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Mistletoe is the common name for obligate hemiparasitic plants in several families in the order Santalales. These plants attach to and penetrate the branches of a tree or shrub by a structure called the haustorium, through which they absorb water and nutrients from the host plant. Therefore, mistletoes are parasitic plants.
The name mistletoe originally referred to Viscum album (European mistletoe, of the family Santalaceae in the order Santalales); it was the only species native to Great Britain and much of Europe. The genus Viscum is not native to North America, but Viscum album has been introduced to California.
Viscum species are poisonous to humans; eating the fruit causes a weak pulse and acute gastrointestinal problems including stomach pain and diarrhea. At least one of the active ingredients is the lectin viscumin, which is an intensely toxic. It inhibits protein synthesis by catalytically inactivating ribosomes. In spite of this, many species of animals are adapted to eating the fruit as a significant part of their diet.
Application of the name mistletoe was extended to related genera and even families, including Phoradendron serotinum, the eastern mistletoe of eastern North America. Eastern mistletoe looks similar to European mistletoe, but has shorter, broader leaves and longer clusters of 10 or more berries.
The largest family of mistletoes, the Loranthaceae, has 73 genera and over 900 species. Subtropical and tropical climates have markedly more mistletoe species; Australia has 85, of which 71 are in Loranthaceae, and 14 in Santalaceae.
Parasitism has evolved at least twelve times among the vascular plants; and of those, the parasitic mistletoe habit has evolved independently five times, in the 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 indicate they evolved independently.
Mistletoe plants grow on a wide range of host trees; they commonly reduce their growth and a large plant stunts and commonly kills the distal portion of branch it grows on. A heavy infestation may kill the entire host plant. Viscum album successfully parasitises more than 200 tree and shrub species.
Technically, all mistletoe species are hemiparasites, because they do perform at least a little photosynthesis for at least a short period of their life cycle. However, this is academic in some species whose contribution is very nearly zero. For example, some species, such as Viscum minimum, that parasitise succulents, commonly species of Cactaceae or Euphorbiaceae, grow largely within the host plant, with hardly more than the flower and fruit emerging. Once they have germinated and attached to the circulatory system of the host, their photosynthesis reduces so far that its becomes insignificant.
Most of the Viscaceae bear evergreen leaves that photosynthesise effectively, and photosynthesis proceeds within their green, fleshy stems as well. Some species, such as "Viscum capense", are adapted to semi-arid conditions and their leaves are vestigial scales, hardly visible without detailed morphological investigation. Therefore their photosynthesis and transpiration only take place in their stems, limiting their demands on the host's supply of water, but also limiting their intake of carbon dioxide for photosynthesis. Accordingly their contribution to the host's metabolic balance becomes trivial and the idle parasite may become quite yellow as it grows, having practically given up photosynthesis.
At another extreme other species have vigorous green leaves. Not only do they photosynthesise actively, but a heavy infestation of mistletoe plants may take over whole host tree branches, sometimes killing practically the entire crown and replacing it with their own growth. In such a tree the host is relegated purely to the supply of water and mineral nutrients and the physical support of the trunk. Such a tree may survive as a Viscum community for years; it resembles a totally unknown species unless one examines it closely, because its foliage does not look like that of any tree. An example of a species that behaves like that is Viscum continuum.
A mistletoe seed germinates on the branch of a host tree or shrub, and in its early stages of development it is independent of its host. It commonly has two or even four embryos, each producing its hypocotyl, that grows towards the bark of the host under the influence of light and gravity, and potentially each forming a mistletoe plant in a clump. Possibly as an adaptation to assist in guiding the process of growing away from the light, the adhesive on the seed tends to darken the bark. On having made contact with the bark, the hypocotyl, with only a rudimentary scrap of root tissue at its tip penetrates it, a process that may take a year or more. In the mean time the plant is dependent on its own photosynthesis. Only after it reaches the host's conductive tissue can it begin to rely on the host for its needs. Later it forms a haustorium that penetrates the host tissue and takes water and nutrients from the host plant.
Species more or less completely parasitic include 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, and the genus Arceuthobium (dwarf mistletoe; Santalaceae) which has reduced photosynthesis; as an adult, it manufactures only a small proportion of the sugars it needs from its own photosynthesis, but as a seedling actively photosynthesizes until a connection to the host is established.
Most mistletoe seeds are spread by birds that eat the (drupes). Quite a range of birds feed on them, of which the mistle thrush is the best-known in Europe, the Phainopepla in southwestern North America, and Dicaeum of Asia and Australia. Depending on the species of mistletoe and the species of bird, the seeds are regurgitated from the crop, excreted in their droppings, or stuck to the bill, from which the bird wipes it onto a suitable branch. The seeds are coated with a sticky material called viscin. Some viscin remains on the seed and when it touches a stem, it sticks tenaciously. The viscin soon hardens and attaches the seed firmly to its future host, where it germinates and its haustorium penetrates the sound bark.
