Who has been leaving presents in the knothole of the tree for the children?

Boo Radley is the anonymous gift giver, who gives Jem and Scout small gifts in the knothole of the oak tree in his yard. Boo Radley ends up giving Jem and Scout several sticks of Wrigley's Double Mint gum, two polished Indian coins, grey twine, soap figures resembling themselves, an old spelling bee medal, an old pocket watch, and an aluminum knife.
Initially, Jem and Scout have no idea who has been leaving them gifts and think that it is someone's hiding spot. The children end up writing a thank you note to the anonymous gift giver, but Jem cannot deliver the note because Nathan Radley fills the knothole of the tree with cement. Being that Boo is an extremely reclusive individual, the gifts were his attempt at developing a friendship with Jem and Scout. Unfortunately, Boo's brother puts an end to their harmless interaction by filling the knothole.