Sentence:  I feel the wool give way / as if six centuries of feet / had worn it back to the hard / earth floor it was made to cover. // Six centuries of Turkish heels / on my spine-dyed back: / madder, genista, <WSD>sumac</WSD>— / one skin color in the soil. 
Meaning of the word marked with <WSD> tags:  (sense 2)Dried and chopped-up leaves and stems of a plant of the genus "Rhus", particularly the tanner's sumac "(see sense 1)", used for dyeing and tanning leather or for medicinal purposes.