Yggdrasil, the tree of life (Norse mythology). Its many roots and limbs keep all the different worlds together: Asgard, Midgard, Helheim, Niflheim, Muspellheim, Svartalfheim, Alfheim, Vanaheim and Jotunheim. Below its roots there is the spring that fills Mimisbrunnr, the well of wisdom, guarded by Mimir.
Tree worship was popular in Celtic thinking. The Tree King through its roots connects with mother earth, reaches heaven and connects it with the Universal Spirit. It brings wisdom and self-knowledge.