The veneration of trees growing beside burial mounds or megalithic monuments was probably a pre-Celtic cult continued by the Celts. The tree embodied the ghost of the person buried under it, but such a ghost could then hardly be differentiated from a tree spirit or divinity. Even now in Celtic districts extreme veneration exists for trees growing in cemeteries and in other places. It is dangerous to cut them down or to pluck a leaf or branch from them, while in Breton churchyards the yew is thought to spread a root to the mouth of each corpse.[679] The story of the grave of Cyperissa, daughter of a Celtic king in the Danube region, from which first sprang the "mournful cypress,"[680] is connected with universal legends of trees growing from the graves of lovers until their branches intertwine. These embody the belief that the spirit of the dead is in the tree, which was thus in all likelihood the object of a cult. Instances of these legends occur in Celtic story. Yew-stakes driven through the bodies of Naisi and Deirdre to keep them apart, became yew-trees the tops of which embraced over Armagh Cathedral. A yew sprang from the grave of Bailé Mac Buain, and an apple-tree from that of his lover Aillinn, and the top of each had the form of their heads.[681] The identification of tree and ghost is here complete. The elder, rowan, and thorn are still planted round houses to keep off witches, or sprigs of rowan are placed over doorways--a survival from the time when they were believed to be tenanted by a beneficent spirit hostile to evil influences. In Ireland and the Isle of Man the thorn is thought to be the resort of fairies, and they, like the woodland fairies or "wood men" are probably representatives of the older tree spirits and gods of groves and forests.[682]