And now the designer knapsack

Necessity used to be the mother of invention. Hats, for example, once protected heads from projectiles in wartime, bright sun in summertime, and cold winds in wintertime. Now, it seems, those practical inventions have become accessory accessories. Witness the ocean of cowboy hats, pith helmets, and fedoras perched a la mode on so many jet-set curls these days.

The trusty knapsack or rucksack has followed the same route.

Representative Ruth Bryan Owen of Florida almost made the knapsack fashionable back in 1931, when she decided women legislators needed to match the carrying capacity of their male counterpart's 13-pocket suits.

Unfortunately, her invention - the knapsack handbag - didn't catch on. There wasn't much demand for an over-the-shoulder sack to carry all the vital items and leave hands free for coping with bills, briefs, and oratorical gestures. Even the silver buckle on the shoulder strap, silver monogram on the bag, and ornamental spiral stitching throughout proved resistible.

A good idea, perhaps, but about 50 years before its time.

In the meantime, women legislators made do with packed purses or I-mean-business briefcases.

Then at some indeterminate point in the past decade, far from Capitol Hill (but somewhere between Hollywood and Madison Avenue), style took over where function and comfort had failed . A simple accessory became a trendy adornment. And Mrs. Owen's knapsack-handbag quietly slipped into fashion without a nod of thanks to its inventor.

Women and men now wear rather than carry knapsacks. The hiker's knapsack has become Mrs. Owen's shoulder pack for personal accessories. It makes a rugged purse for both sexes and is often casually slung over one shoulder (although two straps are still provided) and paraded through city streets and college campuses with such crucial bumper-stickerish social message as ''I'd rather be treking and picnicking in the mountains than walking to work.''

Gone are the days of discreet birdwatchers carrying knapsacks in basic Boy Scouts green or Alpine guide grey. The packs now come in vibrant rainbow colors visible for 10 blocks and are graced with foreign-designer labels.

Whereas the knapsack used to carry extra gear - a camera, sweater, cookies, or a few apples - its crucial contents now include sleek street shoes for office workers who walk to work in their running shoes.

How people maneuver with such a strange item barely hanging on one shoulder is a mystery. Turn a corner, bump into a carrier, and off flies the knapsack - taking stereo headset and sunglasses along with it in a tangle of wires and straps and buckles. Only a raging snowstorm can force fashion aside in favor of practicality.

Warm weather, however, brings out the latest in trendy ornaments.

Sunglasses once kept glare out of the eyes. Every now and then women would flip them up to hold their hair in place, or men would balance them just above the forehead as a slight concession to style.

Now, however, sunglasses - specifically mirrored mountaineering shades on lanyards with leather blinders at the temples to keep out snow and glare - have a new purpose along Atlantic or Pacific beaches and even in major cities.

They simply hang suspended at mid-stomach at a casual and almost permanent distance from the eyes, making their own public announcement, such as ''I could be totally cool and hide behind these, but now we're on the same level, okay?''

A novel necklace for squinting blond beach boys or young professionals in nine-piece business suits.

Such fashionable gimmicks are not limited to the United States.

A recent visitor to Japan, where anything remotely related to sports is the rage, tells of young Japanese fascinated by the idea of surfing, but not actually venturing into the water. ''But they drive around with surfboards on the roofs of their cars,'' he says, ''because it is socially acceptable.''

The young Japanese are also reportedly seen parading past empty tennis courts , decked out in the latest tennis whites and proudly carrying their rackets - without the slightest intention of playing. It is simply the fashionable thing to do.

If, some day, one of these youngsters is ever challenged to a tennis match, perhaps the cycle will come full turn - and fashion will become the mother of necessity.