In Chapter 7 of To Kill a Mockingbird, Scout and Jem find many gifts inside the knot-hole of an old tree on the Radley property. They find sticks of gum, a boy and a girl carved out of soap, and a spelling medal, among other things. Scout and Jem eagerly look forward to each discovery inside of the knot-hole. Boo Radley is the one leaving the gifts in the knot-hole. One day, they go to the tree and discover the knot-hole filled with cement. They ask Mr. Radley about it, and he tells the children that the tree is dying and that the cement will help it:
"Tree's dying. You plug 'em with cement when they're sick. You ought to know that, Jem."
Jem and Scout find this puzzling. They ask their father about this explanation, and he tells them that the tree looks healthy. They realize that Mr. Radley had been lying to keep the gifts out of the knot-hole.
When the tree is filled with cement, it symbolizes the end of Boo Radley's attempts to communicate with the children. He communicates with them through the gifts he leaves for them. These gifts symbolize the friendship he extends to them. When the tree is filled in, it symbolizes an obstacle in their friendship.