Many people would be surprised to learn that the majority of the world's food comes from just 20 plant species - two thirds of them from the grass family alone. This is especially odd, considering that the Earth's diverse flora includes tens of thousands of edible wild plant varieties. Although the seeds of a few thousand exotic crops are still planted by small farmers around the world, many traditional varieties are being abandoned, along with their unique genetic heritage, in favor of modern, high performance crops. And many of the wild ancestors of today's hybrid food crops are also disappearing, as human settlements and pollution destroy their habitats.

To preserve this genetic treasure and protect a potential source of new crops for future farmers governments and international organizations have established so-called "seed banks" that hold hundreds of thousands of seeds in protective cold storage. The U.S. Department of Agriculture operates one of the world's largest and most sophisticated seed banks at the National Center for Genetic Resources Preservation, located on the campus of Colorado State University in Fort Collins.

Plant physiologist Christina Walters is proud of her work at the National Center for Genetic Resources Preservation, where she studies ways to extend the longevity of the Center's massive seed collections. She leads me down long, echoey hallways to the heart of the seed storage lab. I'm soon to discover, it's a frigid heart.

Walters: "This is really what makes the seed lab so special. But it's cold."

Schlender: "Wow! This is cold. We're not going to get stuck in here?"

We're standing in a bitter cold, 34 x 15 meter room, lined with shelves holding row after row of paper sacks. Each sack contains about 5,000 seeds. There are chickpeas, collected from India, wheat from Iran, peanuts from Brazil. I learn that storage rooms on two floors here hold over 400,000 varieties of seeds.

Schlender: "How cold is it?"

Walters: "-18 degrees Celsius. Zero degrees Fahrenheit."

Schlender: "That's really cold. Let's get out of here."

At room temperature, a wheat seed might survive 50 years. Thanks to the arctic air in the storage rooms, some wheat seeds might still sprout 200 years from now. How can scientists be sure? Back in her laboratory, Dr. Walters introduces me to her assistants. Then she lets me examine some rare seeds through a microscope.

Schlender: "They just look like a few little . . . seeds."

Walters: "That's the untrained eye. For us, it's an endangered species that's dear to us."

My untrained eye can't tell whether these seeds will ever spout, and to my dismay, Dr. Walters' eye can't either.

Schlender: "You mean that, between you and me, I can tell just as well as you can?"

Walters: "In fact, you . . . are."

Schlender: "Well, that's pretty bad."

Walters: "Well, that's a really amazing thing of life, Is that here's something that's alive, but you can't tell it. It doesn't have any of the mechanisms that we understand as living. They don't grow, they don't reproduce. They're not metabolizing, so they just don't look alive, but they are alive. I mean, there's some really fundamental questions we're asking."

Scientists at the Colorado seed bank never take the success of their preservation efforts for granted. They send seeds to cultivation centers scattered around the United States and the world, where plant scientists "grow out" the test seed. If too many fail to sprout, the cultivation sites grow extra seeds to replenish the seed bank stores, and scientists study ways to improve seed preservation.

The technology behind seed storage is impressive. Some seeds at the national seed storage lab might remain viable for 600 years. To show me how they could survive so long, Dr. Walters leads me to a room that's relatively warm, though it contains sealed tubs of liquid nitrogen that are 10 times chillier than the big cold storage room. She unseals a cryogenic tub, and when I reach inside, my hands feel surprisingly comfortable.

"Your hands aren't sensitive to that level of cold. It's -196 degrees centigrade. Great for the seeds, but not so good for humans," Dr. Walters says.

To avoid freeze-drying my fingers, I yank them out, while Ms. Walters adds liquid nitrogen to the tank. Out billow clouds of chilly white fog.

"I love this research. It's kind of dramatic," she says.

With enough know-how and liquid nitrogen, Ms. Walters hopes someday to extend seed viability to well beyond 600 years. How much more? "A couple thousand years, plus or minus," she says.

Dr. Walters says her work is important because humans in many parts of the world are rapidly destroying plant diversity. Between now and the year 2050, she predicts, the world is likely to lose one-sixth of the seed varieties it has today. By the year 2010, half again as much could be lost. Meanwhile, soil conditions, climate and pests are changing, too. Centuries from now, we may need the wisdom locked within the seeds' DNA and preserved through cold storage.

"Seed is always revered. It's one thing that goes from one generation to another generation to another," Dr. Shands says.

Henry Shands directs the National Center for Genetic Resources Preservation. He says that every year, the seed lab and its cultivation centers send out 100,000 seed samples to people who request them from around the world. Some go to countries whose crops have been devastated by flood, disease, drought or war. Some go to other seed storage centers as well as university, government and industry labs. They all get mailed out in coin-sized packets, usually 50 seeds at a time. Dr. Shands says each packet is freely given.

"We don't even make them pay for stamp," he says.

Although the Center doesn't store patented or genetically modified seeds, cultivars developed from these seeds by private companies often become patented. Some countries that don't like the patent system and believe seed collectors should be paying them for access to their plant resources -have refused to share their native seeds with this lab. But since many of these countries lack the cold storage and testing methods needed to preserve their own vanishing species, Dr. Shands hopes they will reconsider.

"We are not owners. We are caretakers. And we owe it to our children, and their children, and future generations to make this material available, and freely available for them to use to survive on this planet," Dr. Shands says.

In addition to preserving seeds, the USDA's National Center for Genetic Resources Preservation recently began storing the frozen embryos of important or rare species of livestock another genetic treasure imperiled by human activities.