It is probably the most famous movie line of the 1960s: In the cocktail party scene of "The Graduate," a stuffy businessman comes up to the newly graduated Dustin Hoffman and offers one word of career advice: "Plastics."

That scene resonates with irony for Pedro Cabral da Silveira, because at the same stage of his life he received equally concise career guidance. But in da Silveira's case, the word was "Cork." On that advice, da Silveira has built a happy and prosperous life in the rolling green splendor of southern Portugal's endless oak forests, the world's leading source of corks for wine bottles.

Bobbing along on a recent tidal wave of wine sales, da Silveira has assembled a sprawling farm of more than a million cork oak trees. With future generations in mind, da Silveira is starting thousands of new trees, even though a Quercus suber planted now won't produce a single marketable cork until 2040 or so.

But for da Silveira and the rest of the cork industry, the days of wine and roses may not stretch that far. Suddenly, cork farmers--and the whole Portuguese economy--face a severe challenge.

The threat can be described in a single word: "Plastics."

In the suburbs of Seattle, an ingenious wine buff named Dennis Burns has perfected a plastic cork that both winemakers and wine drinkers could find superior to the traditional product. With Burns's company, Supreme Corq Inc., in the lead, "hundreds of millions of wine bottles" per year are now sealed with synthetic stoppers, the company said.

The plastic cork, widely endorsed by winemakers and oenophiles, is the hottest idea in bottling since the 17th century. It was around 1680 that the legendary Dom Perignon, dissatisfied with the contemporary stopper--a lump of hemp coated with olive oil--first jammed a cork into one of his bottles of champagne. An industry was born, and it has been centered on the sunny montados, the savanna-like landscapes of Portugal's Alentejo region, ever since. Today, the Alentejo turns out about 10 million corks per day.

"For centuries, we have been producing a pure product that benefits the environment," said da Silveira, bouncing and tossing over the forest floor in his mud-spattered Suzuki Grand Vitara. He is a garrulous man with a round, bearded face perpetually encircled by smoke from his Portuguese cigarettes--even though, he insists, "they tasted better in the old days, when the filter was made of cork."

"But now," da Silveira said, "there is an attack to natural cork stoppers. And this attack comes from--it is unbelievable for me!--a sterile rubber lump."

Actually, the synthetic version is made from "medical grade thermoplastic polymer," according to Jerry Zech, president of Supreme Corq.

This high-tech bottle top is more standardized and predictable than any agricultural product can be, Zech said. It costs no more than a natural cork--about 12 cents apiece for bulk buyers. It does not crumble beneath a corkscrew. It can be made in a cork-like brown, or in a rainbow of other colors; the best-seller these days is jet black.

Most important, the polymer is guaranteed free of the notorious chemical TCA, the cork contaminant that turns wine into musty, undrinkable rot. There are angry arguments in the trade about how much wine is actually ruined by TCA; a middle-of-the-road estimate would be that 1 bottle in 12 is wasted due to a tainted cork.

Still, Zech has no interest in getting into a fight with the traditional cork growers. "The wine industry is healthy and growing," he said. "The future is bright for all sorts of stoppers."

For the time being, at least, he has a point. The prosperous 1990s have sparked huge growth in wine sales. This year, about 14 billion bottles will be produced, an increase of about 50 percent over five years ago, according to the Portuguese giant Amorim, the world's biggest manufacturer of wine corks. At the moment, makers of bottle corks, natural or otherwise, are enjoying strong sales and high prices.

But since the gap between planting and product on his farm is more than 40 years, da Silveira takes the long view. And he finds it a little scary: "We can always sell our cork for flooring or bulletin boards. But the wine cork is our most profitable product. Our farms will fail if that plastic blob steals our market."

Natural cork is the soft, fibrous outer bark of a species of oak tree native to the southern Iberian Peninsula. It forms a protective layer around the core of the tree, where growth occurs.

After a tree has grown for four decades, the corky bark is ready for harvest. Highly trained teams of cutters come through the forest and peel away the outer two to three inches of bark. The wine corks are then stamped out of the bark strip. Once a tree has been peeled, it takes nine years before the bark is thick enough for another harvest.

The cork industry is so important to Portugal that it is against the law for a landowner to cut down an oak tree. A special force of "bark police" traverses the forests to make sure that nobody is clearing land or peeling the trees too early in the growing cycle.

But such traditional problems seem minor compared with the threat from Supreme Corq.

Portugal's government and the cork industry are fighting back on several levels. On the negative side, they have distributed research studies vaguely suggesting health threats from plastic. They have recruited environmental groups such as Britain's Royal Society for the Protection of Birds to argue that air pollution will skyrocket and crucial habitats will disappear if cork farmers stop tending the vast oak forests of southern Europe.

To blacken the name of the synthetic competitor, Portugal's media have reported that Supreme Corq is owned by none other than Bill Gates, chairman of Microsoft Corp. Zech chuckled at this but said, "We don't really identify our investors."

Meanwhile, Amorim has launched a worldwide quality offensive. "The biggest argument the plastic side has is [TCA] contamination," said the company's Francisco Brito Evangelista. "And we are going to take away that argument, because Amorim is going to eliminate TCA completely from natural corks."

At Ponte de Sor, Portugal, about 50 bumpy miles through the forest from the da Silveira farm, Amorim is about to open a state-of-the-art cork factory designed to eliminate any chance of contamination. Brito Evangelista said the plant will turn out some 800,000 corks per day.

Since a lot of wine drinkers don't notice what kind of stopper pops out of the bottle, the cork industry this month launched a new "cork mark," which is supposed to do for the product what wool, cashmere and cotton trademarks have done for those natural fabrics.

This year's tree-peeling season has just ended in the Alentejo, and the hills are dotted with cherry-brown trees that look strangely naked, like newly sheared sheep. A healthy tree can live through more than 20 cycles of this every-nine-years indignity, but it is still possible to feel sorry for the oaks, totally exposed to the coming winter.

"Oh, you don't have to worry about the trees," said da Silveira. "I will care for them with love, as long as I can sell my corks. But if my market goes away, what will we gain except one more artificial thing that destroys a great tradition?"