Why didn't Tom leave the scroll for Aunt Polly?

Tom didn't leave the scroll because he came up with an idea.... an idea that would allow the boys to go home.

But something occurred to him, and he lingered con- sidering. His face lighted with a happy solution of his thought; he put the bark hastily in his pocket.

Chapter Fifteen doesn't tell us Tom's plans. You'll have to read the next chapter to find out what he's planning to do.