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Dan Charles

It has become a rite of summer. Every year, a "dead zone" appears in the Gulf of Mexico. It's an area where water doesn't have enough oxygen for fish to survive. And every year, the National Oceanic and Atmospheric Administration commissions scientists to venture out into the Gulf to measure it.

Take a look at the next box of strawberries you find in the store. Depending on where in the country you happen to be, it may have come from Florida. But it won't for much longer.

Why?

In citrus-growing areas, you see lots of old converted school buses on the road; these are company buses, carrying the workers who will harvest oranges and grapefruit. And in the evening, some of those buses roll into a truck stop on a two-lane country road south of the town of LaBelle. Young men scramble out, trot into the store and line up at the taco counter.

This is where I met Esteban Gonzalez and his brother Isaac, from the Mexican state of Veracruz.

It's fall. Time to pick apples. For some of us, that's casual recreation, a leisurely stroll through picturesque orchards.

For tens of thousands of people, though, it's a paycheck. They drive hundreds of miles for the apple harvest in central Washington, western Michigan, the shores of Lake Ontario in upstate New York, and Adams County, Pa.

"The truth is, every apple that you see in the supermarket is picked by hand," says Philip Baugher, who runs a fruit tree nursery in Adams County.

Nearly every plant that we now depend on for food — from wheat to beans to tomatoes — comes from ancestors that once grew wild on hills and in forests.

In most cases, we don't know who, exactly, tamed those plants. We don't know which inventive farmer, thousands of years ago, first selected seeds and planted them for food.

The blueberry, though, is different. We know exactly who brought it in from the wild, and where.

It happened in the pine barrens of New Jersey.