LAS JOYAS, Mexico — For centuries, peasant farmers in the mountains of western Mexico have unwittingly guarded a secret that could turn out to be the botanical discovery of the century: a kind of corn that does not have to be replanted every year and that is naturally immune to half a dozen common crop diseases.

Since researchers learned of the plant a little over a decade ago, \o7 Zea diploperennis \f7 has become the centerpiece of a new kind of nature reserve and an eloquent argument for protecting habitats that may hold undiscovered plants and animals. As scientists learn more about it, the plant is coming to represent yet another cause: preserving traditional farming and a culture that revolves around corn.

Far from being a completely wild species as first thought, the plant appears to be a sort of benevolent weed that thrives in conjunction with organic corn farming, according to scientists at the University of Guadalajara Without the peasant farmers, the plant would probably be extinct.

That conclusion is particularly significant these days, when negotiators from Mexico, the United States and Canada are trying to hammer out a free-trade agreement that includes agricultural commerce. The two northern countries are efficient, low-cost producers that sell corn for about half the price it commands in Mexico.

But in Mexico, corn is not just a commodity. It's a cultural issue. So, the specter of low-priced northern corn inundating the Mexican market is seen not just as a trade issue but as a threat to the national heritage.

Mexicans call themselves the "children of corn," descendants of people whose religious pantheon included three corn gods. Cornstalks were common patterns in ancient Mexican art. Statues of the Aztecs' hairless dogs had corncobs carved in their mouths, the way the English place apples in the mouths of roasted pigs.

The most ancient evidence of human corn consumption was found in caves in Puebla, a state in central Mexico. Archeologists estimate the corn discovered there is 7,000 years old.

Today, corn remains a fundamental element of Mexican culture. Most of the country's corn is grown by subsistence farmers, who sell their excess production at local markets.

The lack of commercial corn farming has meant little use of sophisticated, commercial hybrids. Two-thirds of Mexico's cornfields are planted in native maize varieties, known in the United States as "Indian corn."