To my surprise, "In a Station of the Metro" is really a very short poem. There are just two lines in this poem, so I think it is easier for me to read and think about it.
"The apparition of these faces in the crowd;" After reading some introduction about the poem on line, I know the poem describes an experience for the writer of coming out of a subway. In the first line, he saw a crowd of people without liveliness and any facial expression, but with frigidity when he came out of the subway. The line may make us feel a little despaired and cold. In the modern times, everybody seems to put a mask on his face when he meets others he doesn't know a lot. Take myself for example, I usually keep my face "cool" without showing my feelings in my face when I walk on a street and face strangers. Because I have been taught not to talk to strangers since I was a child. Maybe the reason for why he used "apparition of these faces" instead of "beautiful faces" is that people are taught to keep proper distance with others, especially strangers. So the faces he saw were not with sweet smiles, but cold faces showing nothing.
"Petals on a wet, black bough." Petals are the most beautiful parts of flowers, so it means very beautiful things but without long life. I guess "petals" here mean people whose life are very colorful and various but limited. "wet, black bough" might mean the modern society. These two lines present the cold relationship between person and person. Nowadays, people ought to guard against others to protect ourselves from danger, so we don't show our happiness or other feelings in our face and try to keep our face calm without any facial expression in the modern times.
People live in the practical world, we get scared to show out our true feeling and hide our enthusiasm, sincere and warm when we face with other people who are not close and familiar with us. So the relationship between people and people is colder and colder, more and more distant. This kind of situation really makes me feel disappointed. Maybe each people should behave himself well and not to do harmful things to others, and then we can learn to trust other people more. The relationship may become warmer and better one day.