What two things does Pound compare in the poem "In a Station of the Metro"?

"In a Station of the Metro" seeks to capture a deeply moving experience that Ezra Pound encountered in Paris when exiting the metro. As he surveyed his surroundings, a beautiful face emerged from the crowd. Then another. And another. Women, children—beauty surrounded him. He really struggled, he has said, to adequately capture what this experience meant to him. He finally did so in a poem consisting of only 14 words.
In the poem, Pound compares the faces of the crowd to petals on a wet, black bough (of a tree). The poem is devoid of any verbs, and even the implied looks like is missing from the transition between objects in the comparison.
By doing so, Pound pares down to the visual basics of the comparison. The faces appear as if an "apparition," connoting that the image is fragile and fleeting. They stand out in the crowd like petals, also fragile, but in a colorful, bright contrast to the "wet, black bough" of background scenery.