''Tree's dying... You plug 'em with cement when the're sick.'' What does Nathan Radley mean by saying this line and why does he say this?It's...

Nathan Radley, who quotes Scripture as though it were a textbook, is a Fundamentalist.  As such his thinking in other areas of life lack as much expansiveness as his interpretation of the Bible.  He is pragmatic without being the least bit imaginative.  So, he tells the children the practical reason without divulging the implications of his act. 

In the Radley household, there is an isolationism that separates its inmates from the rest of Maycomb.  Mr. Radley, a fundamentalist who had no flexibility, did not know how to deal with his son Boo when he was younger. So, he confined and restricted Boo, believing. as Robert Frost's speaker in one poem declares, "Good fences make good neighbors."  Nathan Radley perpetuates this conviction. There is no communication with other citizens of Maycomb, for communication can present problems that remaining isolated avoids.