From Wikipedia.

Poppies have long been used as a symbol of both sleep and death: sleep because of the Opium extracted from them
and death because of their commonly blood-red color.
In Greco-Roman myths poppies were used as offerings to the Dead.

Poppies are used as emblems on tombstones to Symbolize Eternal Sleep.
This aspect was used fictionally in The Wonderful Wizard of Oz to create magical poppy fields dangerous because they caused those who passed through them to Sleep Forever.

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