Demeter and Corn

So there's a bit of confusion I see pop up now and again, and that's when corn doesn't mean corn. What is mean is, say you read that the Greek goddess Demeter is a corn goddess. If you live in America or Canada (or a few other places), you're probably thinking of this stuff here:

Image from the Wikipedia article on corn, "Cornheap" by en:User:Pratheepps

The problem is, corn (which for clarity's sake I'm going to refer to as maize), is native to the Americas, the ancient Greeks did not known it. So why is Demeter so frequently called a goddess of corn?

This is where we get into the second definition of corn, that is as a word for any cereal grain, or more specifically, it's sometimes used as a word to describe whatever the local cereal is. So corn can mean barley, oats, wheat, rice, rye, and yep, corn can mean maize as well in that context, since maize is a cereal.

So speaking broadly, yes, Demeter can be associated with maize, because she is a corn goddess, but in the context of ancient Greece corn refers to the local cereal grains, which would have primarily consisted of barley, but also included wheat.