What is a Plant?

Photosynthesis is the process by which plants harvest solar energy and, by way of a complex biochemical process, convert it to biomatter.  We then use this biomatter as food.  Plants are primary producers.  Without plants, human life wouldn't be possible.

I visited an inner-city junior high recently, where (among other things) I attempted to explain the important role that plants play in our lives.  I mentioned the fact that the majority of our food is derived from plants, whether from animals that consume plants, or from the plants themselves.  Several of these kids were thoroughly confused.  They said "But who eats plants?  That's just crazy."  They didn't understand that bread was made from grain, that grain is a seed of a plant.  They had no concept that broccoli was a plant, that fruits like apples grew on trees.  Then there was the issue of meat - these kids didn't see the link.  I asked them where beef came from.  Some of them knew that it came from steer (some said cows).  I then asked them what the animals ate.  They answered "Grass!"  But they had no clue that grass was a plant. 

It made me think - what is a plant?  What is the definition of a plant in common culture? Are these kids' parents just as confused about the origins of their food?  And how was something this basic left out of their education?