ON MAY 21 AND 22 the National Arboretum will be bright with an exhibit of tall bearded irises, shown by the Chesapeake and Potomac Iris Society from 10 a.m. to 6 p.m.

Like the other shows at the arboretum (24th and R Streets NE, which is near Bladensburg Road and New York Avenue) the display is open to the public and free. For those not familiar with the arboretum, let me say that there is plenty of parking space just as you enter the gates on R Street, and the Administration Building where the show is held adjoins that area.

The tall bearded iris - the usual iris of gardens - has been bred to its present remarkable state within the past century or so.

Mature gardeners - I do not say old - can remember when there went no tall yellows. These only turned up in the 1930s, and the first one we went wild about was 'Alta California', which, we thought, was perfection itself. We already had a superb white, 'Purissima,' and a number of blues and lavenders, as well as too many rosy or orchid-colored varieties, and some fine deep purples, so we thought we were all set.

The great white 'Purissima' could grow to shoulder height, though usually somewhat less, and its progeny (chiefly through 'Snow Flurry,' its daughter) now fill the average iris garden.

The iris, as much as any flower and more than most, inspires a curious fanatical enthusiasm among gardeners, possibly because it combines elegance of shape, opulence of colors, delicacy of texture and intricacy of variation, all at the very height of spring.

Some say an iris garden is too much of a good thing. The color is over-whelming and therefore deadening, they say, and I have no argument with those who prefer less gaudiners and show. Nobody, after all, makes them plant irises in the first place.

It is perfectly ture, at the same time, that gardeners who have run through a good many enthusiasms in their day, return at last to the iris as the most beautiful of all flowers, all things considered and all objections overruled. Every gardener should try out a dozen or so current varieties of iris, merely to see if they produce, for him, an unparalleled excitement in mid-May. If they do not no harm is done.

I knew a man who liked roses and irises better than other things, and he was always threatening to cut down on the space given to irises. The roses bloomed into November, but the irises only three weeks in spring.

From time to time he would take some iris space and give it to roses, which he pruned to two inches every February in order to ram as many rose bushes as possible into his small space.

I guess at other times he must have pitched out the roses, because I could never detect any shortage of irises in their season. He liked to send great buckets of roses around town about Nov. 10 or 15, and I always suspected iris fanatics were special targets of his November pails of roses.

His irises, of course, he would not have cut if six stalks would save the Republic from ruin.

I knew another great fanatic of the iris who was a great trial to his wife, an equal fanatic for irises but more methodical than he. He had a field with several thousand irises he raised from seed, and the idea was to choose perhaps two from 5,000 and scrap the rest in order to make room for new seedlings.

The trouble was that although he made a great show of discarding thousands of irises a year, he commonly kept one rhizome of each, so that in a couple of years he was right back where he started.

One year, in a smaller area, he raised a thousand seedlings, none of them outstanding, but so beautiful all together that he left them that way permanently.

There were a lot of soft raspberry creams and buff yellows and lemons and sky blues. Beyond doubt he often crossed varieties merely for their color.

In theory, he was breeding for, say, better yellows, but he always made a lot of crosses that he knew would produce other colors, not because he expected to turn up anything especially fine among them, but merely because he wanted the other colors to please him when he was out working with his yellows.

He would have made greater progress if he had loved his irises less, of course, but then progress is not everything.

The great name, when it comes to irises, is Michael Foster, a 19th-century Englishman, who provided the main push toward combining the genes of the very large wild irises of Europe, resulting (in a nutshell) in the irises of today. Most iris freaks know 50 other names of men and women who bred the great flower in their several springs. Such famous (though long discarded as inferior to their more beautiful descendants) varieties as 'Dominion,' 'W.R. Dykes,' 'Conquistador,' 'Purissina,' 'Souvenir de Madame Gaudichau,' 'Los Angeles,' 'Jean Cayeux' and so forth, are back of everything we now have.

Probably the average gardener is more concerned with iris borers and rot (serious afflictions both) than with great irises of the past which, if seen today, would seem relatively dull, but without which we should never have had the flowers we do.

It is the same, of course, with roses. I could not help noticing a rose I raised from seedn that is quite worthless but which could never have had its superb red color without the old Red Bengal and the anicent roses of China.

Sometimes in mongrel dogs you see one that you have seen before in an illuminated manuscript or even ancient art and I am always glad to see what I call a basic dog.

In the same way among irises and other flowers, you see new flowers that remind you with a shock of old ones. In a sense, a great deal has happened with the irises through the selection of new ones raised from seed, and it is safe to say that America has for many years led the world, quite without a rival, in the production of fine irises. As the English and the French did formerly. All our fine irises spring from them.

The way to see irises is to station yourself in an iris garden shortly before sunrise. The colors are very good them. As the sun's rays first strike the flowers, an electric change occurs and everything leaps, or explodes, into life. There should be a public iris garden where people can see for themselves. After sunset is a fine time for irises, too, when the lavenders set lose their red and turn blue. Iris nuts are content to gaze at a very dull sight for 49 weeks a year, because they know what is going to happen in May. It is an unreasonable sort of gardening. Passion is generally unreasonable, and highly amusing to see in others. To some, though, it is real, if foolish. Very real.