What is an example of alliteration in Ezra Pound's "In a Station of the Metro"?

Alliteration is the repetition of consonant sounds at the beginning of a series of words. This technique can give a poetic line rhythm or musical quality as well as stress important meanings in the poem. In Pound's poem, he uses alliteration in the second line: "Petals on a wet, black bough." The repetition of the "b" with "black" and "bough" is alliterative. 

This is an Imagist poem. And this simply means the poet tried to create an image without any superfluous or unnecessary rhetorical language. Pound wrote it about a succession of beautiful faces he'd seen upon leaving the London subway. Collectively, they seem like apparitions or ghosts. As he looks quickly from one to the next, he sees one and then it is gone, and so on. It's as if, like a ghost, each face disappears as soon as he sees it. He then moves on to the next face. 

He compares the succession of faces with the petals on a "wet, black bough." Note that the comma after "wet" isolates the alliterative "black bough." The poem ends with this image and the alliteration emphasizes the image. The beauty of the petals is attached to a black bough (branch). The beauty of the petals is juxtaposed to the bleakness of the "black bough" and this is perhaps a comparison with the beautiful faces coming out of the dark subway.