Mistletoe is the common name for obligate hemiparasitic plants in the order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they extract water and nutrients from the host plant.
The name mistletoe originally referred to the species Viscum album (European mistletoe, of the family Santalaceae in the order Santalales); it is the only species native to the British Isles and much of Europe. A related species with red rather than white fruits, Viscum cruciatum , occurs in Southwest Spain and Southern Portugal, as well as in Morocco in North Africa and in southern Africa. [1] There is also a wide variety of species in Australia. [2] The genus Viscum is not native to North America, but Viscum album was introduced to Northern California in 1900. [3] [4]
The eastern mistletoe native to North America, Phoradendron leucarpum , belongs to a distinct genus of the family Santalaceae.
European mistletoe has smooth-edged, oval, evergreen leaves borne in pairs along the woody stem, and waxy, white berries that it bears in clusters of two to six. The eastern mistletoe of North America is similar, but has shorter, broader leaves and longer clusters of 10 or more berries.
Over the centuries, the term has been broadened to include many other species of parasitic plants with similar habits, found in other parts of the world, that are classified in different genera and families such as the Misodendraceae of South America and the mainly southern hemisphere tropical Loranthaceae.
The word 'mistletoe' derives from the older form 'mistle' adding the Old English word tān (twig). 'Mistle' is common Germanic (Old High German mistil, Middle High German mistel, Old English mistel, Old Norse mistil). [5] Further etymology is uncertain, but may be related to the Germanic base for 'mash'. [6]
Parasitism has evolved at least twelve times among the vascular plants. [7] Molecular data show the mistletoe habit has evolved independently five times within the Santalales—first in the Misodendraceae, but also in the Loranthaceae and three times in the Santalaceae (in the former Santalalean families Eremolepidaceae and Viscaceae, and the tribe Amphorogyneae). [8]
The largest family of mistletoes, the Loranthaceae, has 73 genera and over 900 species. [9] Subtropical and tropical climates have markedly more mistletoe species; Australia has 85, of which 71 are in Loranthaceae, and 14 in Santalaceae. [10]
Mistletoe species grow on a wide range of host trees, some of which experience side effects including reduced growth, stunting, and loss of infested outer branches. A heavy infestation may also kill the host plant. Viscum album successfully parasitizes more than 200 tree and shrub species.[ citation needed ]
All mistletoe species are hemiparasites because they do perform some photosynthesis for some period of their life cycle. However, in some species its contribution is very nearly zero. For example, some species, such as Viscum minimum , that parasitize 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 it becomes insignificant. [11]
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. [11]
At another extreme other species have vigorous green leaves. Not only do they photosynthesize 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 in this manner is Viscum continuum. [11]
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 meantime 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. [11]
Species more or less obligate 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, [12] 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.[ citation needed ]
Some species of the largest family, Loranthaceae, have small, insect-pollinated flowers (as with Santalaceae), but others have spectacularly showy, large, bird-pollinated flowers.[ citation needed ]
Most mistletoe seeds are spread by birds that eat the 'seeds' (in actuality drupes). Of the many bird species that feed on them, the mistle thrush is the best-known in Europe, the phainopepla in southwestern North America, and Dicaeum flowerpeckers in 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. [13]
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 behavior; 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. [14]
Biochemically, viscin is a complex adhesive mix containing cellulosic strands and mucopolysaccharides. [15]
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. [16] [17]
There are 1500 species of mistletoe, varying widely in toxicity to humans; the European mistletoe ( Viscum album ) is more toxic than the American mistletoe ( Phoradendron serotinum ), though concerns regarding toxicity are more prevalent in the US. [18] The effects are not usually fatal. [19] In parts of South Asia, they are frequently used as an external medicine. [20] The active substances are phoratoxin (in Phoradendron) and tyramine (in Viscum) and their effects include blurred vision, diarrhea, nausea and vomiting. [19] Less commonly they cause cardiac problems; seizures, hypertension, and even cardiac arrest. Toxins are more concentrated in the leaves and berries of the plant, with teas prepared from the plant being particularly dangerous. While adults may suffer little effect, these are more pronounced in small children and in animals. [18]
Mistletoe has been used historically in medicine for its supposed value in treating arthritis, high blood pressure, epilepsy and infertility. [21] [20]
Mistletoes are often considered pests that kill trees and devalue natural habitats, but some species have recently been recognized as ecological keystone species, organisms that have a disproportionately pervasive influence over their community. [22] 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 the phainopepla) while in Australia the mistletoebird behaves similarly. When eaten, some seeds pass unharmed through their digestive systems, emerging in extremely sticky droppings which the bird deposits on tree branches, where some may stick long enough to germinate. As the plants mature, they grow into masses of branching stems which suggest the popular name "witches' brooms".[ citation needed ]
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.
