Parasites are Welcome for Christmas

 by 

Stephen G. Saupe
Biology Department
College of St. Benedict/St. John�s University
Collegeville, MN  56321
320 � 363 � 2782; ssaupe@csbsju.edu
(originally published in Sagatagan Seasons, Winter 2002)

      The Christmas season is rapidly approaching and we will soon be decorating our homes with evergreens, candles, holly, candy canes, parasites, wreathes, and garlands.  Whoa!  Did you say "parasites?"  That�s right � but I�m not referring to those over-bearing relatives who don�t know when they�ve overstayed their welcome, nor even to scourges such as malaria or the intestinal protozoan, Giardia.  The parasite that plays an important role in many yuletide celebrations is mistletoe. 

Mistletoe is a general name for plants in the Mistletoe family (Viscaceae).  Our Christmas mistletoe comes primarily from two species, Viscum album, which is the mistletoe of Europe, and Phoradendron leucarpon which is native to the US.  Mistletoes have evergreen leaves and stems of the mistletoe appear to be jointed because of constrictions at the nodes.  The plant breaks easily at these constrictions.  Mistletoes can parasitize a variety of species, but they are especially common on oaks.   

Mistletoes produce sticky white berries, each with a single seed, that are enjoyed by birds which transport the seeds are to a potential new host.  The seeds often adhere to the bird�s beak and are planted in the bark of a branch as the bird tries to dislodge the sticky hitchhiker.  When the seed germinates, a root-like structure emerges and burrows into the stem of the host and forms a swollen structure called a haustorium.  The haustorium anchors the parasite and serves as a conduit through which the parasite steals water and minerals from its host.  Mistletoes don�t need to obtain sugars, proteins or other nutrients from the host because its photosynthetic green leaves are able to fulfill this need.

 Although there are over 20 species of Phoradendron in the southern and western United States, none live in Minnesota.  In fact, the only mistletoe that occurs in the state is Eastern dwarf mistletoe (Arceuthobium pusillum).  These small, yellowish-colored plants lack chlorophyll and thus, unlike the Christmas mistletoes, are wholly parasitic and rely on their host for both water and food.  Dwarf mistletoe grows on pines and junipers and has been reported in a few counties in northern Minnesota.  Fortunately, our extensive conifer stands in the St. John�s Arboretum seem to have been spared from this parasite that can significantly reduce the health and timber quality of the host plant. 

 Although there aren�t records of either Dwarf or Christmas mistletoes in the CSB/SJU Bailey Herbarium, the St. John�s Arboretum is home to several other parasitic plants including Indian paintbrush (Castilleja), Lousewort (Pedicularis sp.), and Bastard toadflax (Comandra umbellatum).  Like the Christmas mistletoe, these species are green and can make their own food.  In contrast, they don�t grow on the stem of their host, but rather, these species send their haustoria into the roots of the host and appear to be rooted in the soil.  Unless you performed a painstaking excavation of their root system, you wouldn�t even know that these attractive plants are water and mineral thieves. 

 Today, mistletoe is used medicinally, especially in Germany, to treat a variety of conditions including hypotension and cancer.  The plants contain a variety of poisonous proteins (phoratoxins and viscotoxins) as well as cytotoxic glycoproteins (lectins).  Studies have not conclusively demonstrated the efficacy of mistletoe, and because of its high toxicity, mistletoe tea or other preparations are most definitely not recommended for home remedy. 

 The Druids considered mistletoe a sacred and divine plant with great spiritual powers to ward off evil.  These ideas were probably derived from mistletoe�s ability to remain green throughout the winter and because mistletoe seemingly grew in the sky without a connection to the ground.  Druid priests dressed in white robes harvested mistletoe with golden sickles.  To pay homage to the spirits, the harvest was accompanied by the sacrifice of two bulls.  The severed plants were caught by virgins and then distributed to homes to ward off evil.  According to Christian legend, mistletoe was once a normal forest tree.  However, after it was used to make the Cross on which Jesus was crucified, mistletoe shrunk to its current growth form in shame. 

Parasites at Christmas?  You bet!  Mistletoe is hung in a doorway or other prominent location where a young man can sneak a kiss from an unsuspecting maiden who stands beneath it.  This practice, which seems to have originated in Britain, may be the result of a Norse legend.  According to the latter Christianized version of the story, Frigga, the goddess of love had a son, Balder, whom she loved dearly.  She asked all the plants, animals, and non-living things of the world to protect her beloved son, but unfortunately she neglected to ask the mistletoe plant.  Loki, an evil god, was jealous of his archenemy�s invincibility so he fashioned an arrow from the wood of mistletoe and used it to slay Balder.  Frigga was distraught but brought him back to life with a kiss, just as we kiss under the mistletoe today.  The most highly prized mistletoe has numerous white berries.  According to this legend the berries represent the silvery tears of Frigga.  After each kiss, one berry is removed.  Once gone, no more kisses are permitted.

If you decorate your home with a spring of mistletoe this Christmas, just remember that you are following an ancient tradition honoring a plant parasite.  And you could certainly do worse - what if those pesky relatives came to stay instead?

References:  (the following are just two of many terrific references about parasitic plants)

• Kuijit, J (1969) The Biology of Parasitic Plants.  Univ of California PRess, Berkeley.

• Nickrent, DL The Parasitic Plant Connection web site.  Southern Illinois University.

• Celtic Origins of Mistletoe