Urban life
The irony in the title is that where this family lives, on this side of Brooklyn, there are barely any trees at all. Just to have one tree break through the concrete ends up being a sign for life and hope, mainly because the city is so removed from nature, but that doesn't mean city life is bad—it's just removed from nature. As Francie discovers the city more and more, so does the reader, and her world is this urban landscape.
Images of racial prejudice
This one is fascinating. Instead of explaining point blank why Francie feels bullied at school, the author simply displays the situation as she encounters it—with no explanation for what is actually going on. The imagery is of Francie being constantly mistreated by other kids in her school, but she just thinks they're mean. She wants to leave to attend a different school, and although she shouldn't be allowed in, she gets in because of her father, who is Irish.
The reader can deduce from this that what happened is that Francie was zoned to a school where other ethnic groups held strong prejudice against the Irish. In this era, that hatred was still alive and well. The final tell is that Johnny succeeds in convincing the charter school to make an exception (the daughter doesn't know, but it's probably because the school is more open and accepting of Irish students).
Pictures of society
For Johnny and the gang, society means different things to each person. For Francie, she experiences her first taste of real life in public school. She is exposed to new people who form instant opinions of her, and the treat her based on her appearance. This is a clever way of describing what is wrong with Francie's culture at large; her junior version of life is as real and tragic as the adult world.
In the adult world, we see what society is really like. There are those who are willing to work hard at whatever job they can get to help provide for their family, and there are those who will not make those same sacrifices.
Imagery of religion
The final pinnacle imagery system is the Irish Catholic church and its symbology, which is littered throughout the book and also in Francie's upbringing, in her mind and personality. However, the real religious journey begins for her when she decides that for all intents and purposes, she does not believe in God, at least not in the way that everyone hopes she will.
Ironically, that is the first step in religious enlightenment, the rejection of beliefs badly held. In other words, Francie only makes such important religious decisions at such a young age because she would really like to know the truth. For this reason, she takes her religious symbology and meaning, and she puts it away to see what life is like following just her instinct.
