English: Growth on tree trunk Beside footpath to Felbrigg Pond. Large rounded outgrowths on trunk or branches of trees are called burls; the wood cut from burls is often used for decorative purposes as a veneer. Burls develop from one or more twig buds whose cells continue to multiply but never differentiate so that the twig can elongate into a limb. Burls do not usually cause harm to trees. http://www.thefreedictionary.com/burl