How does Nathan Mc Call use imagery in Makes Me Wanna Holler: A Young Black Man in America?

Imagery:

"Down on all fours in rows of vegetables, plucking up food in the sweltering heat, I couldn't help wondering what it must have been like for my ancestors to toil like this with no hope of being freed. Always nearby was the symbolic 'ol mass,' a white, shotgun-toting prison guard."