The imperativeverb "look" instantly shows the power and command she has overherhusband - this is furtheremphasised by the trochaictetrameter.
The commonnoun "serpent" creates a semantic field of sin and lies as it conveysreligiousimagery relating to the story of AdamandEve, where the devil is disguisedasasnake.
This is juxtaposed with the commonnoun "flower" - which symbolises femininity - suggesting Lady Macbeth has alreadybecome a masterofdeception.