Despite being a very short poem, Ezra Pound's "In a Station of the Metro" explores a number of major themes. First of all is the theme of the modern world. This is shown most clearly through the poem's setting: a metro (underground) station in Paris. Pound's speaker is struck by the number of people he sees in this busy station, prompting him to compare the faces to "apparitions." This not only creates a strong visual image but also makes an important point about the modernization and growth of big cities like Paris: people are disconnected; they are just blurred faces in a crowd.
Secondly, Pound also explores another theme, that of the natural world. To show this, Pound uses a metaphor in which he compares the people in the station to wet petals on a "bough" ("a branch of a tree"). That these petals are "black" suggests that modernization, seen here through the Paris metro, has weakened society's relationship with nature. Moreover, by describing the "bough" as wet, Pound argues that this disconnection from nature is a bad thing: it has, quite literally, dampened people's spirits.