How does Papa talking about a tree help develop the theme?

In this section, Papa helps Cassie understand the difference between things to let be and others to demand after she’s been told to call Lillian Jean “Miss.” He points out the way the little fig tree is surrounded by larger trees, "Them other trees all around...that oak and walnut, they’re a lot bigger and they take up more room and give so much shade they almost overshadow that little ole fig." Her father explains that like the Fig Tree, they need to do whatever it takes to survive.

"It keeps on blooming, bearing good fruit year after year, knowing all the time it’ll never get as big as them other trees. Just keeps on growing and doing what it gotta do. It don’t give up. It give up, it’ll die. There’s a lesson to be learned from that little tree, Cassie girl, ‘cause we’re like it. We keep doing what we gotta, and we don’t give up. We can’t.”

This supports the themes of "Doing what you "gotta do".