The alder is a very ancient tree that has grown in the British Isles for thousands of years. The January tree is easily recognized by its regularly spaced branches and its conical shape. Like the willow, it is a water-loving tree. The timber is oily and water-resistant, and is often used for under-water foundations. Parts of Venice and many medieval cathedrals were built on alder foundations. The common alder (Alnus glutinosa (L.) Gaertner) is found along lowland rivers, where it grows with aspens, poplars, and willows. Like willows, alders sprout from stumps. This allows them to regenerate after heavy flooding. In protected areas they may grow to 65 feet tall. Alders are members of the birch family (Betulaceae). A tree associated with several pagan gods, the alder represented the letter F (fearn) in the druidic tree alphabet. It was known in medieval legend as the tree of the Erl King, or alternatively as the tree sacred to the god Bran, brother of Branwen who kept the Cauldron of Regeneration. Thus the tree stood for the idea of resurrection. It bore the same significance in the Odyssey. Legend says that Bran used his body to span the river Linon, forming a bridge to protect his followers from the flooding waters, as alder wood does when used as a building foundation. The beginning of the Celtic solar year was marked by the alder tree. In the territory of Celtic druids there used to be a tribe known as Arverni, 'People of the Alder'. A whistle made of Alder is the basis for the old superstition of whistling up the wind.