A growth on a Tuliptree trunk.
With evidence of several others that have broken off.
Or, uh, been broken off, as in this case. I grabbed it and it snapped right off.
Meanwhile, on an oak.
What are these things? Burls of some kind? Root tissue?
On beech. Same type of growth or different?
Backyard and Beyond 
Mysteries to further explore… I love science! Happy New Year!