Mistletoe is the common name for obligate

hemiparasitic 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 ...

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...

s in the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

Santalales. They are attached to their host tree or

shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from tree ...

by a structure called the

haustorium In botany and mycology, a haustorium (plural haustoria) is a rootlike structure that grows into or around another structure to absorb water or nutrients. For example, in mistletoe or members of the broomrape family, the structure penetrates th ...

, 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 The British Isles are a group of islands in the North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner and Outer Hebrides, the Northern Isl ...

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 Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to A ...

North Africa North Africa, or Northern Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, and it is sometimes defined as stretching from the Atlantic shores of Mauritania in ...

and in southern Africa. The genus ''Viscum'' is not native to

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

, but ''Viscum album'' was introduced to Northern California in 1900. 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 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, ras ...

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 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'' (the W ...

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, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

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, but also in the

and three times in the Santalaceae (in the former Santalalean families Eremolepidaceae and

Viscaceae 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 tax ...

, and the tribe Amphorogyneae). The largest family of mistletoes, the

, has 73 genera and over 900 species.

Subtropical The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north a ...

and

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...

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. Mistleltoe in Lebanon

All mistletoe species are hemiparasites because they do perform some

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

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

succulent In botany, succulent plants, also known as succulents, are plants with parts that are thickened, fleshy, and engorged, usually to retain water in arid climates or soil conditions. The word ''succulent'' comes from the Latin word ''sucus'', meani ...

s, commonly species of

Cactaceae A cactus (, or less commonly, cactus) is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. The word ''cactus'' derives, through Latin, from the Ancient Gre ...

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 that

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 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 Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

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 The hypocotyl (short for "hypocotyledonous stem", meaning "below seed leaf") is the stem of a germinating seedling, found below the cotyledons (seed leaves) and above the radicle ( root). Eudicots As the plant embryo grows at germination, it ...

, 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 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 A seedling is a young sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle (embryonic root), the hypocotyl (emb ...

actively photosynthesizes until a connection to the host is established. Some species of the largest family, Loranthaceae, have small,

insect-pollinated Entomophily or insect pollination is a form of pollination whereby pollen of plants, especially but not only of flowering plants, is distributed by insects. Flowers pollinated by insects typically advertise themselves with bright colours, ...

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 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'' 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

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