This is a large Gall on an oak tree that I came across while hiking the Ouachita National Forest here in Arkansas back in December. I see a lot of these on my hikes but most of them are smaller. I stood next to this one to show the size.
Plant galls are abnormal outgrowths of plant tissues, similar to benign tumors or warts in animals. They can be caused by various parasites, from viruses, fungi, and bacteria, to other plants, insects and mites. Plant galls are often highly organized structures and because of this, the cause of the gall can often be determined without the actual agent being identified. This applies, particularly to some insect and mite plant galls. The study of plant galls is known as cecidology.Wikipedia’s contributors, “Gall,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Gall&oldid=900222568 (accessed June 27, 2019).