Incredible, edible bean?

Back in 1974 a National Academy of Science research team stumbled across a new bean, or more accurately, an old but much underutilized bean that grows in the tropics.

What the scientists found delighted them. ''A supermarket on a stalk,'' is how some described it. Others compared it to the butcher's pig: you use everything but the oink. Still others likened it to an ice cream cone: you eat the whole thing.

In Latin it is called Psophocarpus tetragonolobus, for reasons that are beyond me. In lay circles it is known as the ''winged bean'' and it has received a good deal of attention in the scientific press since its rediscovery.

The bean has edible, potatolike roots, spinachlike leaves, and bean pods. While the bean pods can be eaten like conventional green beans, they can also be shelled, and, while immature, cooked and eaten like green peas, which they resemble in taste. Once mature, they taste somewhat like lima beans. Would you believe it, even the flowers make a tasty meal: When sauteed they have the flavor of mushrooms. Indeed, only the stem is inedible, which leaves precious little for the compost heap.

The bean is this productive in the tropics and subtropics. In more temperate climates the vines grow well enough but do not flower and consequently produce no beans. At least, they are not supposed to. But one vine forgot to play by the rules in the Bucks County garden of Derek Fell last year.

Fell had seen the seeds advertised in the Gurney Seed Company catalog and sent away for some. The Yankton, S.D., company duly sent the seed along with words of caution. Expect a lot of greenery but no beans, it said.

Fell planted four seeds indoors in individual peat pots and transplanted them outside when the soil had warmed up. He had to cover them to save them from the final frost to strike his region that season.

The vines all prospered. In early August first one flower appeared, then another, and another. Bean pods soon followed. Close inspection showed that only one of the four vines was flowering.

Had the one-in-a-million, or more accurately one-in-10 million, chance occurred? Was this a mutant that had adapted itself to a vastly different climate, and would it be the forerunner of a new strain of winged beans that will reproduce as readily in northern Minnesota as it now does in southern Florida?

It remains to be seen if the beans, so carefully saved by gardener Fell, do reproduce true to form this year and for several years to come. Hopes are high. After all, corn, which now grows all the way up into Canada, was once a tropical grass. In very recent years, it was a chance mutation that brought about the now famous sugar snap pea.

Scientists have discovered that the winged bean is sensitive to day length. Summer days are simply too long outside the tropics to trigger flowering in the bean. When a suitable day length arrives in more northern climes, frost invariably cuts it to the ground. If the Fell beans are no longer troubled by length of days a new wonder crop may be on its way for all the world.

Meanwhile, this is the Year of the Bean in the North American world of vegetable growing, which is as good a reason as any to start experimenting with the winged beans. Grow them just as you would conventional pole beans.

If you live far enough south you can expect some beans. Farther north, the very least you will get is another helping of ''spinach.''