The children can’t give a letter to the person who leaves the gifts in the tree because Nathan Radley cements in the hole in the tree.
The Finch children are excited when they begin finding gifts in the Radley tree, but are not sure who is sending them at first. Scout has no idea, but Jem seems to suspect Boo. Scout and Jem decide to write a thank you letter, even though they don’t know who to address it to. They want to show their appreciation for the gestures of friendship. Unfortunately, when they go to the tree they see Mr. Nathan Radley has cemented the hole where the gifts have been left.
Scout is surprised, but Jem is very upset. He understands that the gifts were gestures of friendship and outreach by a timid, reclusive man who just wants a friend.
When we passed our tree he gave it a meditative pat on its cement, and remained deep in thought. He seemed to be working himself into a bad humor, so I kept my distance. (Ch. 7)
Nathan Radley is refusing to allow Boo to reach out to the children. By removing Boo’s one method of communication with the children, Nathan reinforces Boo’s isolation and keeps him under control.
Jem’s understanding of who actually left the presents and why demonstrates his maturity compared to Scout. He feels sorry for Boo, and this I the reason why he cries at the end of the chapter. He considers Nathan Radley mean and misses the interaction with Boo.