I came across this red oak tree with a huge burl on it in Oxford, MA last fall.

Burls have always fascinated me.  Burls usually form on a tree’s trunk but can grow on limbs as well.  We know that burls are the result of unchecked, disorganized growth creating wildly contorted grain and a “bird’s eye” effect caused by aborted adventitious buds, but no one really knows why they form.  Burls should not be confused with cankers and galls, which are usually caused by pathogens or insects.  Burls always have bark on them and only rarely have sprouts; internally they are composed of all sound wood.  Cankers and galls often have internal bark and decay initiated by the agents that cause them and may have sprouts that grow and die as they get larger.