Final answer:
In 'To Kill a Mockingbird', Mr. Radley filling the tree is not because the tree is sick, which is false, but to prevent Boo Radley from communicating with Scout and Jem, symbolizing obstructed communication.
Step-by-step explanation:
The tree that Mr. Radley fills in To Kill a Mockingbird is not actually sick. This is revealed to be false within the context of the novel. Mr. Radley claims he is filling the knothole of the tree because it's sick, but in reality, he does this to stop his son, Boo Radley, from leaving gifts inside it for Scout and Jem Finch, the main characters. The author uses this event to symbolize how communication and understanding are being thwarted in the storyline and to develop the mysterious aura surrounding the Radley family.