Mistletoe

European mistletoe attached to a silver birch

Mistletoe is the common name for a group of hemi-parasitic plants in the order Santalales that grow attached to and within the branches of a tree or shrub. Parasitism has evolved only nine times in the plant kingdom;^[1] of those, the parasitic mistletoe habit has evolved independently five times: Misodendraceae, Loranthaceae, and Santalaceae, including the former separate families Eremolepidaceae and Viscaceae. Although Viscaceae and Eremolepidaceae were placed in a broadly-defined Santalaceae by Angiosperm Phylogeny Group II, DNA data indicates that they evolved independently.

The word 'mistletoe' (Old English mistiltan) is of uncertain etymology; it may be related to German Mist, for dung and Tang for branch, since mistletoe can be spread in the feces of birds moving from tree to tree. However, Old English mistel was also used for basil.

European mistletoe, Viscum album, is a poisonous plant that causes acute gastrointestinal problems including stomach pain, and diarrhea along with low pulse.^[2]

The name was originally applied to Viscum album (European Mistletoe, Santalaceae), the only species native in Great Britain and much of Europe. Later the name was further extended to other related species, including Phoradendron serotinum (the Eastern Mistletoe of eastern North America, also Santalaceae). European Mistletoe is readily recognized by its smooth-edged oval evergreen leaves borne in pairs along the woody stem, and waxy white berries in dense clusters of 2 to 6. In America, the Eastern Mistletoe is similar, but has shorter, broader leaves and longer clusters of 10 or more berries. In the United States, Phoradendron flavescens is commercially harvested for Christmas decorations^[3], as is Viscum album in Europe.

The largest family of Mistletoes, Loranthaceae, has 73 genera and over 900 species.^[4] Subtropical and tropical climates have markedly more Mistletoe species; Australia has 85, of which 71 are in Loranthaceae, and 14 in Santalaceae.^[5]

Life cycle

Mistletoe in winter

Mistletoe plants grow on a wide range of host trees, and commonly reduce their growth but can kill them with heavy infestation. Viscum album can parasitise more than 200 tree and shrub species. Almost all mistletoes are hemi-parasites, bearing evergreen leaves that do some photosynthesis, and using the host mainly for water and mineral nutrients. However, the mistletoe first sprouts from bird feces on the trunk of the tree and indeed in its early stages of life takes it nutrients from this source. An exception is the leafless quintral, Tristerix aphyllus, which lives deep inside the sugar-transporting tissue of a spiny cactus, appearing only to show its tubular red flowers.^[6] The genus Arceuthobium (dwarf mistletoe; Santalaceae) has reduced photosynthesis; as an adult, it manufactures only a small proportion of the sugars it needs from its own photosythesis but as a seedling it actively photosynthesizes until a connection to the host is established.

Some species of the largest family, Loranthaceae, have small, insect-pollinated flowers (as with Santalaceae), but others have spectacularly showy, large, bird-pollinated flowers.

Most mistletoe seeds are spread by birds, such as the Mistle Thrush in Europe, the Phainopepla in southwestern North America, and Dicaeum of Asia and Australia. However, distinguishing between this species and ones of other ecological biomes is not difficult. They derive sustenance and agility through eating the fruits and nuts (drupes). The seeds are excreted in their droppings and stick to twigs, or more commonly the bird grips the fruit in its bill, squeezes the sticky coated seed out to the side, and then wipes its bill clean on a suitable branch. The seeds are coated with a sticky material called viscin (containing both cellulosic strands and mucopolysaccharides), which hardens and attaches the seed firmly to its future host.

Ecological importance

Mistletoe was often considered a pest that kills trees and devalues natural habitats, but was recently recognized as an ecological keystone species, an organism that has a disproportionately pervasive influence over its community.^[7] A broad array of animals depend on mistletoe for food, consuming the leaves and young shoots, transferring pollen between plants, and dispersing the sticky seeds. The dense evergreen witches' brooms formed by the dwarf mistletoes (Arceuthobium species) of western North America also make excellent locations for roosting and nesting of the Northern Spotted Owls and the Marbled Murrelets. In Australia the Diamond Firetails and Painted Honeyeaters are recorded as nesting in different mistletoes. This behavior is probably far more widespread than currently recognized; more than 240 species of birds that nest in foliage in Australia have been recorded nesting in mistletoe, representing more than 75% of the resident avifauna.

A study of mistletoe in junipers concluded that more juniper berries sprout in stands where mistletoe is present, as the mistletoe attracts berry-eating birds which also eat juniper berries.^[8] Such interactions lead to dramatic influences on diversity, as areas with greater mistletoe densities support higher diversities of animals. Thus, rather than being a pest, mistletoe can have a positive effect on biodiversity, providing high quality food and habitat for a broad range of animals in forests and woodlands worldwide.

