What is Demeter's symbol of power?

The main symbol associated with Demeter is a bundle of wheat. Because she rode in a chariot pulled by winged serpents, those serpents are also sometimes symbolically associated with Demeter.

Demeter was the Greek goddess of agriculture and of motherhood. She was the patron goddess of farmers and mothers. Among the lesser gods and attendants associated with her were her daughter Persephone, the goddess of spring; Hekate, the goddess of witchcraft; and various nymphs of trees, flowers and clouds. The holiest shrine built to Demeter was at Eleusis in Attika, Greece, with other shrines to her in Messenia, Greece and Enna, Sicily.