In "Mushrooms," Sylvia Plath personifies the mushrooms and this personifications suggests hypothetical comparisons between the function/life of the mushrooms and people. Plath describes the mushrooms as seemingly insignificant, relatively unnoticed, meek or even perceived as weak, and yet determined to persevere.
Plath uses short lines to emphasize the subtlety and quiet way the mushrooms branch out and get their feet "in the door."
Widen the crannies,
Shoulder through holes. We
Diet on water,
On crumbs of shadow,
Little or nothing.
Again, Plath uses personification with "shoulder" and "asking." The mushrooms are "voiceless" and this seems like a departure from her usage of personification but it could be interpreted that the lack of a voice is another symbol of the way the mushrooms are oppressed. In other words, the personified mushroom has no political voice; or, the mushroom is voiceless relative to the forces around it that try to contain or silence it.
This poem could be interpreted as an analogy between the plight of the mushrooms and the plight of an oppressed group of people; perhaps women or minorities. In this case, "Mushrooms" is an allegory where the extended metaphor speaks of mushrooms but refers to one of these groups. Historically, women have had to fight for the right to vote, to receive equal pay, and gain equal rights. Thus, like the mushrooms, they used to be considered meek and less significant, but over time, they persevered in spite of the oppressive forces around them.