Why does Marian hide the apple?

Marian hides the apple before entering the old ladies' home. This tells us an awful lot about what kind of person she is. For Marian is a very selfish individual. She only visits old ladies' homes and similar institutions to gain points as a Campfire Girl. She doesn't turn up at these places out of the goodness of her heart or because she wants to help people, but simply to gain credit for being charitable. Her selfish motivations are reflected in the act of hiding the apple. Marian doesn't want to share the apple with any of the ladies in the home. They're already taking enough of her time; she'll be damned if they're getting any of her food as well.
The impurity of Marian's intentions is illustrated further when she emerges from the old ladies' home after a truly uncomfortable experience there. Marian retrieves her hidden apple and takes a large bite out of it. Just as Eve in the Garden of Eden sinned by eating of the Tree of Knowledge, so Marian has sinned through her selfishness and lack of concern for any of the old ladies at the home.