What is the symbolism of Janina pulling puffs of milkweed from her pocket and setting them free as the bombs drops?

Janina pulled something from her pocket. It was a milkweed pod. She must have plucked it from the
plant in the alley. It looked empty. She blew into it. Three or four puffs rose into the air. They sailed
up and out of the grave, past Mr. Milgrom and into the rectangle of gray sky and the black falling
teardrops of the bombs.