What does the word choice and the contrast between the two lines suggest in "In a Station Metro" by Ezra Pound?

First, note that there are no verbs in this poem, making
it only a juxtaposition of two images.  I use that word, juxtaposition, not to be
pretentious but to convey a sense of visual placement, as if the poet were a painter and
these images are visibly beside each other as if in a painting. The semicolon is an
interesting choice. Given that Pound described this poem as an equation, and that the
semicolon can be used to (somewhat paradoxically) link related,
independent clauses, it functions as an equal sign, providing two ways of
expressing a similar thing.  Whereas 2 + 2 and 4 are figures or symbols that express the
same "quantity," the "apparition of these faces in the crowd" and "petals on a wet,
black bough" express a similar "quality." 

There are no
linking words such as the simile links "as" and "like."  Pound was trying to focus on
the images themselves, making the leap from one image to another, making the leap from
faces to the petal metaphor a leap of imagination; not a leap that is literally spelled
out on the page as if it were a technical manual.  As there are no verbs, nothing
"happens" in the poem; unless you consider the work of the equation as the work - in
other words, the imaginative leap that the poem
encourages. 

The poem is a hokku, the precursor to the more
widely known haiku.  Hokku was often an introduction to a longer poem but could also be
a pairing of images, usually with no linking words.  This is called parataxis.  With
respect to this poem, it is often referred to (by Pound, I think) as super-position; one
image "on top" of another. "On top" is tricky wording.  While they are paginated one
above the other, think of the abstract nature of a numerical equation; one is not
necessarily above, below, to the right; etc. 2 + 2 and 4 are concepts and therefore
meanings that are related but that relationship is not relegated to certain spaces in
space and time.  They just are: related but independent. In physics, superposition
refers to a particle that can be in two places at one time; again, somehow beyond
space-time. 

As for the words themselves, it is open to the
reader's own semantic interpretation.  But I think it's clear that apparition (ghost,
and suddenly "appearing," French), faces - petals, and wet,
black bough is a rainy backdrop.  Perhaps it is a gloomy, yet somehow inspirational
vision of the sudden "equating" of a ghostly group of beautiful faces before a metro in
the rain with the likewise ghostly petals, if those petals are no longer attached to the
plant.  So, life, or the ephemerality of it, is the connecting metaphor in this
interpretation and the poet "sees" this as suddenly (appear, apparition); as a
revelation which can be as sudden and abstract as 2 + 2 = 4.  The meaning is
subjective.