Grain is symbolic of regeneration and resurrection. There is an obvious association between burial and planting of the seed. Grain, which is the seed from the harvested fall crop, issues forth new life in the spring. The shaft of wheat is a most common symbol seen on old tombstones.
In many ancient societies grain was symbolic of the mysteries of transformation. It was sacred to the goddess Ceridwen, and to Ceres, the Patroness of the Mysteries. Grain because of its generative power to sprout from a seed beneath the soil was believed to possess knowledge of the underworld. Therefore, the seed of the plant also is thought to be magical and contain occult power. A.G.H.
Source: 78, 161.