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. There are hundreds of species which mostly live in tropical regions. 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 fruits, rather than white, Viscum cruciatum, occurs in Southwest Spain and Southern Portugal, as well as in Morocco in North Africa and in southern Africa. There is also a wide variety of species in Australia. The genus Viscum is not native to North America, 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 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 ten or more berries. Over the centuries, the term mistletoe 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.
Etymology The word 'mistletoe' derives from the older form 'mistle' adding the Old English word tān (twig). 'Mistle' is from Common Germanic (cf. Old High German mistil, Middle High German mistel, Old English mistel, Old Norse mistil). Further etymology is uncertain, but may be related to the Germanic base for 'mash'. Online Etymology Dictionary offers a similar theory, noting, "The alteration of the ending... is perhaps from a mistaking of the final -n for a plural suffix after tan fell from use as a separate word, but Oxford finds it a natural evolution in West Saxon based on stress."
Groups 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, but also in the Loranthaceae and three times in the Santalaceae (in the former Santalalean families Eremolepidaceae and Viscaceae, and the tribe Amphorogyneae). The largest family of mistletoes, the Loranthaceae, has 73 genera and more than 900 species. Subtropical and tropical climates have markedly more mistletoe species; Australia has 85, of which 71 are in Loranthaceae, and 14 in Santalaceae.
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 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 much that it becomes insignificant.
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