Mistletoe is the name given to a group of plants which are "hemi-
parasites" on other plants. Most of them fasten themselves to the branches or trunk of a tree, although a few, like the
Western Australian Christmas tree, are free-standing. All of them make use of some other
plant to gain
water and
nutrients, although their green leaves perform
photosynthesis. A
seed of one of these plants, when it lands on a branch, will send out roots which worm their way into crevices of the tree and tap into its water supply. Some types of mistletoe even develop leaves which resemble those of the tree on which they live; one hypothesis is that they are taking in growth hormones along with nutrients, and receive the same "instructions" on leaf shape as their host. There are over a thousand members of the mistletoe family; the largest number live in the
tropics. All of them are in the family
Santalales, but they are not all in the order
Loranthaceae, "showy mistletoes," (as dem bones states above); some are
Misodendraceae, "feathery mistletoes," or
Viscaceae, "mistletoe." And not all Santalales are mistletoes.
The plant usually known as just "mistletoe" is the European mistletoe (Viscum album). It has a great role in European folklore because it seems to survive with green leaves through the winter when other plants are dormant. Its white berries are extremely sticky, and birds that try to eat them often end up with one stuck to the side of their beak. This tactic works fairly well to spread the seeds inside the berries, since a frustrated bird will try to scrape off the berry onto any convenient branch, thus giving the mistletoe seed a place to germinate.
Australian mistletoes have gone one further with this seed-spreading tactic. Their seeds are so sticky that they remain so after passing through a bird's digestive system and losing their berry outside. The mistletoe bird, which digests these with unusual speed, often has to scrape the seed off its rear onto a branch, which again gives the seed a great place for sprouting once it's stuck to the wood. Some of these seeds will not germinate if they haven't been eaten and excreted by birds.
The pine mistletoe, found in the southwest U.S., relies on birds to some degree to spread its seeds, but also has another method. As its berries ripen, pressure builds up within them and their connection to the stalk they grew on gets weaker. Finally, the berry just bursts, and the seed within squirts out at an estimated speed of 45 feet per second; these seeds have traveled fifty feet, and like other mistletoes, they stick to whatever they land on.
Sources:
Attenborough, David. The Private Life of Plants. Princeton, New Jersey: Princeton University Press, 1995.
http://www.sciencenews.org/20001223/bob2.asp
http://www.science.siu.edu/parasitic-plants/
http://www.encyclopedia.com/html/m/mistleto.asp
birding.about.com/library/weekly/aa122000a.htm
http://www.rms.nau.edu/mistletoe/genera/