On her website Myth & Moor, Terri Windling, writer, artist, and book editor, looks at the Green Man in history, architecture, folklore and contemporary art. The blog article includes many photos.
From the blog article:
The Green Man is a pre–Christian symbol found carved into the wood and stone of pagan temples and graves, of medieval churches and cathedrals, and used as a Victorian architectural motif, across an area stretching from Ireland in the west to Russia in the east. Although commonly perceived as an ancient Celtic symbol, in fact its origins and original meaning are shrouded in mystery. The name dates back only to 1939, when folklorist Lady Raglan drew a connection between the foliate faces in English churches and the Green Man (or "Jack of the Green") tales of folklore. The evocative name has been widely adopted, but the legitimacy of the connection still remains controversial, with little real evidence to settle the question one way or the other. Earliest known examples of the foliate head (as it was known prior to Lady Raglan) date back to classical Rome — yet it was not until this pagan symbol was adopted by the Christian church that the form fully developed and proliferated across Europe. Most folklorists conjecture that the foliate head symbolized mythic rebirth and regeneration, and thus became linked to Christian iconography of resurrection. (The Tree of Life, a virtually universal symbol of life, death and regeneration, was adapted to Christian symbolism in a similar manner.)