One of summer's most popular fruits is called a drupe!

It comes in green, yellow, crimson and purple. Combine it with another drupe fruit, the peach, and you end up with a nectarine.

Drupe is the botanist's term for a fruit that has an outer skin, a pulpy, succulent layer (the flesh) and a single seed, or pit.

Today's mystery guest is the plum. It, along with the peach and cherry, fits the description of a drupe.

Plums, in one form or another, have been mentioned in literature for at least 2,000 years and plum pits have been found in ancient Asian ruins. So plums have been around for a long time, but beyond using them to make jellies and jams, and maybe a plum cake or pie, most Americans don't cook with plums. They eat them out of hand. And there's a lot to be said for that, especially if you can find them ripe. There is nothing more disappointing than a luscious-looking plum that turns out to be sour!

Europeans, on the other hand, have found marvelous things to do with plums, cooking them with beef and poultry, turning them into dumplings and tortes. Some think the Yugoslavs, however, have put plums to their finest use - as a powerful brandy called slivovitz . The Chinese, at least those in the United States, do something pretty spectacular with plums, too. They turn them into a rich, tangy plum sauce. Also known as duck sauce, because it was first served with deep-fried duck, it is used in Cantonese restaurants in this country as a dip for spareribs and egg rolls.

Among the better-known plum varieties sold in this part of the country, though not necessarily in the greatest supply, are the Damson, a cooking plum; beach plums, cooking plums that grow wild on the Atlantic coast from Canada to Virginia; Greengage, eaten out of hand and delicious in ice cream; Santa Rosa, Queen Ann and Nubiana, eating and cooking plums; the Italian (also known as the purple or prune) plum, which is well-suited to cooking and eating and can be found even into October. Incidentally, these plums are not Italian, nor are they turned into prunes.

Sometimes plums have an identity crisis since most Americans think of prunes as fresh plums that have been dried. But some species of fresh plums are also called prunes, as in the Italian, or purple plum, which is also called an Italian prune. To further confuse matters, the French call plums prunes , while the French word for prune is pruneau .

July and August and the beginning of September are the height of the plum season, which means they are at their best and cheapest. And, therefore, time to break out of jam and jelly mold and try the plums in other dishes.

When you buy plums, look for those with good color that are from slightly firm to slightly soft to the touch. Avoid those that are hard, shriveled, overmature, bruised or poorly colored. Firm plums will soften if left at room temperature for a day or two. Ripe plums are perishable, lasting only three to five days at the most. PLUM SOUP (6 servings) 1 1/2 pounds prune or Italian plums, pitted

1/2 cup sugar or to taste 1 piece (2 inches) cinnamon stick Peel of 1 lemon

5 tablespoons cornstarch mixed with 1/4 cup cold water 1/2 cup dry white wine Sour cream or yogurt

Combine the plums, sugar, cinnamon, 8 cups water and lemon peel in saucepan and bring to boil. Reduce heat and simmer, covered, for 20 to 30 minutes or until plums are soft. In a blender or food processor, or using a food mill, puree the mixture. Pour into a saucepan and stir in the cornstarch mixture. Cook over moderate heat, stirring, until mixture is thick and smooth. Stir in wine and chill thoroughly. Serve garnished with our cream or yogurt. BEEF AND PLUMS (6 servings) 3 pounds cubed beef Salt and pepper to taste 4 onions, chopped 1/4 cup brandy 2 cups sour cream 1 cup beef stock Grated rind and juice of 1 lemon 3 tablespoons flour 12 ripe Italian, or prune, plums, halved and pitted 1/2 cup ripe olives, halved

Brown meat in heavy casserole in its own fat. Season with salt and pepper. Add onions and cook until they are soft. Heat brandy in small pan; ignite and pour over meat. Shake pan gently until flame dies. Beat the sour cream with the stock, lemon rind and juice. Stir a little of liquid into flour to make a smooth paste. Return to liquid mixture and mix well. Pour over meat. Cover casserole and bake at 300 degrees for 2 to 2 1/2 hours or until meat is almost tender. Add plums and olives and cook, covered 15 minutes longer until meat is tender. PLUM COMPOTE (6 servings) 2/3 cup sugar 1 cup dry white wine 1 cup water 4 whole cloves 1 piece (3 inches) stick cinnamon Peel of 1 lemon 18 ripe red or greengage plums

