A burl (American English) or bur or burr (UK English) is a tree growth in which the grain has grown in a deformed manner. It is commonly found in the form of a rounded outgrowth on a tree trunk or branch that is filled with small knots from dormant buds. A burl results from a tree undergoing some form of stress. It may be caused by an injury, virus or fungus. Most burls grow beneath the ground, attached to the roots as a type of malignancy that is generally not discovered until the tree dies or falls over. Such burls sometimes appear as groups of bulbous protrusions connected by a system of rope-like roots. Insect infestation and certain types of mold infestation are the most common causes of this condition. Maple Burl is not a distinct species of Maple, but rather, it’s a figure that’s occasionally found in Acer saccharum (Sugar Maple) trees.