What is the poem's dominant figure of speech-why does the author compare poetry to fruit to medallions to casement ledges and the flight of birds?A...

In Archibald MacLeish's poem, Ars
Poetica, (The Art of Poetry) one critic states that the reader encounters
didacticism in the guise of ars gratia artis. 
That is, the reader is instructed while the poet gratuitously provides a poem at the
same time.

The poem's dominant figure of speech is its
controlling metaphor: a poem is a timeless, infinite work of the imagination. That is,
the poem is a metaphor for poetry itself. In comparing a poem to the objects that he
does, MacLeish creates the metaphor of universality and timeless essence.  For instance,
in the first stanza, the speaker suggests with the use of the images of fruit,
medallions, stone, and the flight of birds, that a poem should not boldly announce what
it is; instead, it should suggest meanings.

In the second
stanza, the speaker uses the simile of the moon as motionless in time to imply that a
poem, like the moon, is present for whomever reads it at any given time.  It is
universal, fresh, alive, and lighted with truth for any given reader at any given
time.

And, finally, a poem has meaning for whomever reads
it.  That is, a poem may have variances in interpretation and yet be "equal to." For
example,

For
all the history of grief

An empty doorway and a maple
leaf

This metaphor expresses
grief, but some readers may interpret the empty doorway as the loss of a loved one, or
an opportunity, or someone who once stood in this doorway, such as a father, who is now
gone.

Because it is not confined to such specific ideas as
a speech, or an essay, or even a novel, MacLeish contends a poem "should not mean/but
be."  These last two lines are famous, and often quoted in anthologies of
poetry.