Combine the sugar, wine, water, cloves, cinnamon and lemon and bring to boil. Cook for 5 minutes. Add the plums; return mixture to boil; lower heat and simmer plums for about 5 minutes or until they are tender but still hold their shape. Chill in the syrup. PLUM SAUCE GLAZE FOR CHEESECAKE 2 pounds red plums, quartered 1/4 cup water 1/2 to 1 cup sugar 1 tablespoon lemon juice 1 1/2 tablespoons cornstarch

In a saucepan stir together the plums, water, sugar and lemon juice over low heat until juices flow. Bring to boil; then cook gently until plums are tender but still hold their shape, about 10 minutes. Stir a little of the water into the cornstarch; then stir into plum mixture. Cook a few minutes more, until mixture is clear and thickened. Cool. Arrange a few plum quarters from the sauce to top of a 9-inch cheesecake and pass remaining sauce separately.

Sauce is good for vanilla ice cream or plain custard, too. PLUM TORTE (8 servings) 1 cup sugar 1/2 cup butter 1 cup flour 1 teaspoon baking soda Salt 2 eggs 24 halved and pitted Italian or prune plums Sugar Lemon juice Cinnamon

Cream the sugar and butter. Add the flour, baking soad, a sprinkling of salt and eggs. Beat well. Place batter in a 9-inch spring form and cover top with plum halves, skin side up. Sprinkle with sugar to taste, about a teaspoon of lemon juice and 2 teaspoons or more of cinnamon. Bake at 350 degrees for one hour. Serve warm.

This torte freezes beautifully. If frozen, to serve, defrost and warm slightly at 350 degrees for about 10 minutes. Other fruits in season - blueberries, raspberries, peaches - may be substituted. PLUM DUMPLINGS (Make about 16) 3 cup flour 1 teaspoon salt 2 eggs, lightly beaten 1/2 stick butter, softened 1/2 to 3/4 cup milk 1/4 cup sugar 1 teaspoon cinnamon 1 teaspoon nutmeg 16 Italian or purple plums, pitted and sprinkled with lemon juice 7 or 8 quarts salted water Topping: 1/2 cup ground walnuts or 1/2 cup breadcrumbs, fried for a few minutes in 1/2 stick butter

Sift together flour and salt to medium bowl. Beat eggs into softened butter until they are well blended. Then stir into the flour, adding just enough milk to form a stiff dough. Remove the dough to a well-floured board and knead for about 10 minutes or until it feels smooth and satiny. Roll the dough into a ball. Cover and let rest for 30 minutes.

In a small bowl combine the sugar, cinnamon and nutmeg and mix. Divide the dough in half and roll each section out to a thickness of about 1/8 inch. Use a 3-inch round cookie-cutter to cut out rounds. Place a plum (2 halves) in the center of each round.Sprinkle with the sugar mixture and cover with another round of dough. Seal the edges firmly by pressing down all around with the tines of a fork.

Bring the salted water to a boil and drop in 8 to 10 dumplings. Simmer for 12 minutes. Remove with slotted spoon and place them in a well-buttered, ovenproof dish. Repeat until all are done. Top with buttered walnuts or breadcrumbs and sprinkle with any remaining cinnamon sugar. Serve warm.

Note: Hungarians substitute 1 1/2 cup of mashed potatoes for half the flour and serve these with sour cream as an unusual Sunday breakfast treat. Apricots, cherries or peaches can be substituted for the plums.

From "The Dumpling Cookbook" by Maria Polushkin PLUM BUTTER (8 pints) 5 pounds ripe red plum 1/2 cup water Sugar 2 1/2 teaspoons almond extract

Combine washed plums and water in heavy pot. Simmer over low heat until plums burst, juice flows freely and pit separates. Shake pot frequently to prevent sticking. Push fruit through a sieve to make a puree. If it is too thin, put back over low heat and continue to cook until the puree is thick enough to mound on a spoon. Measure puree. For each cup, add 2/3 cup sugar, stirring well. Heat to boiling point and boil vigorously, stirring constantly. When mixture forms 2 drops on the edge of a metal spoon (this is called sheeting) it is cooked. It takes about 30 minutes. Add the almond extract. Pour immediately into hot, sterilized jars. Seal.