Mistletoe is the only completely parasitic plant. It can kill the host tree it grows upon. It has green leaves and white berries.
Mistletoe is important in druidic belief. It was considered sacred, especially when growing on an oak tree. It was used as a ward against evil, and it was a sign of fertility. (The tradition of kissing under the mistletoe is a remnant of this belief.)
Mistletoe has long been used medicinally to treat epilepsy, among other ailments. The druids believed it could cure poisoning and restore fertility to farm animals.
While mistletoe is a reputation for being highly toxic, in reality very few people get severely ill from ingestion. According to a 1992 study, "Eight of 10 people who consumed five or more berries had no symptoms, and 3 of the 11 people who consumed only leaves had upset stomachs."