What is the rhyme scheme of the poem "Ars Poetica"?

“Ars Poetica” is written in free verse (sometimes called "vers libre") which characteristically has no set rhyme or metrical scheme.

This doesn't mean, however, that there's nothing to say about the metrical, rhythmic or rhyme properties of the poem: often the absence of rhyme and meter usually has a role to play in itself. 

In "Ars Poetica" there are rhymes ("mute" and "fruit" in the first couplet, for example--though, it is not strictly a "rhyming couplet" because the lines are of different metrical lengths) but they are unexpected, shifting and unusual precisely because of the way that the poem's form does not fit any usual poetic patterns. 

Note too the way that free verse plays with the reader's expectations: after the first three couplets rhyming ("mute/fruit", "dumb/thumb", "stone/grown"), Macleish suddenly throws in a couplet that doesn't rhyme at all:

A poem should be wordless
As the flight of birds

If the first line were "A poem should be words" then "words" and "birds" would rhyme--but "wordLESS" provides a little shock to the ear, because the expectation of a rhyme is not delivered by the poem. You might argue, even, that this departure from the established rhyme thus far--a little flight into the air, perhaps?--is exactly the sort of unusual, shimmering effect that free verse best creates.