French mistletoe sends British hearts aflutter. You know it's Christmastime when French mistletoe arrives in London

Mistletoe is as much a part of the British tradition as high tea. No proper home would be complete at Yuletide without a green sprig above the doorway. But what Englishmen likely don't realize is that most of the mistletoe that decks their halls comes from their age-old rival, France. Not that mistletoe won't grow in Britain. ``It grows very nicely near Windsor Castle, for example,'' says Adrian Whiteley of the Royal Horticultural Society. But mistletoe is a plant that few people want to cultivate. It's a parasite that grows in clumps on apple, poplar, and oak trees, sapping their vigor and sometimes killing the host plants.

Few people bother to harvest the evergreen plant. But Bernard Plainfoss'e not only gathers mistletoe. He also exports it from the northwestern French city of St. Malo.

Mr. Plainfoss'e points out that France is the only country in Europe to export the plant. This year it will ship more than 80 tons abroad - 66 to the United Kingdom, 10 to Sweden, and seven to Denmark. This amount marks two-thirds of the total consumed in Britain.

Plainfoss'e is four years past the French retirement age, but he continues harvesting mistletoe out of love for the tradition. His family has been in the mistletoe trade since 1912. ``When I was young, schoolchildren used to gather it for us,'' he recalls. ``They used the pocket money to buy orange- and lemon-flavored bon-bons in the little corner grocery.''

England was always the major market for this produce, and when Plainfoss'e was old enough, he began driving the delivery trucks. He would arrive in an unfamiliar English city and ask directions from the local bobbies. When they discovered what he was bringing, they would escort him - ``like the president of France,'' says Plainfoss'e with evident satisfaction. ``It was always a holiday when the mistletoe arrived.''

Those were the golden days of the mistletoe industry, when small grocery shops greeted its arrival with jubilation. Now, giant supermarket chains distribute the product.

In other ways, too, the trade has changed. When Plainfoss'e was a boy, there were 20 exporters in St. Malo alone. But World War II dealt a severe blow to the trade by cutting off regular crossings of the English Channel. Now there are only two exporters - Plainfoss'e and Charles Le Provot, a 77-year-old rival.

If the marketing has changed, the methods of harvesting have not. Today a gypsy clan collects the mistletoe for Plainfoss'e. They set off from their camps at 4 a.m. and return in the evening, each team bearing as much as a ton of mistletoe for a day's work.

They begin Nov. 1 and carry on until the middle of December. ``Only the traveling people have the courage to go out and gather mistletoe in this weather,'' says Tonia Demeulester, one of the clan.

Adults use long pruning hooks to cut the clumps from low-growing apple trees. Children shin the trunks of poplars to reach the higher bunches. The tender sprigs are torn from the larger branches and packed into 22-pound shipping crates. Each crate will bear a bright red sticker with Plainfoss'e's export tax number and the greeting, ``Merry Christmas to our English friends.''

Plainfoss'e inspects each crateload. ``It's important for it to be good quality, or the couples who kiss under it won't marry,'' he says. After 36 years of wedlock, Plainfoss'e swears by the romantic properties of mistletoe.

The custom of kissing beneath it arises from Norse mythology, which held that the sun god Balder was killed by a dart fashioned from mistletoe. When the underworld agreed to release him from its bondage, Balder's mother was so joyful that she kissed everyone under boughs of the same plant. For centuries, whenever a Scandinavian warrior met an enemy beneath a sprig of mistletoe, he was supposed to lay down his weapons and embrace his foe.

Some 50 years ago British scientists totaled up the cost of all the mistletoe purchased in Great Britain and divided it by the estimated number of kisses that took place under it. The cost, they found, was very reasonable.