MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
A cross section through a Cork Oak ( Quercus suber ) bark burl or burr. Burls occur when an insect or mold creates stress in the tree through virus or fungus. The tree produces a growth of wood and bark that is severely contorted. These are prized for their patterns. Cork oak naturally has a thick coating of bark and so do the burls.
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