If "Mary's" is a constituent or not?

[reginaregina]

Hello, everybody. I have a question about the sentence " Mary's book is on the table." According to what I've learned from class, the diagram of the subject should be [Mary ['s book]]. As "Mary's" is not under the same node in the tree diagram, it should not be one constituent. But if I use the stand olone test, I can ask the question "Whose book is on the table?"
The answer could be"Mary's". In this way "Mary's"seems to be a constituent. I am confused. Thank you very much.