The water hyacinth is a flower of such texture and general elegance that it is hard to believe it is one of the world's great pests.

I was aware the St. Johns River in Florida had these lovely flowers in such mats and masses as to impede traffic, but only after I ran into Dr. Noel Vietmeyer of the National Academy of Sciences did I learn other wonderful things about this outrageous plant.

It comes from Brazil, he believes, but evidently behaved itself in former times or else stayed back in the Amazon jungle where gardeners rarely penetrated, since it is not mentioned until 1824 when the German botanist Karl von Martius mentioned it following an expedition to that country.

In our own country the plant had a marvelous boost in 1884 when it was given as a souvenir to a lot of visitors attending the Cotton States Exposition in New Orleans. In no time it was appearing in the warm wild waters of the South as an escape. By 1895 it could be found 600 miles to the east in the St. Johns River, where it extended 200 feet from the shore on each side of the stream, in patches (not solid) running 100 miles.

A fortunate gale in 1896 blew the water hyacinth up the river, blocking it for 25 miles and wrecking an important local lumber industry when logs could no longer be floated down to market.

By the turn of the century the plant was in India. In the 1950s it was to be found in more than one African river and it appeared in Australia and Central America in the 1960s. Today it is well established in all kinds of rivers including the Nile, the Mississippi, the Ganges, the Zambezi, the Congo and the Mekong, to say nothing of the Amazon.

One year in my former garden I set one plant out in my lily pool the end of May, and by July I was using a spading fork to lift out the surplus. A single plant, Vietmeyer reports, can spread prodigiously. In a Louisiana sample it was found that two plants increased to 300 within 23 days. Ten plants can cover an acre of water in 10 months.

I am not, of course, the only gardener to consider this one of the loveliest of all flowers. The king of Siam's wife introduced it to the palace gardens in Bangkok in 1901 and there it grew admirably until a flood washed some of them into the canals, where it has run wild.

The water hyacinth has inflated stems, like ping-pong balls fitted with a dense mat of fine roots that hang in the water, and a leaf like a sail on top. Stolons are produced from the parent plant, each with a new plant at the tip, and in time the stolon rots and the new plant can sail off on its own.

It is a fine plant for fish fry to shelter in, and unfortunately it is also ideal for mosquito larvae. In a small ornamental pool it does no damage, and goldfish soon eat the mosquito infants, but in frost-free climates the plant forms such dense mats and so much rotting detritus that oxygen levels are reduced and fish virtually vanish. Then the mosquitoes flourish indeed.

Probably the water hyacinth was present in every Deep South state before Congress passed a law forbidding its transport across state lines. Up here the plant dies over winter unless brought indoors, so there seems little reason to forbid importing it for aquariums and fish pools, but since the law went into effect some years ago I do not know where you can get it now.

You used to see it commonly in places where they sell small ornamental tropical fish, but several gardeners tell me they cannot find it in aquariums now. If you find a fellow gardener with some, all you need is one small plant, and by fall you will have enough to stock the world. I no longer have any, having let mine die in the usual way through the winter.

The plant can gang up and in a storm can knock down bridges, and in tropical regions where they need irrigation for crops the hyacinth can clog up pumps. It can be a disaster in rice paddies, certainly inhibiting and often destroying the rice seedlings.

Back of dams it can be a terror. Within two years of the completion of a great dam in Suriname, the hyacinth had blanketed 12 square miles of the reservoir. In the Egyptian delta the plant was common around Alexandria but did no great damage since the Nile floods washed it out to sea every year. But since the Aswan High Dam was built, controlling and rationing the Nile flow, the plant has become a monster to reckon with in the delta.

The hyacinth will endure salt water for a short time, and cases are known in which it was washed out to sea from one river only to float and find a new home up a different river. It has what you might call a real zest for life.

Naturally, humans being what we are, all anybody has been able to think of (in tropical regions where it is never frozen back) is how to get rid of the water hyacinth. Vietmeyer can rattle off heroic measures taken against the plant, but it has never been exterminated, he tells me, in any place where it has once taken hold.

It finally occurred to a few people to harvest it instead of cursing it. It has been estimated an acre of the plant can produce 60 tons of dry organic stuff, which could be fed to animals. The trouble is the water hyacinth is 95 percent water, so the trick is to get the water out of the plant before you start hauling it. Work is afoot on machines to press the water out. It has been tried experimentally as cattle fodder with good results.

Ingenious fishermen in Bangladesh and the Philippines stick poles in the water to make a circle 10 feet in diameter, and let the water hyacinth grow there. The fish flock to the shade provided by the plant, and the men catch them in nets.

In Burma they heap rich soil on floating mats of the water hyacinth and plant vegetables. Eggplant, cucumbers and a number of other things grow vigorously in these gardens.

The plant is also valuable for purifying water. Runoff into rivers carries a lot of phosphates -- very valuable fertilizer, of course, but a major pollutant of our streams -- and the water hyacinth traps, as you might say, a lot of it, and if harvested and applied back to the soil it would provide humus and fertilizer and also clean up the river.

It is not a total monster, in other words. Things that beautiful rarely are.