Jem goes "stark white" when he finds the knothole where Boo has been leaving gifts for the two children is filled with cement. This is a shock to him, as he has been enjoying the gifts that he and Scout have been finding there and the communication being established. He is also disturbed because he's worried that the thank you letter he left in the knothole might have been the cause of it being filled in.

In fact, he asks Nathan Radley if he plugged up the hole, and Nathan says yes. He says, falsely, that he did it because the tree was sick. But the tree, as Jem notes, is perfectly healthy. Jem must now realize that his letter tipped Nathan off to what was going on, motivating him to block this method of communication.

In Chapter 7, Jem and Scout write a note to the anonymous person who has been leaving them gifts in the knothole of the Radley tree. The next morning on their way to school, Jem runs ahead of Scout to put the note into the tree and discovers that the knothole has been filled in with cement. When he turns around to face Scout, she says, "I saw him go stark white" (Lee 39). Jem is in shock and extremely depressed after finding out that the knothole has been filled in. Jem did not expect to see the cement inside of the tree which is why he looked like he had seen a ghost. The next day, Jem asks Nathan Radley if he put the cement in the tree and Nathan lies to him by telling Jem that the tree was dying. Nathan's actions suggest that he was attempting to stop communication between Boo and the children.