What can you say about an apple named granny smith? People have been known to buy it just because it sounds cute. Other people refuse to buy it because it is too silly. (Washingtonians can't stand silly.) But the fact is that the granny is hot.

Last year Americans ate 2 1/2 million bushels; this year that will be doubled. The figures have been doubling every year since they began keeping track of the apple here in 1981, and it is getting ready to climb right up behind delicious and golden delicious. Worldwide, it is the third or fourth biggest variety. In Australia, it is No. 1.

But then, Australia is where it came from, and therefore it is not so amazing that there is a memorial park dedicated to Granny Smith in Ryde, a suburb of Sydney.

To Granny herself, you understand, not the apple. She was Maria Ann Smith, and she lived with her husband Thomas in Ryde in the 1850s. They had a small orchard and drove their cart into Sydney Market to sell their wares.

There are several stories, but the best is this: One day Granny got hold of a discarded gin case at the market and brought it home to use for packing apples. Some rotten Tasmanian French crabapples lay in the bottom, and she tipped them out onto the compost pile at the foot of her garden.

Next thing she knew, a tree had sprouted. Eventually it bore big bright green apples, speckled with white and wonderfully tart. And crisp. Crisp! (Krispy hadn't been invented yet.) And the apples kept forever, and the neighborhood kids were always stealing them.

An old horticulturist recalls being told by one T. Small that, as a boy in 1868, he had visited the Smiths with his father, a fruit grower named E.H. Small. His father had checked out the apple, for growers are eternally searching for new varieties, and pronounced it a good cooker. Young T., however, took more than the one professional bite. He ate it. Chomped it, chewed it, delighted in the brisk snap as each new chunk cracked off between his teeth. "I say," he said. "This apple is good to eat!"

The rest is destiny. All the granny smith apples that are or were or ever will be come from that one tree in the Smith garden.

Which is a lot of trees. Grannies are grown all over the world now, notably in New Zealand, South Africa, Argentina, Chile and France, and increasingly in this country from Pennsylvania to South Carolina to Arizona. Because it thrives in warm climates as well as cool it has an extra long season, and because it keeps well it is found in stores year-round.

The granny does have its problems, as the Australian Apple and Pear Corp. freely admits. For one thing, it takes 180 or more days to mature, at least a month longer than most. And it is a sucker for apple scab, known in Australia as Black Spot. You also have to watch it for woolly aphis, dimple bug, green crinkle, bitter pit and mites. Luckily, the original granny tree was healthy.

For, as you probably know, apple varieties are like mules. Plant seeds from a granny, or a mcintosh, or a northern spy, or a york, or a rome, or a king david, or a pippin, or a golden grimes, or any apple you ever heard of, and what will come up will not be a granny, mcintosh, northern spy, york, etc., but merely a generic apple, so to speak. A particular variety must be passed on by grafting or budding or some other artificial insemination technique.

The golden russet is America's most aristocratic apple, tracing its family tree back 350 years.

One reason you no longer see the thousands of varieties you knew as a kid -- each one of which came about as a sport, a lucky chance -- is that it is simply too much expensive bother to reproduce them all, especially the ones that go mushy too soon, or are too sour or otherwise unattractive to the mass market. There does seem to be, lately, a move to restore some of the old favorites.

But who knows how many other varieties never made it at all, bloomed and faded undiscovered in a less carefully watched compost pile, in some abandoned New England farmyard long since gone to forest, or killed off by the green crinkle? It makes you think.

The Aussies are pushing a new long-keeping variety, the lady williams, which seems to be a sort of red granny. It hasn't been exported much yet but has been gradually finding new fans in Australia since 1935, when it was discovered on the farm of Arthur L. Williams in Paynedale, Western Australia. Williams' wife was named Maud, but the neighbor's little girls couldn't pronounce it so they called her Lady Williams.

The apple marches on.