In Frank Norris' short story "A Deal in Wheat", the wheat symbolizes natures power over man.
True to the characteristics of the Naturalist, Norris shows the power of nature over man. The Naturalist believes that nature will always be superior over man. No matter what man does, he will always fail if nature is involved.
That being said, the price of wheat is what all involved in the story depend upon. If the price goes up, man can be successful. If the price goes down, man will fail. The power of nature, therefore, is seen in the promising abundant crop or the failure associated with a bad crop.
Regardless, it is up to nature whether or not man will profit. Only nature can decide if the wheat will yield a good or bad harvest. In the end, wheat simply symbolizes the power of nature.