Mistletoe is the common name for plants that depend on other plants for some nutrients. These plants belong to the order Santalales. They attach to their host tree or shrub using a special structure called the haustorium, which allows them to take water and nutrients from the host plant. There are hundreds of mistletoe species, most of which live in tropical areas.

The name "mistletoe" originally referred to the species Viscum album (European mistletoe), which is part of the family Santalaceae in the order Santalales. This species is the only mistletoe native to the British Isles and much of Europe. A related species, Viscum cruciatum, has red fruits instead of white ones. It is found in Southwest Spain, Southern Portugal, Morocco, and southern Africa. Many other species of Viscum grow in Australia. The genus Viscum is not found naturally in North America, but Viscum album was brought to Northern California in 1900.

The eastern mistletoe native to North America, Phoradendron leucarpum, belongs to a different genus within the family Santalaceae.

European mistletoe has smooth-edged, oval, evergreen leaves that grow in pairs along its woody stems. It produces waxy, white berries in clusters of two to six. The eastern mistletoe of North America is similar but has shorter, broader leaves and longer clusters of ten or more berries.

Over time, the term "mistletoe" has come to include many other parasitic plants with similar habits found worldwide. These plants are classified in different genera and families, such as the Misodendraceae in South America and the Loranthaceae in tropical regions of the Southern Hemisphere.

Etymology

The word "mistletoe" comes from the older form "mistle" combined with the Old English word "tān," which means "twig." The word "mistle" is linked to a group of older languages called Common Germanic, which includes Old High German, Middle High German, Old English, and Old Norse. The rest of the word's history is not completely clear, but it may be connected to the Germanic word for "mash."

The Online Etymology Dictionary suggests that the change in the ending of the word might have happened because people confused the final "-n" with a plural ending after "tan" stopped being used as a separate word. Oxford suggests that this change was a natural part of how the West Saxon language developed, influenced by where the stress in the word was placed.

Groups

Parasitism has developed at least twelve times among vascular plants. Scientific studies show that the mistletoe lifestyle has developed separately five times within the Santalales—originally in the Misodendraceae family, and also in the Loranthaceae family. It has also developed three times in the Santalaceae family, which includes the former Santalalean families Eremolepidaceae and Viscaceae, and the tribe Amphorogyneae.

The largest mistletoe family, Loranthaceae, includes 73 genera and over 900 species. Subtropical and tropical regions have many more mistletoe species. In Australia, there are 85 mistletoe species, with 71 belonging to the Loranthaceae family and 14 to the Santalaceae family.

Life cycle

Mistletoe species grow on many types of host trees. Some host trees may experience problems like slower growth, shorter branches, or even death if the mistletoe infestation is too heavy. Viscum album can parasitize over 200 types of trees and shrubs.

All mistletoe species are hemiparasites, meaning they can perform some photosynthesis during part of their life cycle. However, in some species, photosynthesis contributes very little. For example, Viscum minimum, which grows on succulents like cacti or euphorbias, develops mostly inside the host plant, with only the flower and fruit visible. Once the mistletoe attaches to the host’s circulatory system, its photosynthesis becomes almost unimportant.

Most mistletoe species have evergreen leaves that perform photosynthesis well. Their green, fleshy stems also help with photosynthesis. Some species, like Viscum capense, live in dry areas and have tiny, scale-like leaves that are hard to see. These species rely mostly on their stems for photosynthesis, which limits their need for water from the host but also reduces their ability to take in carbon dioxide. This makes their contribution to the host’s metabolism very small, and the mistletoe may turn yellow or golden as it grows.

At the other end of the spectrum, some mistletoe species have large, green leaves that perform photosynthesis actively. In severe cases, mistletoe infestations can take over entire branches of a host tree, sometimes killing the tree’s crown and replacing it with mistletoe growth. In such cases, the host tree only provides water, nutrients, and physical support. These trees can survive for years as part of a mistletoe community, appearing almost like a different species unless closely examined. An example is Viscum continuum.

Mistletoe seeds germinate on the branches of host trees or shrubs. In their early stages, mistletoe plants are independent of their hosts. They often have two or four embryos, each producing a hypocotyl that grows toward the host’s bark under the influence of light and gravity. Each embryo may develop into a mistletoe plant. The seed’s adhesive darkens the bark, possibly helping guide growth away from light. The hypocotyl, with a small root tip, then penetrates the bark, a process that may take over a year. During this time, the plant relies on its own photosynthesis. Only after reaching the host’s conductive tissues can the mistletoe begin relying on the host for water and nutrients. Later, a haustorium forms to take water and nutrients from the host.

Some species, like the leafless quintral (Tristerix aphyllus), live deep inside the sugar-transporting tissue of spiny cacti, showing only their red flowers. Other species, like those in the genus Arceuthobium (dwarf mistletoe), have reduced photosynthesis as adults but actively photosynthesize as seedlings until they connect to a host.

Many mistletoe species in the Loranthaceae family have small, insect-pollinated flowers, while others have large, showy, bird-pollinated flowers.

