The title, "Metaphors," directly states the poem's purpose: to provide a series of metaphors. The first line notes that these metaphors comprise a riddle; the solution is to derive what the metaphors refer to and what the significance is behind those metaphors.
The first line, "I'm a riddle in nine syllables," states the structure of the metaphorical poem and it is also the first clue. The title has nine letters in it and each of the nine lines contains nine syllables. As the metaphors all refer to the speaker's feelings on being pregnant, the significance of "nine" refers to the nine months of pregnancy. The poetic devices are effective because the structure (based on "nine") and form reinforce the metaphors themselves, which refer to the duration of the nine months of pregnancy.
In the second and third lines, the speaker playfully refers to her increase in size due to being pregnant. She feels like an elephant or a melon on two tendrils (two legs). The third line refers to these images and reveals that the speaker, while seemingly playful, has conflicted emotions about being pregnant. To humans, the inside fruit is the valuable part of the melon and the ivory is the valuable part of the elephant. The speaker compares herself as the relatively worthless receptacle carrying the supposedly more valuable items; the fruit and ivory clearly being compared to the child.
In line 6, she compares herself to a purse and the child as the money. This is another comparison where the child is valuable and she, the carrier, is just a means of transporting the valuable goods.
In the last two lines, the speaker notes that being pregnant is essentially irreversible.
I've eaten a bag of green apples,
Boarded the train there's no getting off.
She's "eaten a bag of green apples." This could infer that she's taken an irreversible step as Eve did by eating the fruit in the Garden of Eden. It might also refer to the fact that she's full of things that are not yet ripe but soon will be. The last line indicates her feelings of being trapped, that there is no way out of being pregnant.
The form and themes of the poem reinforce each other. The poem itself is contained and structured in terms of nine. Likewise, the speaker feels contained, trapped, and in some ways devalued by the physical and psychological implications of being pregnant.