Corn-on-the-cob has been a popular American favorite since the days of the Pilgrims. Today there are more than 200 varieties and hybrids available to gardeners. And, something wonderful is taking place in the development of new varieties that are not only sweeter but stay sweet much longer.

With the usual kinds, unless you grow it yourself or can get it from someone who does, you may lose out on much of the pure joy of eating it. It stays fresh and sweet for only a very short time. The gourmet wants it cooked and on the table within an hour after picking.

This new kind of sweet corn is not conditioned by the sugary-1 gene but by a similar genetic factor (designated shrunken-2), according to Dr. E. V. Wann, research geneticist and laboratory director, U.S. Vegetable Laboratory, Agricultural Research Service, Charleston, S.C.

Shrunken-2 gene conditions an even higher level of sugar in the kernels, giving them a sweeter taste and the sweetness is maintained for a prolonged period perhaps even as much as two or three days without refrigeration.

The different types of corn should never be planted together at the same time, Dr. Wann says. Pollen from dent corn or popcorn will contaminate sweet corn, causing the kernels to be starchy and not sweet.

Nor should the standard sweet varieties be interplanted at the same time with the extra sweet (shrunken-2) varieties, as the pollen from one will contaminate the other - destroying the quality of both.

Sweet corn varieties differ in the way their growth is affected by day length. Early maturing varieties developed for the North are not recommend for the South. They are adapted to the long, cool summery days in the North and do not make satisfactory growth in the Deep South.

Conversely, the southern varieties are not adapted to the North. When planted in the North they will not silk and tassel until they reach 8 to 12 feet in height, and it is too late for them to produce edible corn before frost. Therefore, specific varieties are recommended for different sections of the country.

Sweet corn requires full sunlight. It does best planted in rows 30 to 36 inches apart with plants spaced 12 to 16 inches apart in row. Planting 4 to 5 short rows is better than one long row to insure complete pollination. It needs an adequate supply of moisture and should be watered when there is less than an inch of rainfall during the week.

Sweet corn should be harvested when the kernels are in the milk stage. At this stage the silks are brown and dry beyond the husks and the ear has enlarged enough to fill the husks tightly to the tip. The kernels are about as large as they will become, but they are still soft, tender and filled with an opaque milky juice.

The husks should never be disturbed to peek at the corn as this will permit insects and birds to invade the ear.

Sweet corn passes through its prime maturity very quickly. With uniform hybrid varieties the harvest of a single planting will last only 4 or 5 days. If harvest is delayed the kernels become tough, starchy and lose their sweet flavor.

Successive plantings are recommended in order to provide a steady supply of fresh corn throughout the practical harvest season. An early and also a full season yariety may be planted at the same time to give a prolonged harvest season.

Sweet corn loses its quality rapidly after it has been picked from the plant. For best quality, the corn should be picked early in the morning and refrigerated immediately. The sooner it is prepared for serving, the better, but it can be held in a refrigerator (35 to 40 degrees) for two to three days with only a moderate reduction in eating quality.

Many of the modern hybrids under optimum growing conditions will produce two nice ears per plant. The top ear will be the dominant one, and it will reach prime maturity a day or so ahead of the second ear.

Under such conditions, a 100-foot row should yield 100 to 120 nice ears.