In chapter 7, Jem and Scout write a letter to the anonymous gift-giver who has been leaving small presents in the knothole of the Radleys's tree. Scout is too naïve and young to realize that Boo Radley is the person leaving them gifts in the tree but notices that her brother has a hunch about the person's identity. Scout even mentions,

He [Jem] had been on the verge of telling me something all evening; his face would brighten and he would lean toward me, then he would change his mind. (Lee, 63)

After they write a letter thanking the gift-giver, Jem attempts to leave their note in the knothole of the Radley tree and discovers that Nathan had filled the knothole with cement. When Jem asks Nathan why he filled the tree with cement, Nathan lies to him and tells Jem that he filled the knothole with cement because the tree is dying. After speaking to Atticus about Nathan's reason for filling the knothole with cement, Atticus confirms that the tree looks healthy, and Jem realizes that Nathan told him a lie.

It is implied that Jem understands Nathan's true intentions for filling the knothole with cement. Jem knows that Nathan wants to prevent his brother, Boo Radley, from communicating with them. Jem is also aware that his chances of developing a friendship with Boo Radley are over, which is why he cries by himself on the front porch.

Jem recognizes, as most readers probably do, that Nathan Radley cements the secret knothole of the oak tree to prevent further contact between his brother, Boo, and the Finch children. When Jem asks Boo's brother, Nathan tells him that the

"Tree's dyin'. You plug 'em with cement when they're sick. You ought to know that, Jem.

It is Nathan's way of symbolically treating the sickness that he believes still affects Boo. Somehow, Nathan must have witnessed Boo placing items in the tree late at night; or possibly, he saw Jem and Scout retrieving the items. Nathan may have even questioned Boo about it. In any case, he closed the opening, preventing Boo's only form of contact outside the Radley House. Although Jem figures it was out of pure meanness, Nathan probably thinks that Boo's contact with children might be considered improper, so he took the action he did. Jem isn't sure about the reason until Atticus points out that the tree is healthy.

"Look at the leaves, they're all green and full, no brown patches anywhere--... The tree's as healthy as you are, Jem."