The hills step off into whiteness.
People or stars
Regard me sadly, I disappoint them.
The train leaves a line of breath.
O slow
Horse the color of rust,
Hooves, dolorous bells--
All morning the
Morning has been blackening,
A flower left out.
My bones hold a stillness, the far
Fields melt my heart.
They threaten
A representative of death: the memory of life passed. 
 Also, it is one flower ‘left out’, for example, of a bouquet. This could be a metaphor for Sylvia Plath herself, one person removed from life and beauty as represented by the flowers. 
 Alternatively, left out possibly refers to being left out  in the open air after picking, causing it to wilt.
Sheep in Fog