Culture, folklore, and mythology

Each arrow overshot his head (1902) by Elmer Boyd Smith, depicting the blind god Höðr shooting his brother, the god Baldr, with a mistletoe arrow

European mistletoe, Viscum album, figured prominently in Greek mythology, and is believed to be The Golden Bough of Aeneas, ancestor of the Romans.^[9]

In the 13th century Prose Edda, due to the scheming of Loki, the god Baldr is killed by his brother, the blind god Höðr, by way of a mistletoe projectile, despite the attempts of Baldr's mother, the goddess Frigg, to have all living things and inanimate objects swear an oath not to hurt Baldr after Baldr had troubling dreams of his death. Frigg was unable to get an oath from mistletoe, because "it seemed too young" to demand an oath from.^[10] In the Gesta Danorum version of the story, Baldr and Höðr are rival suitors, and Höðr kills Baldr with a sword named Mistilteinn (Old Norse "mistletoe"). In addition, a sword by the same name appears in various other Norse legends.

In cultures across pre-Christian Europe, mistletoe was seen as a representation of divine male essence (and thus romance, fertility and vitality), possibly due to a resemblance between the berries and semen.^[11][12]

Mistletoe postcard, circa 1900

According to Pliny the Elder, the Celts considered it a remedy for barrenness in animals and an antidote to poison.^[13]

Mistletoe is commonly used as a Christmas decoration, though such use was rarely alluded to until the 18th century.^[14] Viscum album is used in Europe whereas Phoradendron serotinum is used in North America. According to custom, the mistletoe must not touch the ground between its cutting and its removal as the last of Christmas greens at Candlemas; it may remain hanging through the year, often to preserve the house from lightning or fire, until it was replaced the following Christmas Eve.^[15]. The tradition has spread throughout the English-speaking world but is largely unknown in the rest of Europe.

According to Christmas custom, any two people who meet under a hanging of mistletoe are obliged to kiss. The custom may be of Scandinavian origin.^[16]

Mistletoe (Phoradendron flavescens) is the state floral emblem for the state of Oklahoma. The state did not have an official flower, leaving mistletoe as the assumed state flower until the Oklahoma Rose was designated as such in 2004.^[17]

Medicinal use

Available clinical evidence does not support claims of anti-cancer effect for mistletoe extract; quality of life and other outcomes measures have likewise shown no or weak improvement in rigorous trials.^[18][19][20]

Mistletoe leaves and young twigs are used by herbalists, and it is popular in Europe, especially in Germany, for treating circulatory and respiratory system problems.^[21][22][23] Use of mistletoe extract in the treatment of cancer originated with Rudolf Steiner, the founder of Anthroposophy. He compared the parasitic nature of the mistletoe plant to that of cancer, and believed that cancer represents a faltering of the body's spiritual defenses.^[20][24] Some anthroposophical mistletoe preparations are diluted homeopathically. Mistletoe extract is sold as Iscador, Helixor, and several other trade names.^[20]

Public interest in the United States was spurred in 2001 following actress Suzanne Somers' decision to use Iscador in lieu of chemotherapy following her treatment for breast cancer using surgery and radiotherapy.^[25][26]

Other uses

The sticky juice of mistletoe berries was used as adhesive to trap small animals or birds.^[27] In South Africa it is called "Bird lime" in English and voelent in Afrikaans. A handful of ripe fruits are chewed until sticky, and the mass is then rubbed between the palms of the hands to form long extremely sticky strands which are then coiled around small thin tree branches where birds perch. When a bird lands on this it gets stuck to the branch and is then easy to catch by hand.

References

Images

Mistletoe in an apple tree in Essex, England	Mistletoe in an apple tree in Essex, England	Mistletoe bush on a Eucalyptus tree	Mistletoe attached to Eucalyptus host
Mistletoe fruits	The sticky seed of the mistletoe on a branch	Red mistletoe, New Zealand	Mistletoe in San Bernardino Mountains
Mistletoe berries in Wye Valley	Mistletoe in abundance in Wye Valley	Mistletoe in North Central Texas

External links

• The Mistletoe Center
• About mistletoe
• Parasitic Plant Connection. See families Misodendraceae, Loranthaceae, Santalaceae, and Viscaceae
• Introduction to Parasitic Flowering Plants by Nickrent & Musselman
• Mistletoe phylogenetics; 2000 WIFDWK pdf by Nickrent
• Encyclopedia of Life Sciences 2001 pdf by Nickrent
• Phoradendron serotinum images at bioimages.vanderbilt.edu
• More Information
• Scientific Studies, Research and Clinical Trials on Mistletoe Treatment in Cancer
• Deck the halls with wild, wonderful mistletoe, West Virginia Department of Agriculture