Specialist mistletoe eaters have adaptations that expedite the process; some pass the seeds through their unusually shaped digestive tracts so fast that a pause for defecation of the seeds is part of the feeding routine. Others have adapted patterns of feeding behaviour; the bird grips the fruit in its bill and squeezes the sticky-coated seed out to the side. The seed sticks to the beak and the bird wipes it off onto the branch.
Once a mistletoe plant is established on its host, it usually is possible to save a valuable branch by pruning and judicious removal of the wood invaded by the haustorium, if the infection is caught early enough. Some species of mistletoe can regenerate if the pruning leaves any of the haustorium alive in the wood.
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. 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. In western North America their juicy berries are eaten and spread by birds (notably Phainopepla, or silky-flycatcher). When eaten, some seeds pass unharmed through their digestive systems; if the birds’ droppings happen to land on a suitable branch, the seeds may stick long enough to germinate. As the plants mature, they grow into masses of branching stems which suggest the popular name "witches’ brooms". 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 owl and the marbled murrelet. In Australia the diamond firetail and painted honeyeater 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 birds.
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. 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.
The word 'mistletoe' (Old English mistiltan) is of uncertain etymology. It may be related to the German Mist, for dung and Tān for branch, since mistletoe can be spread in the droppings of birds moving from tree to tree.
According to the 13th century Prose Edda, because of the scheming of Loki, the god Baldr was 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. 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).
When Christianity became widespread in Europe after the third century AD, the religious or mystical respect for the mistletoe plant was integrated to an extent into the new religion. In some way that is not presently understood, this may have led to the widespread custom of kissing under the mistletoe plant during the Christmas season. The earliest documented case of kissing under the mistletoe dates from 16th century England, a custom that was apparently very popular at that time.
Mistletoe is commonly used as a Christmas decoration, though such use was rarely alluded to until the 18th century. Viscum album is used in Europe whereas Phoradendron serotinum is used in North America. Both are commercially harvested. 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 throughout the year, often to preserve the house from lightning or fire, until it is replaced the following Christmas Eve. The tradition has spread throughout the English-speaking world, but is largely unknown in the rest of Europe.
The type of mistletoe used during Christmas celebrations is of the same type as that believed to be held sacred by ancient druids, but, outside northern Europe, the plant used is not of the same species. The mistletoe that is commonly used as a Christmas decoration in North America (Phoradendron flavescens) grows as a parasite on trees in the west as also in those growing in a line down the east from New Jersey to Florida. In Europe, where the custom originates, the 'original' mistletoe, Viscum album, is still used. The European mistletoe is a green shrub with small, yellow flowers and white, sticky berries which are considered poisonous. Ancient druids considered the Viscum album plant holy, but had no knowledge of the Phoradendron flavescens. Modern druids focus on the parasitic habitat on oak (where it is very rarely found) as being the definer of a sacred mistletoe, and use Phoradendron flavescens as well as other mistletoe species.
According to an old Christmas custom, a man and a woman who meet under a hanging of mistletoe were obliged to kiss. The custom may be of Scandinavian origin. It was alluded to as common practice in 1808 and described in 1820 by American author Washington Irving in his The Sketch Book of Geoffrey Crayon, Gent.:
The mistletoe is still hung up in farm-houses and kitchens at Christmas, and the young men have the privilege of kissing the girls under it, plucking each time a berry from the bush. When the berries are all plucked the privilege ceases.
See also The Holly and the Ivy.
Mistletoe (Phoradendron serotinum) 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.
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. 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. Some anthroposophical mistletoe preparations are diluted homeopathically. Mistletoe extract is sold as Iscador, Helixor, and several other trade names.
Available clinical evidence does not support claims of anti-cancer effect, quality of life, or other outcomes from the use of mistletoe extract. Research has likewise shown little or no improvement in rigorous trials. 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.
Mistletoe's use for Christmas decoration is discussed in a previous section (see above, Culture, folklore, and mythology.)
The sticky juice of mistletoe berries was used as adhesive to trap small animals or birds. In South Africa it is called "bird lime" in English and voëlent 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 and 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.
Mistletoe bush on a Eucalyptus tree
Mistletoe attached to Eucalyptus host
Mistletoe in San Bernardino Mountains
Mistletoe berries in Wye Valley
Mistletoe in abundance in Wye Valley
Mistletoe in North Central Texas
Mistletoe in France Versailles, France
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