In this poem, Pound is comparing "faces in the crowd" to petals <span>that are laying against a tree branch. </span>This is exactly what he says in the poem - "petals on a wet, black bough." A bough is the main branch of a tree, so this is definitely the only appropriate answer in this case. He wants to say that people resemble these petals.
I would say that the intended meaning of that hyperbole is that the speaker thinks that the colonists are being unfairly treated by the British government.
