Ezra Pound likely based this poem on the myth of Apollo, the Sun God, and Daphne, a nymph. The traditional myth is that Apollo insulted Eros (or Cupid, his Roman name), saying he was not worthy of his warlike bow and arrow. In response, Eros angrily shot Apollo with an arrow to induce his love, and then shot the nymph Daphne with an arrow to make her feel hatred. Apollo fell head over heels for Daphne and continuously followed her, while she loathed him (and all men), desperate to shake his pursuit. Finally, Eros intervened to help Apollo catch Daphne, but she begged her father, Peneus, to change her form. He agreed, and thus Daphne transformed into a tree. "A Girl" details her transformation. In the poem, Apollo accepts Daphne as she is, but laments her foolish choice to transform into a tree in the last two lines: "A child—so high—you are/and this is folly to the world."
Ezra Pound chose to employ split narration in this poem. The first five-line stanza reads as if Daphne is narrating. She closely details her transformation, describing the feeling of the tree entering her hands and growing in her breast. However, Pound wrote the second half of the poem from the perspective of a third-person onlooker, likely Apollo.
While this poem has a strong basis in mythology, and Pound clearly wrote it with this particular story in mind, there are more contemporary interpretations, as well. The first narrator could be an older child detailing her figurative transformation into a tree, letting her imagination run wild. The second stanza could be from the perspective of an adult who understands her need to escape into reverie, assuring the girl that even if the world finds her imagined transformation to be "folly," she shouldn't let that dampen her creative instincts.
The free verse form of this poem is extremely effective, particularly since it's so short. The lack of rigid structure makes it easier to picture this poem as a conversation between the two different narrators. The free verse also adds to the whimsical, childish sense of the interpretation of the poem that does not center around mythology; a child's imagination is not constrained by any sort of structure, so neither is this poem.
Though critics and scholars continue to argue over whether the true interpretation of this poem lies in mythology or is a lesson on childhood imagination, it is possible that Pound had both meanings in mind. Pound was probably using the well-known myth of Apollo and Daphne to relay a wider message about the way society looks at imagination and creativity.