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. [23]
Mistletoe is relevant to several cultures. Pagan cultures regarded the white berries as symbols of male fertility, with the seeds resembling semen. [24] The Celts, particularly, saw mistletoe as the semen of Taranis, while the Ancient Greeks referred to mistletoe as "oak sperm". [25] [26] Also in Roman mythology, mistletoe was used by the hero Aeneas to reach the underworld. [27] [28]
Mistletoe may have played an important role in Druidic mythology in the Ritual of Oak and Mistletoe, although the only ancient writer to mention the use of mistletoe in this ceremony was Pliny. Evidence taken from bog bodies makes the Celtic use of mistletoe seem medicinal rather than ritual. [29] It is possible that mistletoe was originally associated with human sacrifice and only became associated with the white bull after the Romans banned human sacrifices. [30]
The Romans associated mistletoe with peace, love and understanding and hung it over doorways to protect the household. [31]
In the Christian era, mistletoe in the Western world became associated with Christmas as a decoration under which lovers are expected to kiss, as well as with protection from witches and demons. [32] Mistletoe continued to be associated with fertility and vitality through the Middle Ages, and by the 18th century it had also become incorporated into Christmas celebrations around the world. The custom of kissing under the mistletoe is referred to as popular among servants in late 18th-century England. [33] [34]
The serving class of Victorian England is credited with perpetuating the tradition. [35] The tradition dictated that a man was allowed to kiss any woman standing underneath mistletoe, and that bad luck would befall any woman who refused the kiss. [36] [37] One variation on the tradition stated that with each kiss a berry was to be plucked from the mistletoe, and the kissing must stop after all the berries had been removed. [35] [37]
From at least the mid-19th century, Caribbean herbalists of African descent have referred to mistletoe as "god-bush". [38] In Nepal, diverse mistletoes are used for a variety of medical purposes, particularly for treating broken bones. [20]
Mistletoe is the floral emblem of the US state of Oklahoma and the flower of the UK county of Herefordshire. Every year, the UK town of Tenbury Wells holds a mistletoe festival and crowns a 'Mistletoe Queen'. [31]
The Santalales are an order of flowering plants with a cosmopolitan distribution, but heavily concentrated in tropical and subtropical regions. It derives its name from its type genus Santalum (sandalwood). Mistletoe is the common name for a number of parasitic plants within the order.
Viscaceae is a taxonomic family name of flowering plants. In this circumscription, the family includes the several genera of mistletoes. This family name is currently being studied and under review as in past decades, several systems of plant taxonomy recognized this family, notably the 1981 Cronquist system.
Loranthaceae, commonly known as the showy mistletoes, is a family of flowering plants. It consists of about 75 genera and 1,000 species of woody plants, many of them hemiparasites. The three terrestrial species are Nuytsia floribunda, Atkinsonia ligustrina, and Gaiadendron punctatum Loranthaceae are primarily xylem parasites, but their haustoria may sometimes tap the phloem, while Tristerix aphyllus is almost holoparasitic. For a more complete description of the Australian Loranthaceae, see Flora of Australia onlineArchived 2018-04-01 at the Wayback Machine., for the Malesian Loranthaceae see Flora of Malesia.
The Santalaceae, sandalwoods, are a widely distributed family of flowering plants which, like other members of Santalales, are partially parasitic on other plants. Its flowers are bisexual or, by abortion, unisexual. Modern treatments of the Santalaceae include the family Viscaceae (mistletoes), previously considered distinct.
The genus Arceuthobium, commonly called dwarf mistletoes, is a genus of 26 species of parasitic plants that parasitize members of Pinaceae and Cupressaceae in North America, Central America, Asia, Europe, and Africa. Of the 42 species that have been recognized, 39 and 21 of these are endemic to North America and the United States, respectively. They all have very reduced shoots and leaves with the bulk of the plant living under the host's bark. Recently the number of species within the genus has been reduced to 26 as a result of more detailed genetic analysis.
Viscum album is a species of mistletoe in the family Santalaceae, commonly known as European mistletoe, common mistletoe or simply as mistletoe. It is native to Europe and western and southern Asia.
Viscum is a genus of about 70–100 species of mistletoes, native to temperate and tropical regions of Europe, Africa, Asia and Australasia. Traditionally, the genus has been placed in its own family Viscaceae, but recent genetic research by the Angiosperm Phylogeny Group shows this family to be correctly placed within a larger circumscription of the sandalwood family, Santalaceae. Its name is the origin of the English word viscous, after the Latin viscum, a sticky bird lime made from the plants' berries.
Phoradendron is a genus of mistletoe, native to warm temperate and tropical regions of the Americas. The center of diversity is the Amazon rainforest. Phoradendron is the largest genus of mistletoe in the Americas, and possibly the largest genus of mistletoes in the world. Traditionally, the genus has been placed in the family Viscaceae, but recent genetic research acknowledged by the Angiosperm Phylogeny Group shows this family to be correctly placed within a larger circumscription of the sandalwood family, Santalaceae.
