Here in southern New England the corn is already waist high and growing so avidly you can almost hear the creak of stalk and leaf as the plants stretch toward the sun. The ears of sweet corn are just starting to show up on local farm stands, inaugurating one of the ceremonies of an American summer. These days the nation’s nearly 80 million-acre field of corn rolls across the countryside like a second great lawn, but this wholesome, all-American image obscures a decidedly more dubious reality.