A note on the importance of larry's letter in the plot of all my sons?

The letter implies suicide. Larry's letter to Ann adds that he loves her but that she must not wait for him.Keller is quiet. He understands. He calls for the car and is ready to go upstairs to get a jacket. Mother tries to stop him, saying that Larry would not have sent him to jail. But Keller says that this is exactly what Larry is saying in the letter. "I think to him they were all my sons"--all the pilots who died. He goes upstairs. Mother turns to Chris and pleads with him not to take Keller to jail, but Chris says that nobody could stop Keller now. Mother says that the war is over--all these things are over--he cannot take away her husband. Chris responds that Keller should not just feel sorry; Larry died not just for that. She asks what more could be done, and Chris gives her a way to become better: "Once and for all you can know there's a universe of people outside and you're responsible to it, and unless you know that, you threw away your son because that's why he died."