Demeter is the goddess of the eartn, harvest and grain. Corn is a grain; thus, she also "reigns" over that element. She is often pictured with a wreath of braided corn ears around her head, and the Greeks would pray to her and offer "gifts" of the harvest to her during ancient times.
When we are studying Greek mythology, one of my students' favorite myths is that of Persephone (Demeter's daughter) and her fate in the Underworld. Demeter plays quite a large role in that myth.