Editing Mistletoe (section)

== Life cycle ==
Mistletoe species grow on a wide range of [[Host (biology)|host tree]]s, 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.{{cn|date=October 2023}}

All mistletoe species are [[parasitic plant|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 [[Succulent plant|succulent]]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 much that it becomes insignificant.<ref name="Visser-1981">{{cite book |author=Visser, Johann |title=South African parasitic flowering plants |publisher=Juta |location=Cape Town |year=1981 |isbn=978-0-7021-1228-7 }}</ref>

[[File:Mistleltoe in Lebanon.JPG|thumb|Mistletoe in winter]]
Most of the Viscaceae bear evergreen [[leaf|leaves]] that [[photosynthesis]]e effectively, and photosynthesis proceeds within their green, fleshy stems as well. Some species, such as ''[[Viscum capense]]'', are adapted to [[Semi-arid climate|semi-arid]] conditions and their leaves are [[vestigial]] scales, hardly visible without detailed [[Morphological typology|morphological]] investigation. Therefore, their photosynthesis and transpiration only take place in their stems, limiting their demands on the water supply of its host, but also limiting their intake of [[carbon dioxide]] for photosynthesis. Accordingly, their contribution to the metabolic balance of its host becomes trivial and the idle parasite may become quite yellow or golden as it grows, having practically given up photosynthesis.<ref name="Visser-1981"/>

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''.<ref name="Visser-1981"/>

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 toward 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 may 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.<ref name="Visser-1981"/>

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,<ref>Susan Milius, "Botany under the Mistletoe" ''Science News'' '''158'''.26/27 (December 2000:412).</ref> and the genus  ''[[Arceuthobium]]'' (dwarf mistletoe; Santalaceae) that 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.{{Cn|date=January 2021}}

Some species of the largest family, Loranthaceae, have small, [[insect pollination|insect-pollinated]] flowers (as with Santalaceae), but others have spectacularly showy, large, [[bird pollination|bird-pollinated]] flowers.{{Cn|date=January 2021}}

Most mistletoe seeds are spread by birds who eat the 'seeds' (in actuality [[drupe]]s). 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]]'' [[flowerpecker]]s 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 and causing the bird to have to wipe it off onto a 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.<ref>{{cite book|title=Zulu Journal|year=1959|url=https://archive.org/details/zulujournalfield0000cowl|url-access=registration|access-date=17 May 2013|publisher=University of California Press|pages=[https://archive.org/details/zulujournalfield0000cowl/page/114 114]–|id=GGKEY:5QX6L53RH1U}}</ref>

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 and consumes the remainder of the fruit.<ref name="Burton-2002">{{cite book|author1=Maurice Burton|author2=Robert Burton|title=International Wildlife Encyclopedia|url=https://books.google.com/books?id=P0_AD0v7vl0C&pg=PA869|access-date=17 May 2013|year=2002|publisher=Marshall Cavendish|isbn=978-0-7614-7266-7|pages=869–}}</ref> An example of a bird with this adapted method is the [[blackcap]] (''Sylvia atricapilla'').<ref name="mistletoe - blackcap">{{cite web |last1=Elias |first1=Emily |title=Why track mistletoe? |url=https://www.oxfordsparks.ox.ac.uk/podcasts/why-track-mistletoe/ |website=Oxford Sparks |access-date=6 January 2025}}</ref>

Biochemically, viscin is a complex adhesive mix containing [[Cellulose|cellulosic]] strands and [[mucopolysaccharide]]s.<ref name="IntSocHortSci-2009">{{cite book|author1=International Society for Horticultural Science. Section for Ornamental Plants|author2=International Society for Horticultural Science. Commission on Landscape and Urban Horticulture|author3=International Society for Horticultural Science. Working Group on New Ornamentals|title=Proceedings of the VIth International Symposium on New Floricultural Crops: Funchal, Portugal, June 11–15, 2007|url=https://books.google.com/books?id=ZEAfAQAAMAAJ|access-date=17 May 2013|year=2009|publisher=International Society for Horticultural Science|isbn=978-90-6605-200-0}}</ref>

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.<ref>{{cite web|url=http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7437.html|title=Mistletoe|publisher=University of California - Davis|access-date=November 30, 2014}}</ref><ref>Torngren, T. S., E. J. Perry, and C. L. Elmore. 1980. Mistletoe Control in Shade Trees. Oakland: Univ. Calif. Agric. Nat. Res. Leaflet 2571</ref>