holiday decorations are still hanging at Valentine's Day!)

Photo credit: Flickr/kqedquest