Nathan Radley filled it with cement
Scout found two pieces of chewing gum minus their outer wrapper.
Mr. Radley fills in the knot hole
The knot hole in the tree likely rotted away over time due to exposure to the elements, insects, and decay processes. As the wood around it deteriorated, the knot hole eventually disappeared or collapsed, leaving a void in the tree.
The Radleys' tree getting filled with cement is described in Chapter 8 of "To Kill a Mockingbird" by Harper Lee. In this chapter, a mysterious figure fills the hole of the tree with cement, which upsets Jem and Scout.
Boo puts them there to please the children
The meaning of gnarl when referring to a tree is the knot protruding from a tree. This can be memorized easily by remembering that gnarl mainly means knot.
They found some Indian-head pennies
burl
gnarl
It is not a species of tree- it is a tree that has a small hollow where a branch died. Where branches grow, it leaves a round marking in the grain of the wood- a "knot". If the knot falls out, you have a knothole.
Boo Radley was the one who was leaving gifts for Jem and Scout in the knot hole of the oak tree.
To tie a knot around a tree, start by wrapping the rope around the tree and crossing the ends over each other. Then, bring one end under and through the loop created by the other end. Pull both ends tight to secure the knot around the tree.