The gravestone made of wheat in the story could symbolize a lot of things. For one, it might be a representation of the impermanence of human achievements. A gravestone is usually a marker of a person's life and legacy, but if it's made of wheat, it won't last long. This could be a commentary on how our worldly accomplishments are fleeting. It could also be a way of showing the simplicity and purity associated with rural or agricultural life. Wheat is a staple of the countryside, and using it for a gravestone gives a sense of down - to - earth simplicity. Maybe it also ties in with the idea of a person being 'of the earth' and returning to it, as wheat is part of the earth and decomposes back into it.
Well, perhaps it symbolizes the fragility of life. You see, wheat is a delicate plant. In the context of a gravestone, it shows how fragile human existence is. Also, it could signify the cycle of life. Wheat is harvested and then new wheat grows, similar to the cycle of birth and death.