Culture : For Mexicans, Growing Corn is a National Heritage : Seven thousand years of evolution have yielded an astonishing variety. For this nation, maize has deep symbolic values.

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, Zea diploperennis 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.”

Too poor to tolerate waste, farmers use every part of the corn plant. Cobs and stalks are fed to animals, burned as fuel or woven into roofing material. Many foods are steamed in cornhusks. Corn silk can be brewed into a diuretic tea.

The multiple practical uses of corn have imbued the grain with magical properties. Kernels are tossed and read like tea leaves. Figures made from husks--such as the cornhusk mules sold on Corpus Christi day--are part of religious ceremonies and holiday celebrations.

But most important, corn is the basis of the Mexican diet.

Mexican cuisine includes more than 600 corn dishes. Every neighborhood has its tortilleria, where housewives line up to buy steaming tortillas by the kilogram as quickly as automated operations can press the dough and move it through specially designed ovens.

Everything from tamales , a sort of corn mush with a filling steamed inside a cornhusk, to atole , a thick, hot drink, is made from corn.

But not from the same corn. The hominy-like white kernels found in pozole , a spicy soup, could no more be used to make cornudos , a triangular corn puff, than popcorn could be used for corn bread. Differences in corn varieties are a big part of the personality of regional dishes.

Between the Guatemalan and U.S. borders grow at least 32 varieties of corn, each with various sub-varieties, developed in response to climate and soil differences.

“This is 7,000 years of evolution,” says Suketoshi Taba, head of the seed bank at the International Maize and Wheat Improvement Center outside Mexico City, picking up an ear from one of 11,000 bins, each containing a different type of corn, stored at the center.

The center collects corn from peasant farmers all over the world.

“This is the resource to combat disease in the future, to make a new crop, to fight hunger,” Taba says of the native corn.

That point was driven home when Rafael Guzman, a University of Guadalajara botany student, found a wild relative of corn growing in a Mexican village in the Manantlan Highlands, a four-hour horseback ride from the nearest road.

Because of the plant’s tiny ears, with only half a dozen kernels, people here call it milpilla, or little corn. They occasionally mix the kernels into corn meal but mainly use the plant for fodder. They have transplanted milpilla to various locations throughout the highlands where cattle graze.

But what caught the attention of scientists at the University of Wisconsin when Guzman showed them milpilla is that it does not have to be replanted every year and can easily be crossed with other varieties, raising the possibility of a perennial corn hybrid. Scientists also are interested in the plant’s immunity to a variety of diseases and insects that infest commercial corn.

“The advantages of growing a perennial corn are obvious,” says L. R. Nault of the Ohio Agriculture Research and Development Center, which is studying milpilla. “Once planted, the crop could be harvested for several years without replanting, offering considerable savings in time and money.”

While scientists are eagerly working on crossbreeds with milpilla, they say it usually takes 30 years or more to develop a new commercial hybrid.

Meanwhile, in an effort to preserve milpilla in a country short on both arable land and federal money, Mexico established a new kind of nature reserve here in Las Joyas, where people continue to own and use the land but work with scientists to maintain the area.

Through that partnership, researchers here have learned that the plant grows best in fields left fallow, a technique used to restore the land when farmers cannot afford fertilizer. However, if farmers abandon a field, truly wild plants, such as raspberries, begin to choke out the milpilla.

So, milpilla needs farmers--but farmers who do not use weed-killers or fertilizers. It could have survived only in the kind of traditional corn culture found in these hills near the Pacific coast, southwest of Mexico City.

While the inhabitants of the area are Indians, traditions and ceremonies connected with corn are among the few obvious remnants of their traditional culture. Dress and hairstyles are European, and only the oldest villagers still speak an Indian language.

But, like their ancestors, people in Las Joyas use corn in religious ceremonies and herbal cures. They still plant native corn varieties, crossing them with each other and, they say, with milpilla to combat blight and insects.

Conservation of those ancient methods preserved milpilla and a variety of other plants. But the methods do not produce the same yields as those in cornfields planted in commercial hybrids, then fertilized and fumigated.

While ecologically sound, the organic farming methods used here are economically disastrous, leaving the keepers of milpilla among the poorest people in the nation. That presents a special challenge for the scientists charged with preserving milpilla and the other plants and animals native to the Manantlan Highlands.

“We cannot just leave the existing system in place, including the peasant farmers dying of hunger,” says Enrique Jardell, director of the conservation project.

Researchers emphasize that finding and implementing solutions will take time. Opening the borders to a flood of corn imports will only exacerbate the problems, they believe.

Abrupt changes in corn trade policy would radically alter the country’s economy, culture and even nutrition.

Government officials say that is one reason why Mexico has not removed trade barriers for corn, leaving the grain among the 3% of products whose import still requires a special permit.

So the question here is whether that policy can or should be maintained under the pressure of free-trade negotiations. The outcome will determine where the ancient corn culture ranks on the list of national priorities.