Throughout A Thousand Splendid Suns, Mariam often compares herself—or is compared by other people—to a weed. The first instance of this motif occurs in Chapter 2, initiated by Nana:

"To Jalil and his wives, I was a pokeroot. A mugwort. You too. And you weren't even born yet."

"What's a mugwort?" Mariam asked

"A weed," Nana said. "Something you rip out and toss aside."

As a harami, a bastard born to an unwed mother, society views Mariam as something pesky and dirty, an errant plant growing where it has no right to grow. Nana affirms this sentiment out of the gate, constantly reminding Mariam of their shared shame as mother and illegitimate daughter. 

Yet Mariam, despite the shame and indignities of life, grows persistently, refusing to die off unregarded. The weed motif comes full circle at the end of Chapter 47—coincidentally, also the end of Mariam's life:

She thought of her entry into this world, the harami child of a lowly villager, an unintended thing, a pitiable, regrettable accident. A weed. And yet she was leaving the world as a woman who had loved and been loved back. She was leaving it as a friend, a companion, a guardian. A mother. A person of consequence at last.

Mariam grows in defiance of those who consider her worthless. She loves deeply and is loved in return. Arguably, she is the most important person in Laila's life—more important even than Tariq—because she saves Laila's life, rescuing her from certain death at Rasheed's hands.