From How Stuff Works:
Mistletoe (Phoradendron flavescens or Viscum album) is a parasitic plant that grows on trees, particularly hardwood trees like oak and apple. A parasite is a plant or animal that needs another plant or animal to survive. As mistletoe grows on a tree, it sends out its roots right into the tree's bark and takes nutrients from the tree. Sometimes, mistletoe can harm a tree and cause deformities in a tree's branches, but usually it doesn't kill its host. If the host dies, the mistletoe dies.

Mistletoe produces its own food by photosynthesis, and is able to live on its own although it is mostly found in trees. Ingesting mistletoe can cause severe stomach cramps and diarrhea, and in some cases can be fatal.
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