Symbolism, Imagery, Allegory
When Isabel and Ruth leave the Finch home, Isabel defies Robert's orders to leave everything behind and takes a handful of seeds that Momma kept in a jar. She later plants some of them outside her quarters at the Locktons'. "I did not know what they would grow into," she explains, "but planting them deep in the cool earth was a comfort" (13.49). Think about how much this parallels the new life she and Ruth have been forced into—Isabel has no idea what this life will grow into, either.
The next time we encounter the mystery seeds, they've begun to sprout. Isabel tells us their stalks are "two inches tall but gave no clue about their identity" (17.4). "Identity" is a funny word for Isabel to use here—after all, isn't that a major part of her struggle throughout the story? She certainly asks a lot of questions about who and what she is—slave or free, Loyalist or Patriot, property or person.
So in a way, Isabel herself is a lot like those plants. They're beginning to grow, to feel their way into the world and see where they fit, but there's still no evidence to tell what kinds of plants they really are. Just like our main girl.
Aside from being a symbol of Isabel's journey toward making her own life, the seeds also represent her faithfulness to her family. By taking Momma's seeds and planting them, Isabel demonstrates her desire to care for Ruth in the absence of her parents and raise her up in a way that would be pleasing to them.
When Ruth is sold, though, Isabel experiences guilt and grief for failing their parents, saying that she'll "never forgive [herself]" (20.41) for falling asleep that night. She experiences this same remorse later that winter, when she discovers the plants grown from the seeds are dead from the cold: "A lump of mud stuck in my throat. I had forgotten to care for them" (33.9). Just as she's filled with regret and shame for letting Ruth be taken—for Ruth's growth and care eluding her grasp—so, too, Isabel feels angry at herself for letting Momma's plants die.
The plants might be dead, but we don't think Isabel—or Ruth, for that matter—is done growing yet.