An old Irish saying has it that two things are too serious to joke about -- potatoes and marriage.

Much of the world seems to agree that at least half of that sentiment is valid. Since the Spanish conquistadors came across the potato four centuries ago, it has been exported to virtually every corner of the world.

Today the potato is grown in almost all of the world's important farming countries. It is described by experts as the most important home-grown food in the United States and Europe.

No one know precisely how and where the potato originated. High in the Andes Mountians of Peru and Bolivia, where the potato has been a staple for thousands of years, there is a legend that it was a gift of the gods.

According to the legend, the gods gave the Indians seed that produced beautiful plants, which the invading enemies ate and became sick. The gods then told the Indians to dig up the roots and eat them. On doing so, they grew strong and were able to vanquish the invaders.

It is no legend that the Spanish took the potato home in the holds of their galleons, and no legend that the value of potato production today is many times that of all the treasure the Spanish stole from the Inca Empire. And it is no legend that the potato helped to change the history of the Western world.

Studies on file at the International Potato Center here attribute the European popluation explosion of the 18th century to the introduction of the potato.

"Production from a small plot of ground provided enough food for a family and usually a surplus as sell," one study says. "This made the Industrial Revolution possible."

Potatoes were once so important to Ireland's economy that "Irish potato" became a common name for the ancient Peruvian tuber. When a fungus disease destroyed Ireland's potato-crop in 1845 and 1946, the resulting famine touched off waves of immigration that helped the ethnic makeup of the United States.

These and other historic anecdotes are of interest to the potato center's scientists, who are determined to use the potato to overcome a modern oppressor -- hunger.

The area from 15 degrees north of the equator to 15 degrees south of it is the area of greatest need for more nutrition, Dr. Richard L. Sawyer, head of the center since it was founded in 1971, said in a recent interview. "Potatoes originally were never even thought of as a solution," he added, "but we believe the potato can be a solution, and a large part of our budget is designed to get our knowledge into the countries that can benefit."

Potatoes, Sawyer said, yield more portien and calories per unit of growing time and per unit of planted area than rice, wheat or corn, the other three major food crops.

One of the centers main accomplishments is the development of a potato that will mature in less than 70 days in hot, tropical climates -- a sharp break with the tradition that the potato should be grown in the cooler temperate zones.

"Medical researchers have found that the potato is better than milk for children with malnutrition," Sawyer said. "Starving children lose the capability to digest milk. Potatoes turn out to be the replacement, providing high-quality portein that's similar to milk."

The potato remains a staple of the Indian diet in the chilly valleys and plateaus of the upper Andes. The indians freeze-dry and preserve their potato crops, using a primitive process developed about 8,000 years ago.

Professor Norman R. Thompson of Michigan State University, who spent a sabbatical year at the center here, explained the process: "They spread the potatoes on rock ledge 10,000 to 14,000 feet above sea level, where heavy night frosts freeze them solidly. Intense solar radiation thaws them next day. Alternate freezing and thawing breaks the cell structure, and moisture begins to leave, helped by tramping on them with bare feet. The result is a freeze-dried product that keeps indefinitely."

The potato center carries out its research in three location in Peru and in 11 other countries around the world. It is one of nearly a dozen international agriculture research efforts supported by a long list of donor countries, development banks, foundations and agencies known jointly as the Consultative Group on International Agriculture Research.

"The goal of the center is to feed people," Sawyer said. "And there's a big difference in the way we look at things now from before.

"We used to examine percentages of protein per acre and stop there. Now we look at the amount of protein produced per acre per day. Only soybeans, beans and peas yield more protein than potatoes per unit of area per unit of time."

The center's work in devising ways of combating late blight and a host of other fungus, virus and insect enemies of the potato helps the big traditional growers in the temperate zones as well as subsistence farmers now planting potatoes for the first time in the tropics. In another break with tradition, the center is working to perfect a commercially satisfactory "true," or botanical, potato seed.

The center has major seed experiments under way in Peru, the Philippines, India and Nepal. Other researchers work in cooperation with the center in New Zealand.

"Seed development is not yet on a commercial basis, but it's not really far from that stage," Sidki Sadik, head of the center's physiology department, said. "We are bombarded by reports from the United States, Canada and other countries where a lot of work is going on in private industry because everyone can see the potential for it."