Free Verse
"Mushrooms" doesn't follow an established structure, but there's no denying that every inch of it is carefully crafted. For starters, the eleven stanzas all have exactly three lines, with each line having exactly five syllables. Spend five minutes trying to write a poem of your own like that, and you'll see how tough it is to pull off such tidiness and have the thing still make sense. Why does Plath choose these exact numbers? There's no telling, but the tight, short stanzas definitely help us feel (and subtly hear) the restraints these tiny mushrooms are pushing against.
The poem also doesn't have any kind of regimented rhythm, but you can totally feel how Plath expertly manages the pace of the poem. When she wants it to slow down and move all creepily, it does. Read the stanza below out loud and you'll hear what we're talking about:
Nobody sees us,
Stops us, betrays us;
The small grains make room. (7-9)
Do you feel how the first two lines kind of steadily speed up, but then the third one brings it back down again? It's because words like "nobody" are ones we just naturally say faster. It's all one word, so it rolls off the tongue. But then the third line is all individual words, so the pace slows down a bit, really emphasizing the line. Later, when Plath really wants to emphasize something she pulls this trick:
Little or nothing.
So many of us!
So many of us! (22-24)
By repeating the line twice, she makes us super-intensely feel the many voices—the exclamation points don't hurt either.
Before we peace out of this section, we should probably point out that, since we're dealing with free verse, there's no formal rhyme scheme to this poem either. However, it's positively loaded with assonance, consonance, and alliteration, which unite the whole thing sonically in an awesome way. You'll find more details on that over in "Sound Check."