MISTLETOE IS AMERICA'S MOST POPULAR PARASITE

by Whit Gibbons

December 19, 2010

One of my daughters suggested I choose a topic for this week associated with the holiday season. I've written about holly and reindeer in recent years, so let's return to mistletoe, a plant that has flowers pollinated by insects, has seeds transported by birds, and takes its water and minerals from trees.

"Mistletoe" refers to any of more than 200 species of semiparasitic shrubs found worldwide. Mistletoe lives throughout the southern United States, from the Atlantic Coast to California, and on every continent except Antarctica. Like true parasitic plants, mistletoe is devoid of roots. Instead, the dark green shrub has extensions called holdfasts that grip the host tree. These rootlike anchors suck water and nutrients from the tree. Thus, mistletoe is only found on living trees, which are essential to the mistletoe's survival. In contrast, Spanish moss uses a tree, dead or alive, only for support, extracting water and nutrients from the atmosphere.

In the South, tiny yellow flowers bloom on the evergreen mistletoe from fall to winter. The familiar white berries, which begin to form soon after pollination, resemble little packets of glue around tiny indigestible seeds. A mistletoe plant can be either male or female and, like a holly tree, only the female plant has berries. Eating mistletoe berries may be potentially lethal for humans, but birds seem to be immune to any toxicity.

Birds' immunity to mistletoe's poisonous qualities is essential to the welfare of the plant. The dispersal and propagation of mistletoe is largely dependent on birds that eat the berries but do not digest the seeds. Ecological studies suggest that seeds are most likely to survive and grow if a bird deposits them on the same species of tree on which the parent plant lived. During spring migration, a flock of cedar waxwings can result in newly developing mistletoe plants being far away from where the seeds were ingested. Mistletoe thrives in bright sunlight in the uppermost branches of big oaks and is typically absent from pines and from evergreen hardwoods such as magnolias with needles and leaves that would shade the mistletoe.

A parasitic lifestyle is unusual among flowering plants. Nonetheless, many aspects of mistletoe ecology are well understood. Competition to obtain water, minerals and even space itself, is highly intense among most plants, but mistletoe does not encounter such problems. Tree limbs are a ready source of water and minerals for this unusual little plant, and its absence from the uppermost branches of a tall oak is probably because no bird has dropped a seed there, not because of competition with other mistletoe plants.

Over the ages, mistletoe has been credited with some intriguing qualities, perhaps, in part, because of its many unusual ecological properties. According to Scandinavian legend, mistletoe was the only organism in the world from which Odin's son Baldur was not protected, and a mistletoe dart was the cause of his death.

Mistletoe is also associated with the Druids, the mysterious, oak-worshiping sect that inhabited the British Isles centuries ago. The Druids considered mistletoe a plant of honor and power. According to legend, when the plant was found growing in an oak tree, the Druids performed sacrificial ceremonies at the tree on the sixth day after a full moon. The Druids reportedly used a golden sickle when harvesting mistletoe from a sacred oak. Although the berries appear just in time for Christmas, mistletoe is not used in churches. One reason may be because of its association with the Druids.

Using mistletoe as a romantic lure was common in England at least as early as the 1500s. In 1520, William Irving wrote that a young man should pluck a berry each time he kissed a girl beneath the mistletoe. A version of that tradition persists today in secular Christmas decorations. And though mistletoe may be excluded from wreaths and floral displays in churches, it will be found in many a home during this season, hanging in a doorway and enticing people to exchange a holiday kiss.

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