Mistletoe is the common name for a group of hemi-parasitic plants in the order Santalales that grow attached to and within the branches of a tree or shrub. Parasitism evolved only nine times in the plant kingdom;Job Kuijt, Biology of Parasitic Flowering Flants (University of California) 1969. of those, the parasitic mistletoe habit has evolved independently five times: Misodendraceae, Loranthaceae, Santalaceae (formerly considered the separate family Eremolepidaceae), and Santalaceae (formerly treated as the separate family Viscaceae). Although Viscaceae and Eremolepidaceae were placed in a broadly-defined Santalaceae by Angiosperm Phylogeny Group 2, they originated independently, according to DNA sequences analysed by Dan Nickrent, Southern Illinois University.

The word 'mistletoe' is of uncertain etymology; it may be related to German Mist, for dung and Tang for branch, but Old English mistel was also used for basil.

Christmas () is an annual holiday that celebrates the birth of Jesus. It refers to both the day commemorating the birth, and also the Christmastide season which that day inaugurates, concluding with the Feast of the Epiphany. The date of the celebration is traditional, and is not considered to be Jesus' actual date of birth. Christmas festivities often combine the observation of the Nativity with various cultural customs, many of which have been influenced by earlier winter festivals. Although nominally a Christian holiday, it is also observed as a cultural holiday by many non-Christians. Of note not all Christian denominations, like Jehovah's Witnesses, certain Seventh Day Adventist churches, and Members of the Living Church of God celebrate christmas.

In most places around the world, Christmas Day is celebrated on December 25. Christmas Eve is the preceding day, December 24. In Germany and some other countries, the main Christmas celebrations commence on the evening of the 24th.

The day following Christmas Day, December 26, is called Boxing Day in the United Kingdom and many countries of the Commonwealth, and called St. Stephen's Day or the Feast of Saint Stephen in Catholic countries.

The Armenian Apostolic Church observes Christmas on January 6. Eastern Orthodox Churches that still use the Julian Calendar celebrate Christmas on the Julian version of 25 December, which is January 7 on the more widely used Gregorian calendar, because the two calendars are now 13 days apart.

The prominence of Christmas Day increased gradually after Charlemagne was crowned on Christmas Day in 800. Around the 12th century, the remnants of the former Saturnalian traditions of the Romans were transferred to the Twelve Days of Christmas (25 December ? 5 January). Christmas during the Middle Ages was a public festival, incorporating ivy, holly, and other evergreens, as well as gift-giving.

Modern traditions have come to include the display of Nativity scenes, holly and Christmas trees, the exchange of gifts and cards, and the arrival of Father Christmas or Santa Claus on Christmas Eve or Christmas morning. Popular Christmas themes include the promotion of goodwill and peace.

Santalales is an order of flowering plants, with a cosmopolitan distribution but heavily concentrated in tropical and subtropical regions.

Most have seeds without a testa, which is unusual for flowering plants. Many of the members of the order are parasitic plants, mostly hemi-parasites, able to produce sugars through photosynthesis, but tapping the stems or roots of other plants to obtain water and minerals; some (e.g. Arceuthobium) are obligate parasites, have low concentrations of chlorophyll within their shoots (1/5 to 1/10 of that found in their host's foliage) and derive the majority of their sustenance from the host's vascular tissues (water, micro- and macro-nutrients, and sucrose). Mistletoe is the common name for a number of parasitic plants within the order Santalales.

The APG II system of 2003 (unchanged from the APG system of 1998) uses the following circumscription:

* order Santalales

:* family Loranthaceae

:* family Misodendraceae

:* family Olacaceae

:* family Opiliaceae

:* family Santalaceae

The AP-Website indicates that Balanophoraceae should also be included in this order (post APG II). It also indicates that Olacaceae (sensu APG II) is not a good family and should be split. Furthermore, it indicates doubt about Santalaceae.

The Cronquist system (1981) used the following circumscription:

* order Santalales

:* family Medusandraceae

:* family Dipentodontaceae

:* family Olacaceae

:* family Opiliaceae

:* family Santalaceae

:* family Misodendraceae

:* family Loranthaceae

:* family Viscaceae

:* family Eremolepidaceae

:* family Balanophoraceae

The families Viscaceae and Eremolepidaceae are included in the family Santalaceae by the APG. The genera Dipentodon (Dipentodontaceae) and Medusandra (family Medusandraceae) are regarded as unplaced by APG II (as is the family Balanophoraceae, now likely to be reincluded; see above).

The family Medusandraceae consisted of two genera: Soyauxia and Medusandra. Molecular evidence place the former within the family Peridiscaceae of the order Saxifragales, and the latter within the order Malpighiales close to Passifloraceae-Turneraceae-Malesherbiaceae. The Chinese monotypic genus Dipentodon is close to Tapiscia and is proposed to constitute the new order Huerteales with Tapisciaceae and the genus Perrottetia formerly placed in the family Celastraceae.

A parasitic plant is one that derives some or all of its sustenance from another plant. About 4,100 species in approximately 19 families of flowering plants are known.Nickrent, D. L. and Musselman, L. J. 2004. Introduction to Parasitic Flowering Plants. The Plant Health Instructor. DOI: 10.1094/PHI-I-2004-0330-01 [http://www.apsnet.org/education/IntroPlantPath/PathogenGroups/Parasiticplants/] Parasitic plants have a modified root, the haustorium, that penetrates the host plant and connects to the xylem, phloem, or both. Parasitic plants are characterized as follows:

*1a. Obligate parasite ? a parasite that cannot complete its life cycle without a host.

*1b. Facultative parasite ? a parasite that can complete its life cycle independent of a host.

*2a. Stem parasite ? a parasite that attaches to the host stem.

*2b. Root parasite ? a parasite that attaches to the host root.

*3a. Holoparasite ? a plant that is completely parasitic on other plants and has virtually no chlorophyll.

*3b. Hemiparasite ? a plant that is parasitic under natural conditions and is also photosynthetic to some degree. Hemiparasites may just obtain water and mineral nutrients from the host plant. Many obtain at least part of their organic nutrients from the host as well.

For hemiparasites, one from each of the three sets of terms can be applied to the same species, e.g.

*Nuytsia floribunda is an obligate root hemiparasite.

*Rhinanthus (Yellow rattle) is a facultative root hemiparasite.

*Mistletoe is an obligate stem hemiparasite.

Holoparasites are always obligate so only two terms are needed, e.g.

*Dodder is a stem holoparasite.

*Hydnora spp. are root holoparasites.

Plants usually considered holoparasites include broomrape, dodder, Rafflesia, and Hydnoraceae. Plants usually considered hemiparasites include Castilleja, mistletoe, Western Australian Christmas tree and yellow rattle.

Image:Hyobanche sanguinea-PICT2581.jpg|3.a Holoparasite Hyobanche sanguinea, Richtersfeld, Namaqualand, northern cape, South Africa

Image:Hyobanche sanguinea-PICT2580.jpg|3.a Holoparasite Hyobanche sanguinea, Richtersfeld, Namaqualand, northern cape, South Africa

Image:Hyobanche sanguinea.JPG |3.a Holoparasite Hyobanche sanguinea, Goegap N.R., Namaqualand, northern cape, South Africa