Project Gutenberg's Plant Lore, Legends, and Lyrics, by Richard Folkard. "Myths, Traditions, Superstitions, and Folk-Lore of the Plant Kingdom."
Found while cleaning out my bookmarks, and decided to share. Has a comparison of various world trees across cultures, plants in the bible, plants used by fairies, devils, and witches, the doctrine of signatures, floral emblems of Shakespeare, the language of flowers, mummies, and more.