A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called the haustorium, which penetrates the host plant, connecting them to the host vasculature – either the xylem, phloem, or both. For example, plants like Striga or Rhinanthus connect only to the xylem, via xylem bridges (xylem-feeding). Alternately, plants like Cuscuta and some members of Orobanche connect to both the xylem and phloem of the host. This provides them with the ability to extract resources from the host. These resources can include water, nitrogen, carbon and/or sugars. Parasitic plants are classified depending on the location where the parasitic plant latches onto the host, the amount of nutrients it requires, and their photosynthetic capability. Some parasitic plants can locate their host plants by detecting volatile chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500 species of parasitic plants in approximately 20 families of flowering plants are known.
Phoradendron leucarpum is a species of mistletoe in the Viscaceae family which is native to the United States and Mexico. Its common names include American mistletoe, eastern mistletoe, hairy mistletoe and oak mistletoe. It is native to Mexico and the continental United States. It is hemiparasitic, living in the branches of trees. The berries are white and 3–6 millimeters (0.12–0.24 in). It has opposite leaves that are leathery and thick. Ingesting the berries can cause "stomach and intestinal irritation with diarrhea, lowered blood pressure, and slow pulse". This shrub can grow to 1 meter (3.3 ft) by 1 meter (3.3 ft).
Phoradendron californicum, the desert mistletoe or mesquite mistletoe, is a hemiparasitic plant native to southern California, Nevada, Arizona, Sonora, Sinaloa and Baja California. It can be found in the Mojave and Sonoran Deserts at elevations of up to 1400 m.
Dendrophthoe falcata is one of the hemiparasitic plants that belong to the mistletoe family Loranthaceae. It is the most common of all the mistletoes that occur in India. At the moment reports say that it has around 401 plant hosts. The genus Dendrophthoe comprises about 31 species spread across tropical Africa, Asia, and Australia among which 7 species are found in India.
Osyris compressa is a facultatively hemiparasitic, mainly South African plant of the sandalwood family, Santalaceae. Until recently, the favoured binomial name was Colpoon compressum, but around 2001, the genus Colpoon was included in Osyris on the basis of comparative DNA studies. That assignment is not final, however, and according to the Kew Gardens plant list, Colpoon compressum P.J.Bergius, though still in review, is the accepted name.
Phoradendron tomentosum, the leafy mistletoe, hairy mistletoe or Christmas mistletoe, is a plant parasite. It is characterized by its larger leaves and smaller berries than dwarf mistletoe. Leafy mistletoe seldom kill but they do rob their hosts of moisture and some minerals, causing stress during drought and reducing crop productions on fruit and nut trees. Leafy mistletoe has the ability to photosynthesize on its own but it relies on other plants in order to obtain its nutrients. It attaches itself to a tree and then grows haustoria, in order to get the food and water it needs.
Amyema preissii, commonly known as wireleaf mistletoe, is a species of mistletoe, an epiphytic, hemiparasitic plant of the family Loranthaceae. It is native to Australia where it has been recorded from all mainland states. The flowers are red and up to 26 mm long. The fruits are white or pink, globose and 8–10 mm in diameter. Its habitat is sclerophyll forest and woodland where it is often found on wattles. On Victoria's Bellarine Peninsula its hosts include coast wirilda, golden wattle and drooping sheoak. Its sticky seeds are eaten and dispersed by mistletoebirds.
Lysiana exocarpi, commonly known as harlequin mistletoe, is a species of hemiparasitic shrub, endemic to Australia. It is in the Gondwanan family Loranthaceae and is probably the most derived genus of that family with 12 pairs of chromosomes. The Loranthaceae is the most diverse family in the mistletoe group with over 900 species worldwide and including the best known species in Australia. Mistletoes are notable for their relationships with other species. In an early reference to the group in Australia Allan Cunningham explorer and first Director of the Royal Botanic Gardens, Sydney, wrote in 1817: "The Bastard Box is frequently much encumbered with the twining adhering Loranthus aurantiacus which 'Scorning the soil, aloft she springs, Shakes her red plumes and claps her golden wings'."
Arceuthobium pusillum is a perennial, obligate parasitic plant in the sandalwood family. Its common names include Dwarf mistletoe or Eastern dwarf mistletoe. It is one of the most widespread dwarf mistletoes within its range which covers the eastern United States and Canada, from Saskatchewan to Nova Scotia and New Jersey. The species name "pusillum" derives from Latin "pusillus", meaning very small.
Viscum minimum is a species of mistletoe in the family Santalaceae. It is a parasitic plant native to South Africa.
Daniel Lee Nickrent is an American botanist, working in plant evolutionary biology, including the subdisciplines of genomics, phylogenetics, systematics, population genetics, and taxonomy. A major focus has been parasitic flowering plants, particularly of the sandalwood order (Santalales). His interest in photographic documentation and photographic databases has led to several photographic databases including Parasitic Plant Connection, Phytoimages, Plant Checklist for the Rocky Mountain National Park, and Plant Checklist for the Crab Orchard National Wildlife Refuge.
Amyema miraculosa, also known as the fleshy mistletoe and the round-leaf mistletoe, is an Australian native mistletoe found in all states except Tasmania. It is a woody, hemiparasitic plant, in the Loranthaceae family. Being hemiparasitic, it draws water and minerals from its host, however it photosynthesises to manufacture its own supply of carbohydrates.