Poppies are one of the many different types of flowers that can be found etched onto a gravestone. Eternal, peaceful, restful sleep is symbolized by this flower.
One can't help but wonder if L. Frank Baum, the author of The Wonderful Wizard of Oz, and the directors and producers of the film The Wizard of Oz knew of the symbolism of poppies. As the Wicked Witch of the West conjures a spell to make Dorothy and her comrades fall into a never ending sleep, her spell takes the form of a field of poppies. It certainly is an interesting coincidence!