Talking of yews, there was apparently an excavation in 1961 by the Chalice Well, they found flints and Roman pottery, but also interestingly, the stump of a yew tree dated to the roman period. Yews of course are 'sacred' as well. The stump was 4 metres below the medieval well house....
Also in the 19th century someone put forward the theory that it was'nt a stick but a Syrian haw (fruit of the hawthorn) that was planted, later monks embellishing the story to fit in with the story of Joseph of Arimathea....