They symbolize hope within the novel. When Tom Robinson is on trial and is eventually killed, the children experience a loss of innocence which is later followed by a period of recovery, similar to that of the camellias. At first, Jem is extremely distraught by Robinson's death, thinking that the court would treat all men equal, but the children do not let that event make them think all of the world is evil and prejudiced and eventually heal, knowing that there will be evil in the world yet they can be the good in it. Jem, who is older than Scout, experiences a regrowing after Robinson's death which is juxtaposed against the camellias he destroyed earlier in the book. I hope my explanation helps! ^u^