The Fountain has a very close connection to the tree in Druid mythology. They both come out of the earth. It is the sacred spring. It's the Cauldron of Dagda. It's the Horn of Plenty. It's the Holy Grail. It's the life as a rejuvenating force of nature.
The fountain can also be found in Alchemy and greek mythology and probably all over the world, as the spring of life.