Christian traditions suggest the prickly leaves of the holly represents the thorns of Christ's Crown. The perennial green leaves represent eternal life and the red berries represent the blood that was shed. Ivy , another evergreen, often symbolizes the female and the holly represents the male. The first to be brought into the house indicates who would rule the house in the coming year. This is an older tradition and can be heard in many Yuletide verses.
Poinsettias are native to Mexico.
Legends tell of a poor Mexican girl and her brother on their way to church on Christmas Eve. They had nothing to give the Christ child so they gathered some weeds to make a bouquet. Another version tells of a poor child who prayed for a gift to present to the Christ child. His prayers were sincere and as he knelt the poinsettia grew at the altar. Poinsettias were brought to the United States by Dr. Joel R. Poinsett who adapted them for use in the US and are their namesake.