The oak tree is a sacred symbol of the Greeks, Romans and Celts. Its strength and hardness make it a symbol of immortality. The tall tree, which attracts lightning, has been associated with gods such as Zeus and the Norse Thor. The Celts considered it to be the world axis, the tree of life, mediating between heaven and earth. Its tannin-rich leaves, bark and acorns were used as a primary astringent by the ancient Greeks. The leaves are made into a stomachic and diuretic tea. A decoction of its bark is a good remedy for diarrhoea, dysentery, cystitis and tracheitis.