Most mistletoe seeds are spread by birds that eat the drupes (which look like seeds). Birds such as the mistle thrush in Europe, the phainopepla in North America, and Dicaeum flowerpeckers in Asia and Australia help disperse the seeds. Depending on the bird and mistletoe species, seeds are regurgitated, excreted, or stuck to the bird’s beak, which the bird then wipes onto a branch. The seeds have a sticky coating called viscin. When viscin touches a stem, it hardens and attaches the seed firmly to the host, where it germinates and sends a haustorium into the bark.

Some birds have special adaptations to help spread mistletoe seeds. For example, the blackcap (Sylvia atricapilla) grips the fruit, squeezes the seed out, and wipes it onto a branch before eating the rest of the fruit.

Biochemically, viscin is a complex adhesive made of cellulose strands and mucopolysaccharides.

If mistletoe is detected early on a valuable branch, it may be saved by pruning and carefully removing the infected wood. However, some mistletoe species can regrow if any part of the haustorium remains in the wood after pruning.

Toxicity

There are 1,500 types of mistletoe, and their harmfulness to humans varies greatly. European mistletoe (Viscum album) is more harmful than American mistletoe (Phoradendron serotinum).

The main harmful substances in American mistletoe are called phoratoxins. These can cause symptoms like blurry vision, diarrhea, nausea, and vomiting, though these effects are rare. Phoratoxins harm the body by damaging cell membranes, which can lead to cell breakdown and death at high levels.

In European mistletoe (Viscum), the most dangerous harmful substance is viscumin. Viscumin stops cells from making proteins, which causes the affected cells to die quickly. This can lead to tissue damage, organ failure, and even death, depending on how much viscumin someone is exposed to. The early symptoms depend on how the substance enters the body because the first cells it touches will be the first to stop making proteins. Viscumin works in the same way as ricin and other similar toxins, but it enters cells differently and can harm cells that are resistant to ricin.

Mistletoe has been used in the past for treating conditions such as arthritis, high blood pressure, epilepsy, and infertility.

Ecological importance

Mistletoes are sometimes seen as pests because they can harm trees and reduce the value of natural areas. However, some types of mistletoe are now recognized as ecological keystone species, meaning they have a major impact on their environment. Many animals rely on mistletoe for food, eating its leaves and young growth, helping to spread pollen, and dispersing its sticky seeds. In western North America, birds like the phainopepla eat mistletoe berries and spread the seeds. In Australia, the mistletoebird performs a similar role. When birds eat the fruit, some seeds pass through their bodies unharmed and are excreted in sticky droppings. These droppings often stick to tree branches, allowing the seeds to grow into new mistletoe plants. As the plants grow, they form dense, branching clusters known as "witches' brooms."

The thick, evergreen witches' brooms created by dwarf mistletoe (Arceuthobium species) in western North America provide good places for birds like the northern spotted owl and marbled murrelet to roost and nest. In Australia, the diamond firetail and painted honeyeater are also known to nest in mistletoe plants.

A study of mistletoe in juniper trees found that more juniper berries grow in areas where mistletoe is present. This is because mistletoe attracts birds that eat both mistletoe berries and juniper berries.

Cultural importance

Mistletoe has been important in many cultures throughout history. Pagan groups saw the white berries as symbols of male fertility, as the seeds looked like semen. The Celts believed mistletoe was the semen of a god named Taranis, while the Ancient Greeks called it "oak sperm." In Roman mythology, mistletoe was used by the hero Aeneas to enter the underworld.

Mistletoe may have been part of Druidic rituals, such as the Ritual of Oak and Mistletoe. The only ancient writer who wrote about mistletoe in this ceremony was Pliny the Elder. Evidence from bodies found in bogs suggests that the Celts may have used mistletoe for healing rather than rituals. It is possible that mistletoe was first linked to human sacrifices, but later became connected to a white bull after the Romans stopped such practices.

The Romans believed mistletoe represented peace, love, and understanding. They hung it over doorways to protect their homes.

When Christianity began in the Western world, mistletoe became linked to Christmas. It was used as a decoration under which people were expected to kiss. It was also believed to protect against witches and demons. Mistletoe remained connected to fertility and health during the Middle Ages. By the 1700s, it was part of Christmas celebrations worldwide. In late 18th-century England, it was common for servants to kiss under mistletoe.

In Victorian England, the tradition of kissing under mistletoe was popular. It was said that a man could kiss any woman standing under mistletoe, and refusing a kiss might bring bad luck. One version of the tradition required removing a berry from the mistletoe with each kiss, and the kissing would stop when all berries were gone.

From the mid-1800s, Caribbean herbalists of African descent called mistletoe "god-bush."

In Nepal, different types of mistletoe are used to treat broken bones.

Mistletoe is the state flower of Oklahoma and the flower symbol of Herefordshire in the UK. Each year, the town of Tenbury Wells in the UK holds a mistletoe festival and crowns a "Mistletoe Queen."