Plath uses a variety of devices in her poem "I Am Vertical."
To begin, here is the second stanza of the poem:
Tonight, in the infinitesimal light of the stars,
The trees and flowers have been strewing their cool odors.
I walk among them, but none of them are noticing.
Sometimes I think that when I am sleeping
I must most perfectly resemble them--
Thoughts gone dim.
It is more natural to me, lying down.
Then the sky and I are in open conversation,
And I shall be useful when I lie down finally:
The the trees may touch me for once, and the flowers have time for me.
In the bolded statement, the "none of them" she refers to is the trees and the flowers. She gives them the human capacity to notice. This is personification. Again, she argues that the sky is able to hold a conversation, which we know is only a human capability. This likewise demonstrates personfication.
Finally, Plath makes a great reference to Death. This is called an allusion. Without directly telling us that she is talking about death, she mentions those things we associated with death, being buried, lying down, and remaining for a time long enough to have roots growing down to the body.