Swapping seeds. Saving pink tomatoes and purple potatoes - all in a day's work for grass-roots seed exchange group

When Kent Whealy got married a decade ago, his newly aquired grandfather-in-law gave him some unusual seeds - a sort of dowry you might call it - that the family had brought with them from Bavaria four generations back.

They were for a potato-leaved pink tomato, a prolific pole bean, and a bright purple morning glory.

It wasn't long into his marriage before Mr. Whealy began to fully appreciate the gift. These were no ordinary seeds. He had in fact been entrusted with some family heirlooms - and saddled with an almost awesome responsibility.

These were not seeds that could be simply replaced with a packaged variety bought at the garden center. One failed crop in the home garden and - short of maybe stumbling across something similar in a remote Bavarian garden - a unique variety would be lost forever.

Mr. Whealy also realized that probably hundreds of families across the nation were in similar situations: Season after season they had saved one or perhaps several vegetable and flower varieties that had been in the families for generations.

It would be to their collective advantage, he believed, if these avid seed savers could get together. So in 1973 he began advertising in horticultural publications for similarly minded gardeners and the Seed Savers Exchange was born. The gardener-members of this grass-roots organization save and swap the seeds of these heirloom varieties, primarily for vegetables but also some flowers.

Today the exchange boasts almost 500 members. From an initial six-page circular listing the available varieties, the Seed Savers Winter Yearbook is now 200 pages thick. All told, the membership now maintains 4,000 different varieties, ''about half of them beans and another quarter tomatoes,'' says Mr. Whealy.

More recently Mr. Whealy spent many months in front of his computer terminal listing every open-pollinated variety (species whose seeds reproduce similar offspring) available from some 150 US and Canadian seedsmen. The Vegetable Variety Inventory is available for a small fee from the exchange.

As a result of the vigilance of Seed Savers Exchange members, some varieties that had been thought to be extinct have been found, propagated by exchange members, and so removed from the danger list. One such variety was the Moon-and-Stars watermelon. The watermelon, with its distinctive bright yellow spots, was found in a Missouri garden a few years back. Today it flourishes in several gardens throughout the United States. Mr. Whealy's concern, however, is that ''for every variety we find, two are being lost for good.''

In 1981 experienced gardeners among the Seed Savers Exchange membership were organized into the Growers Network. These gardeners undertake to multiply and return seed from particularly endangered species, such as the Moon-and-Stars watermelon. Some of the returned seed is placed in Mr. Whealy's refrigerated storage.

Talk to any seed saver and you quickly understand why the Growers Network is needed. The task of saving the dwindling variety of fruits and vegetables is beyond any one gardener or even a score of them. Take John Withee of Lynfield, Mass., for example. His search for the yellow-eyed bean he had known as a boy in Maine saw him uncover more than a thousand other threatened bean varieties.

His personal collection now numbers 1,000. Growing the beans is not all that time-consuming, but keeping accurate records and labeling the seeds is. Mr. Withee needed help and through the Growers Network he got some.

Then there is Robert Lobitz, whose 300-variety potato collection includes some with skins and flesh of black, buff, brown, blue, pink, purple, lavender, russet, red, yellow, and white, of course. He would appreciate help as well. So would Carl Barnes, whose many corn varieties include one grown from 800-year-old seed uncovered in a Utah cave, and Leroy Schmidtbauer, whose 50-variety tomato collection includes one shaped like a ''giant banana pepper.''

If you are interested in joining the Seed Savers Exchange or contributing through the Growers Network, send a self-addressed stamped envelope to Seed Savers Exchange, 203 Rural Avenue, Decora, Iowa 52101.

Next week: How to save home-grown seeds