Mythical Mistletoes

Among the Holiday Season's iconic plants, none are more modest than a sprig of mistletoe. Strategically attached in a doorway, mistletoe lurks in the shadows compared to the brightly lit decorations of Christmas, Hanukkah, Kwanzaa, and New Year's Eve. Nonetheless, it reliably makes its way into tens of millions of homes around the world.

Despite that popularity, mistletoe doesn't belong to any individual holiday. It belongs to an entire season. Mistletoe gives permission for a stolen kiss as early as the office holiday party in late November all the way to the wee hours of January 1st revelry.

How has it come to pass that such an unassuming plant has such cultural power?

The fact that all mistletoes are parasites undoubtedly called them to the attention of the ancients. Strictly speaking, they are not fully parasitic, but hemi-parasitic. Hemi-parasitism refers to a condition in which one species grows on another and absorbs some, but not all, of its nutrients from the host. In the case of mistletoes, they depend on their host tree or shrub for water and mineral nutrients. They obtain these directly through root-like haustoria that tap directly into the host's vascular system. However, mistletoes also photosynthesize, thereby creating their own carbon-based nutrition (carbs, fats, proteins, etc.). Consequently, mistletoes are evergreen and are apparent on their deciduous hosts in the middle of the freezing winter.

Various cultures reasoned that such a vigorous plant must have unusual properties. The ancient Druids, Greeks, and Romans respected the plant, using it for medicinal and ceremonial purposes. The Romans, in particular, connected the plant with peace, love, and understanding, hanging mistletoe over doorways for protection. The current use of mistletoe as a stimulus for romantic activity is often attributed to a story from Norse mythology:

When the goddess Frigg gave birth to her son Baldr, she had a prophetic dream that he would die before his time. She solicited promises from all things, living and not, to protect him from harm. However, she forgot the mistletoe. One day, when the gods were having a party, they amused themselves by hurling and shooting things at Baldr, marveling at his inability to be hurt. However, the mischief-maker, Loki, tricked Baldr's blind brother, Hother, into shooting him with an arrow made with a mistletoe shaft. The arrow took its toll. (After all, it's Norse mythology. You have to expect tragedy.)

Frigg and the other gods were devastated. Frigg's tears turned into mistletoe berries, and Baldr was resurrected through the efforts of the assembled gods. With Baldr reborn, mistletoe was forgiven, and no longer forgotten. Frigg blessed it with the power of vitality and love.

By the late 1700s, obligate kissing under the mistletoe had become a tradition of the English service class. Each kiss was to be accompanied with removing a berry from the motivating sprig. Once the berries were depleted, the sprig lost its power. Variations on the tradition include that a kiss under the mistletoe would result in true love.

Legend and tradition notwithstanding, the botany of mistletoe is equally compelling. First things first, let's define what we are talking about. Mistletoes are a heterogenous group of more than a thousand related species. The European mistletoe of legend is Viscum album. The North American mistletoe that is likely to end up in your home is Phoradendron leucarpum. It is found sporadically throughout the continent, including earning the honor of the official "floral emblem" of Oklahoma. The European and North American mistletoes are remarkably similar, frequenting hardwood trees and bearing white berries (technically, one-seeded "drupes"). The two species, with some of the other mistletoes, were once in their own family, but genomic evidence has revealed them to be in the larger sandalwood family, the Santalaceae, a family whose entire membership is hemi-parasitic.

Santalaceae is a member of a tight group of seven families, all of whom are hemi-parasitic or fully parasitic. One of these, the Loranthaceae, is comprised of what are known as the "showy mistletoes". Coincidentally, Western Australia's "Christmas Tree" is a member of the Loranthaceae. Officially named Nuytsia floribunda, this hemi-parasitic tree bursts into an impressive floral display of brilliant orange inflorescences in the midst of the austral summer, that is, at Christmastime.

In California, P. leucarpum grows in relatively moist portions of the state. For example, I have frequently seen it decorating sycamores that grow along rivers and streams. In California's deserts, the leafless desert mistletoe P. californicum grows exclusively on shrubs and trees of the legume family, such as acacia and mesquite. Genetic research conducted in my lab has shown that desert mistletoe may be a cryptic complex of host-specific species. In some parts of the California and Arizona deserts, entrepreneurs sell this species as a substitute for P. leucarpum. At higher elevations in California, conifers are parasitized by various species of dwarf mistletoes in the genus Arceuthobium. The genus is notable for dispersing its sticky seeds at nearly 50 miles per hour. The final mistletoe occurring in California is the OG V. album of Europe. This species was introduced to Sebastopol in Sonoma County by Luther Burbank in 1900. Its distribution is slowly spreading, but whether it will become invasive remains to be seen. The European species has not been introduced elsewhere in North America.

The question of toxicity always arises in the discussion of holiday plants. Toxicity for mistletoe is much like that of another holiday icon, poinsettia. Both poinsettia and commercial mistletoe contain toxins and should not be consumed, especially by children and pets. Neither is terribly toxic; fatalities are extremely rare. European mistletoe is considerably more toxic than American mistletoe.

Beyond its celebratory uses, mistletoe species can have other benefits. Both P. leucarpum and V. album have been employed in traditional and folk medicine. Chemicals isolated from European mistletoe have antitumor properties and have been included in certain cancer therapies for decades. Likewise, ecologically, mistletoes are considered keystone species. Many mistletoe species are animal-pollinated. Their foliage and especially fruits are eaten by a wide variety of animals. Birds will often eat the fruits and later defecate the sticky seeds on a branch. That dispersal may account for the origin of mistletoe's Anglo-Saxon name: "mistle" = dung + "tan" = twig. "Dung on a twig". Those ancients were observant!

Humans often have simple ideas about the natural world, particularly plants, but each species has a lot more going on than we initially suspect. In the case of the modest mistletoe, there's more than meets the lips.

This article is an expanded version of one written for the University of California Botanic Gardens Newsletter.