Along this straight, gray artery of the San Joaquin Valley, America's richest market basket, rests a reminder of the dry hole it once was and could be again.

The highway's unirrigated perimeter nurtures nothing but yellow-brown grass and a few scattered California live oaks, the unpromising vista that greeted the valley's European discoverer, Pedro Fages, in 1772. The few native inhabitants lived off the acorns, baking them into bread after crushing the meat and washing out the tannic acid.

They did not understand that they lived in a natural greenhouse. They did not know what wonders would result if they brought down water from the mountain and added it to the blessings of constant temperature, rainless summers and rich soils.

Mickey George, his brother, father and cousins now grow acres of plums, peach and citrus trees in Tulare County, but Mickey George remembers what an old cowboy told him. Around the turn of the century, the old man said, "Some buddies and I decided to ride one Saturday night to the King's River and back about 30 miles . We agreed when we got back there wasn't a homestead site worth the trouble."

Today, that land is the agricultural heart of the valley, the richest vein of a $16 billion annual gold mine of fruits and vegetables. It produces more than any other state and most nations of the world. A troublemaker like the Mediterranean fruit fly is as welcome as the serpent in the Garden of Eden.

Californians lump the San Joaquin, and the smaller Sacramento Valley north of it, into the one term, Central Valley. It is a place of windless days, warm, moist nights, huge tomatoes at county fairs and fat cows lowing in the early morning, but its residents like to talk about it in numbers.

Of the state's $15.7 billion in crops and by-products last year, the valley produced about $10 billion. The second-place contenders, Texas and Iowa, are far behind.

In the San Joaquin Valley alone there are 28,000 farms producing 250 different crops, a diversity equaled nowhere. "Anything that is grown in the United States, we produce here," said associate dean James Lyons of the University of California at Davis agricultural college.

California, with the valley as its central growing spot, produces all the nation's nectarines, 97 percent of its apricots, 92 percent of its grapes, 89 percent of its avacados, 85 percent of its lemons, 81 percent of its plums, 80 percent of its tomatoes, and 64 percent of its peaches.

The valley's richness and value to the American economy only increase the cost of the fruit fly invasion. A study by Charles E. Hess, dean of the agricultural college at Davis, estimates it would take about $1 billion the first year if the Medfly got loose in the valley and full-scale spraying and fumigating had to get under way.

The Medfly, which does not bite or sting humans, would hardly be noticed in the breadbasket of Kansas and Nebraska, since it leaves such crops as wheat alone.

But this valley is America's premier fruit and vegetable garden, and Medflies love fruits and vegetables. The creature lays eggs -- which produce fruit-destroying maggots -- in more than 200 varieties of produce, most of them grown in the valley.

Farmers here have contended with insects before: the Oriental fruit fly appears somewhere in California nearly every year, but that pest can be easily isolated and exterminated with a potent chemical that draws the male Oriental fly. The Medfly resists such simple traps.

While the Medfly cannot kill the valley, the failure of the valley's elaborate system of dams and canals could. The Sierra Nevadas, overlooking the floor of what was once an inland sea, hold down the winds and collect the snow, but it has taken some time for men to learn how to trap the mountain water and bring it into every part of the valley.

The Spanish who followed Fages did little with the valley. They preferred the coastal towns that could be reached by sea. The Americans who came were more interested in farming, but for several decades they were discouraged by the disputes over land titles and distracted by the rush for gold in the mountains.

Cattlemen found the yellow-brown grasses useful. By 1860 the valley had 3 million head of cattle. But a drought from 1862 to 1864 nearly wiped out the cattle herds, and the wheat farmers took over. A Yankee merchant from Boston, James Lloyd LaFayette Warren, began to sell seeds and agricultural instruments and organize fairs. More farmers appeared from the east.

A sordid mix of extraordinary land concessions to the major railroad companies, speculation and poorly supervised sales of public lands resulted in huge single-owner land holdings. To this day, this seems to some a blessing and to others a curse, but the large tracts owned by huge companies did accelerate the valley's development. They could afford to finance and use the largest and most modern harvesting equipment, or hire cheap labor in great numbers to do the work.

That attention to quality, technology and the economies of scale, plus the valley's natural beauties, brought in more young talent and more growth. Carl A. Pescosolido Jr., now a major orange grower in Exeter, turned up in 1959 with an economics degree from Harvard, drawn to a place "I saw as a great grainhouse."

The University of California attracted the best engineers and scientists. They developed in 1961 a tomato picker so gentle it left a fresh egg uncracked and a lettuce picker with a mechanical thumb that pressed each lettuce head, decided if it was ready to pick, then picked it.

The machines have never really replaced human pickers, however. The migrant laborers who have flooded the valley provide its sadder stories, of sickness, exploitation and heartbreak.

The homeless and the impoverished, from both devastated areas of the American South and Midwest and several other countries have been coming here for work for a century.

Prosperous Mickey George of Tulare County recalls that his grandfather, who came to the valley from the Portuguese islands of the Azores, "went to his grave an illegal alien."

Today, the issue of immigrants flares again as the U.S. government considers reviving a program to let in "guest workers" from Mexico. So many who have come here have found money and homes that the flow can probably never be stemmed.

The United Farmworkers Union, representing many of the Mexican immigrants, has grown strong. Young Mexican-American laborers have found new pride in their Hispanic roots and new strength in their numbers, which are leading popular historians to talk about the creation of "Mexamerica" that includes the valley.

As the sons and daughters of older valley residents continue to move off the land, the younger first-generation immigrants and their children appear to be the ones most likely to inherit it, along with the old fights for secure sources of water and against the challenges of small insects like the Medfly.