Plath may doesn't drop a regular rhyme scheme on us with this poem, but she goes nuts with poetic devices of assonance, consonance, and alliteration. In case you don't know already, assonance is repeated vowel sounds within words, while consonance is repeated consonant sounds. Alliteration is a particular kind of consonance where the repeated consonant sound is at the beginning of the words. Almost every stanza has an example of these bad boys of poetic devices.
We'll do some surgery on the first stanza to show you how it works. First, let's look at the consonance of the repeated V sounds:
Overnight, very
Whitely, discreetly,
Very quietly. (1-3)
The repetition of the V sound at the beginning of "very" also counts as alliteration. Notice that we have more consonance with the reoccurring R sound, which usually pops up in close proximity to the V, because "ver" is repeated three times. Although, there is one rogue R in "discreetly." As if that wasn't enough, the speaker also gives us more consonance with a repeated T sound ("Overnight," "Whitely," "discreetly," and "quietly").
Well, there's no doubt this lady loved her consonants, but Plath's also having fun with vowels. Check out the assonance of the repeated long I sounds here, too ("Overnight," "Whitely," and "quietly"). Is it just us, or is that one incredibly tightly constructed stanza? She does this kind of stuff for the entire poem. It's really pretty amazing. Let's pick apart another of our favorite stanzas. (What? We just like doing this.)
Soft fists insist on
Heaving the needles,
The leafy bedding, (10-12)
First, we notice consonance with the way we have F and T sounds repeated in close proximity. Cool right? It's kind of percussive and really makes those soft fists seem more insistent, beating at a sonic level. Did you notice that the S is repeated a lot as well? Hey, us too. When you've got a bunch of repeated S sounds, it's actually a special case of consonance called sibilance.
But what about assonance, you're wondering. Is there some of that here too? You bet your sweet bippy (whatever that is). In the first line ("Soft fists insist on"), you have short O and I sounds. "Fists" and "insist" actually come very close to making an internal rhyme, but they're just a hair off—or an S off actually—making them a heck of a close slant rhyme. The last two lines of the stanza ("Heaving the needles, / The leafy bedding") are crammed with long E sounds, as well. Okay, we're pretty sure you get the picture. Plath ties the entire poem neatly together with this expert use of devices.
But why, you ask? Well, we think that, just as the form gives the impression of a tight, close-off, oppressive space, these sound games of repeating vowels and consonants create the same effect. Things are tightly packed together, even on the level of sound, but that won't last for long. Look out, world, the mushrooms are poking through and they're not taking no for an answer.