create a steady heartbeat rhythm, which abruptly halts with the shortened, unmetered lines that follow them. It is as if the uneven form is saying that while modern poetry came from a tradition of fixed metrical forms, that tradition is dead.

Elsewhere, MacLeish plays with other metrical patterns, such as the trochees found in line 12. Line 11 has an unsteady metrical pattern that begins with a trochee (stressed/unstressed), and then progresses into an anapest (unstressed/unstressed/stressed), and then includes an iamb (unstressed/stressed), and finally ends on an extra syllable, a single feminine ending (or an unstressed syllable). But line twelve switches back into a traditional metrical pattern where it suddenly shifts into trochaic tetrameter, with an extra stressed syllable at the end (or a masculine ending).

LEAV-ing, |as the MOON |re-LEASE-|es
TWIG by |TWIG the |NIGHT en-|TANG-led |TREES,

This intermittent appearance of metrical forms reminds the reader of traditional fixed form aesthetic theories of poetry, which largely grew out of ancient Greek poetic tradition. For instance, in his Manifesto, “Ars Poetica,” or “The Art of Poetry,” Horace theorizes that Homer’s use of dactylic hexameter for epics set the standard for poetry of epic content. This is just one example of where Horace argued that there are fixed forms more suitable to one subject matter than another. MacLeish’s choice to use free verse in this poem, with intermittent metrical forms, seems to put the poem in opposition to Horace’s outdated aesthetic ideas about poetry. Where lines briefly touch and break away from a fixed metrical pattern, MacLeish seems to argue that a poem’s form should be free to whimsically change from moment to moment.

“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.

Archibald MacLeish’s poem ‘Ars Poetica’, first published in 1926, is written in free verse and therefore does not have a rhyme scheme. A number of major modern poets including T. S. Eliot have used free verse where poetic meter and rhyme are deliberately neglected. This poem by MacLeish does the same. It should also be noted in this regard that the 'Salem on Literature' section of eNotes clearly states the following: ““Ars Poetica” is a short poem in free verse, its twenty-four lines divided into three stanzas of four couplets each.” The link given below provides further details on the poem.