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Mistletoe is the common name for
obligate{{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym '' facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe 300px, Aerobic and anaerobic bacteria can be identified by growing them in test tub ...
hemiparasitic A parasitic plant is a plant that derives some or all of its nutritional requirement from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All Parasite, parasitic plants have modified roots, called ha ...

hemiparasitic
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s in the
order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the habit of achieving a ...
Santalales The Santalales are an order of flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Order(biology), orders, 416 Family (biology), families, ap ...
. They are attached to their host tree or shrub by a structure called the
haustorium In botany Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes k ...
, through which they extract water and nutrients from the host plant. The name mistletoe originally referred to the species ''
Viscum album ''Viscum album'' is a species of mistletoe in the family Santalaceae, commonly known as European mistletoe, common mistletoe or simply as mistletoe (Old English ''mistle'').D. Zuber (2004). Biological flora of Central Europe: ''Viscum album'' L. F ...

Viscum album
'' (European mistletoe, of the family
Santalaceae The Santalaceae, sandalwoods, are a widely distributed family of flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Order(biology), orders, 4 ...
in the order Santalales); it is the only species native to the
British Isles The British Isles are a group of islands in the North Atlantic off the north-western coast of continental Europe Continental Europe or mainland Europe is the contiguous continent A continent is any of several large landmasse ...

British Isles
and much of Europe. A separate species, ''
Viscum cruciatum ''Viscum cruciatum'', commonly called the red-berry mistletoe, is a species of Viscum, mistletoe in the family Santalaceae. It is native to Lebanon, Morocco, Palestine (region), Palestine, Portugal, Spain, and Syria. The plant has small leaves. T ...

Viscum cruciatum
'', occurs in Southwest Spain and Southern Portugal, as well as in Morocco in North Africa and in southern Africa. The genus ''Viscum'' is not native to North America, but ''Viscum album'' was introduced to Northern California in 1900. European mistletoe has smooth-edged, oval, evergreen leaves borne in pairs along the woody stem, and waxy, white
berries A berry is a small, pulpy, and often edible fruit. Typically, berries are juicy, rounded, brightly colored, sweet, sour or tart, and do not have a stone or pit, although many pips or seeds may be present. Common examples are strawberries, raspbe ...

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. The eastern mistletoe native to North America, ''
Phoradendron leucarpum ''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 Mexic ...
'', belongs to a distinct genus of the family
Santalaceae The Santalaceae, sandalwoods, are a widely distributed family of flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Order(biology), orders, 4 ...
. 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 Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
and families such as the
Misodendraceae ''Misodendrum'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer ...
of Chile and Argentina and the mainly southern hemisphere tropical
Loranthaceae Loranthaceae, commonly known as the showy mistletoes, is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). Th ...
.


Etymology

The word 'mistletoe' derives from the older form 'mistle' adding the Old English word (twig). 'Mistle' is common Germanic (Old High German , Middle High German , Old English , Old Norse ). Further etymology is uncertain, but may be related to the Germanic base for 'mash'.


Groups

Parasitism Parasitism is a close relationship between species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, ...

Parasitism
has evolved at least twelve times among the vascular plants. Molecular data show the mistletoe habit has evolved independently five times within the Santalales—first in the
Misodendraceae ''Misodendrum'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer ...
, but also in the
Loranthaceae Loranthaceae, commonly known as the showy mistletoes, is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). Th ...
and three times in the Santalaceae (in the former Santalalean families
Eremolepidaceae Eremolepidaceae is treated in some systems as a family of flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Order(biology), orders, 416 Famil ...
and
Viscaceae Viscaceae is a taxonomic family name of flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Order(biology), orders, 416 Family (biology), fami ...
, and the tribe Amphorogyneae). The largest family of mistletoes, the
Loranthaceae Loranthaceae, commonly known as the showy mistletoes, is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). Th ...
, has 73 genera and over 900 species.
Subtropical The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones located to the north and south of the tropics, Torrid Zone. Geographically part of the Geographical zone#Temperate zones, ...

Subtropical
and
tropical The tropics are the region of Earth surrounding the Equator. They are delimited in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S; these latitudes correspond to ...

tropical
climate Climate is the long-term pattern of weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a la ...

climate
s have markedly more mistletoe species; Australia has 85, of which 71 are in Loranthaceae, and 14 in Santalaceae.B. A. Barlow (1983) A revision of the Viscaceae of Australia. ''Brunonia'' 6, 25–58.


Life cycle

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. All mistletoe species are 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 ''Viscum minimum'' is a species of mistletoe in the family Santalaceae. It is a parasitic plant native to South Africa. Description In its native habitat ''Viscum minimum'' uses two species of succulents, ''Euphorbia polygona'' and ''Euphorbia h ...
'', that parasitize
succulent ''. In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the ...
s, 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. Most of the Viscaceae bear evergreen
leaves A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit together form the shoot system. Leaves are ...

leaves
that
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
e 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 is a vestigial structure; it has lost much of its ancestral function. Vestigiality is the retention during the process of evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations o ...
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 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''. 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 Diagram of Scouler's willow (''Salix scouleriana'') seed, indicating position of hypocotyl. The hypocotyl (short for "hypocotyledonous stem", meaning "below seed leaf") is the stem of a germinating seedling A seedling is a young sporophyte ...
, 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. Species more or less obligate include the leafless quintral, ''
Tristerix ''Tristerix'' is a genus of mistletoe in the Family (biology), family Loranthaceae, native to the Andes, ranging from Colombia and Ecuador to Chile and Argentina. They are Woody plant, woody perennial plant, perennials usually occurring as aerial ...
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 The genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific cla ...
'' (dwarf mistletoe; Santalaceae) which has reduced photosynthesis; as an adult, it manufactures only a small proportion of the
sugars Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides o ...
it needs from its own photosynthesis, but as a
seedling A seedling is a young sporophyte '' Tortula muralis''. In mosses, the gametophyte is the dominant generation, while the sporophytes consist of sporangium-bearing stalks growing from the tips of the gametophytes A sporophyte () is the dip ...

seedling
actively photosynthesizes until a connection to the host is established. Some species of the largest family, Loranthaceae, have small, flowers (as with Santalaceae), but others have spectacularly showy, large, bird-pollinated flowers. Most mistletoe seeds are spread by birds that eat the 'seeds' (in actuality
drupe In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the A ...
s). Of the many bird species that feed on them, the
mistle thrush The mistle thrush (''Turdus viscivorus'') is a bird common to much of Europe, Palearctic, temperate Asia and North Africa. It is a year-round resident in a large part of its range, but northern and eastern populations bird migration, migrate so ...

mistle thrush
is the best-known in Europe, the '' Phainopepla'' in southwestern North America, and ''
Dicaeum '' Dicaeum '' is a genus of birds in the flowerpecker family Dicaeidae, a group of passerines tropical southern Asia and Australasia from India east to the Philippines and south to Australia. The genus is closely related to the genus ''Prionoch ...
'' 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 {{Short pages monitor