The ancient Celts honered nature as the ultimate teacher.  Living plants and animals were revered for thier ability to convey wisdom through thier behavior in the natural world.  Trees were very crucial to the Celts for practical and spiritual valve, with the branches reaching high into the heavens - touching the divine spiritual and invisible forces of nature.  The roots reaching deep into the physical earthly plane, grounding deep with tangible reality.  These distinctly separate forces cannot support life on thier own, but when these energies combine, life blossoms from this union.