Basil: There are 45 Different Varieties : Seasoning: The spice can now be found in salads, soups, sauces and cooked vegetables.

Not so very long ago, basil was, for all intents and purposes, something dried that came in a jar and was used in making spaghetti sauce. Now fresh basil is constantly called for--in salads, soups, sauces, and as a seasoning for cooked vegetables. Pesto--the Italian sauce made of basil, Parmesan cheese, garlic, olive oil and pine nuts--was a novelty less than a decade ago. Now it’s so ubiquitous it sometimes seems as conventional as catsup.

Not surprisingly, fresh basil has become a year-round commodity on the order of fresh parsley. Still, it is at its sweetest and best, its most abundant and least expensive, when grown under the hot summer sun.

There are about 45 varieties of Ocimum basilicum, most of them edible and most of them delicious. In addition to the familiar sweet basil, there are:

* Lettuce leaf basil, milder than the standard but with leaves so large they can be used as wrappers for such items as giant shrimp or strips of chicken breast.

* Piccolo basil, a very ornamental plant whose tiny, highly aromatic leaves are perfect in herbed oils, pasta salads and similar foods where the bits of basil should be small but the darkening cut edges of larger leaves would be unattractive.

* Opal basil, whose primary claim to fame is the rich burgundy color of leaves and stems. The flavor is there, but it’s less pronounced and less sweet, and many find purple pesto more disconcerting than delightful. It does make pretty herb vinegar, though, and is a fine garnish for pale foods.

* Flavored basil is known primarily to gardeners, though well-stocked farm stands or fancy greengrocers may offer lemon basil, anise basil and cinnamon basil, all of which do taste like their namesakes. Holy basil is sacred in India, where it is used in religious rituals, but its fine fragrance is not accompanied by good flavor, and it isn’t (or shouldn’t be) used in cooking.

* Thyrsiflora basil, also called Thai basil, is the sweetest and most aromatic of all. Since Thai cuisine seems to be sweeping the country, we can hope the bright-green leaves of Thyrsiflora will soon be more widely marketed.

Meanwhile, big bunches of good old sweet basil are everywhere--at supermarkets, farm stands, green markets. This is the season to make big tomato and basil salads, to shred leaves into the sauteed zucchini and stuff them under the skins of chicken breasts. Now is the time to put up the pesto. Don’t forget to leave out the garlic if you’re going to freeze it; frozen garlic can turn bitter after only a month or two.

And now is the time to experiment a bit, using basil with fruit, for instance, or to moderate between hot and sweet. It does both in the following, a very pretty one-dish summer dinner that takes much longer to tell than it does to make.

Since preparation is quick and cooking is even quicker, be sure to have all ingredients (including the diners) ready before you turn on the heat. SLIGHTLY THAI WILTED SALAD

1/2 pound lean pork (tenderloin or leg), cut in 1/8-inch-thin strips

2 fresh hot red chiles (5 inches long and 1/3-inch wide at stem end)

1 1/2 teaspoons cornstarch

1/2 cup chicken stock

1 tablespoon lime juice

1 tablespoon soy sauce

1 teaspoon brown sugar

2 tablespoons peanut oil

1 tablespoon minced ginger root

2 large cloves garlic, coarsely minced

1 scant cup Thyrsiflora basil or 1 heaping cup sweet basil (small leaves and tips only)

2 medium nectarines, cut in 1/2-inch chunks (about 1 cup)

Tender inner leaves of 1 medium head romaine lettuce, shredded into ribbons (about 8 cups)

1/4 cup thinly sliced green onions, including about 1 inch green part

Plain rice or thin egg noodles

Place pork strips on paper towels in single layer and pat dry. Slit hot chiles, remove half of seeds and cut pods crosswise into thin strips. Place cornstarch in small bowl, stir in chicken stock, lime juice, soy sauce and brown sugar.

Heat large wok over high heat. Add oil, hot chiles and ginger. Cook, stirring constantly, until chiles are rich brown and ginger is golden. Remove with slotted spoon and add to chicken stock mixture.

Quickly add pork to wok and stir-fry just until meat is white. Stir in garlic, basil and stock mixture. Bring to boil. Add nectarines. Continue to cook, stirring, just until fruit is hot and sauce is slightly thickened. Add about 2/3 of lettuce, stir well and remove wok from heat.

Make bed of remaining lettuce on serving platter. Arrange pork mixture on top and sprinkle with green onions. Serve at once with rice or noodles. Makes 4 servings.