The kids decide to leave a note in the knothole of the Radley tree. However, when they get there, Mr. Nathan Radley is filling up the hole with cement. Because he's there, they can't leave the note. When Jem asks Nathan why he's filling it up, he says,
"Tree's dying. You plug 'em with cement when they're sick. You ought to know that, Jem."
But as Jem discusses this with Atticus, Atticus tells him that the tree looks just fine--green leaves and all. After Jem was by himself for awhile, Scout noticed that he'd been crying.
"His face was dirty in all the right places."
Jem had realized that his "friend" (Boo) had been taken away from him. He now had no chance at communicating with him anymore, and he realizes this. Scout, however, is clueless. She has no idea who was leaving the gifts.
They wanted to send a letter to their friend who was leaving them gifts at the tree. This friend in fact was Boo Radley. This was his only way of communicating with the children and his brother sealed up that tree in order for communication to stop. However, Boo's brother pretended that the tree was dying so cement was needed.
- When Jem tries to leave the note for the person who has been recently leaving gifts for him and Scout he is unable to do so because Mr. Nathan Radley has already sealed up the knothole with cement because “The tree’s dying.” When Jem asks Atticus about the tree’s state of condition, he tells him that it seemed quite healthy because the leaves were green and full. Jem then realizes that Mr. Nathan must have sealed the hole to cut off the interaction between the kids and Boo. After thinking about this Jem begins to cry.