"Apparition" (line 1)
The word "apparition" often refers to the sudden and unexpected appearance of something spooky and supernatural, like a ghost or a phantom. Pound doesn’t directly call the faces "apparitions"; he only uses the word to describe the way they seem to come and go in a flash. But the word makes the faces seem like ghosts from some other realm.
"Petals on a wet, black bough" (line 1)
Are the faces on the subway even faces at all? Maybe not. Pound uses the break between the first and second lines to create confusion over two levels of reality – the reality of everyday things and the reality of spiritual visions. It may be that the faces really are petals, at least to this particular observer.