What explanation does Mr. Nathan give for sealing up the knot-hole?

Boo Radley leaves presents for Scout and Jem in the knot-hole of an old tree by the Radley house.  They find gifts ranging from sticks of gum to carved soap to an old spelling bee medal.  The children eagerly reach into the knot-hole to find their surprises.  One day, they go to the knot-hole and Jem notices something different about it.  He calls to Scout and she sees a strange expression on his face.  He...

Boo Radley leaves presents for Scout and Jem in the knot-hole of an old tree by the Radley house.  They find gifts ranging from sticks of gum to carved soap to an old spelling bee medal.  The children eagerly reach into the knot-hole to find their surprises.  One day, they go to the knot-hole and Jem notices something different about it.  He calls to Scout and she sees a strange expression on his face.  He is shocked because "someone had filled [their] knot-hole with cement" (To Kill a Mockingbird, Chapter 7).

They wait for Mr. Nathan Radley so they can ask him why cement is in the knothole.  The next day, they spot him.  They greet their neighbor and ask him if he was the one who put the cement in the knot-hole.  He tells them that yes, he had done it:

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

Jem and Scout are still uncertain, so they ask their father.  They point the tree out to him, and ask him if it is indeed dying.  Their father looks at the tree, and he seems puzzled by their question.  He tells them that the tree is healthy: "Look at the leaves, they're all green and full, no brown patches anywhere..."