Plants have long been associated with pagan, and later Christmas, traditions. While the most famous are holly, rosemary¹ and mistletoe², there are other lesser known plants that figure into these traditions. While some of these continue to be familiar Christmas symbols others have faded to the realm of "little known facts". In terms of little known facts, for the sake of children and pets, holiday decorators should know a little about Christmas plant safety, too.
Ancient Ivy³ 

    Ivy is perhaps the most well known of Christmas symbols.