Pedro gives Tita the roses to show his love for her no matter what Mama Elena says. This act of Pedro shows love because although their love is forbidden and they cannot marry because of the De La Garza tradition he still shows his love to Tita and reminds her that he still loves her and will always love her although is is marrying Rosaura.
In this scene, all of those who are on the ranch decide to sleep outside because of the extremely hot weather. Mama Elena cuts the watermelon and puts it in a bowl of ice cold water so that if anyone wakes they can have a cool refreshing midnight snack. This particular scene shows amorous behavior between Tita and Pedro.
In the final scene of the book, Tita and Pedro are allowed to show their love for each other without someone having to criticize them and their love for each other. Nacha lights candles and sets up the room for Tita and Pedro to make love for the last time with no one there to witness. They died making love to each other to show that true love is real and no matter what if you are meant to be with someone then you will be with them.