Three men's dedication to one small corner of the natural world. What makes someone devote his career to one kind of plant?

Some people devote their lives to motor-racing, or violin-playing, or studying planets. But who gives his all to rhododendrons? Peter Cox does. His large family house in Perthshire, Scotland, is surrounded by rhododendrons. The 10 acres of garden and nursery are a mass of color in May and June, the main flowering season for these widely popular shrubs. But to Peter Cox the flowers are just a bonus, a luxury. For him the plant's interest is continual.

He is one of three British horticulturists I have asked: What makes someone dedicate his career to a single kind of plant? What are the fascinations, aims, and satisfactions of long-term commitment to such a select little corner of the natural world?

One of Mr. Cox's driving forces is his sheer knowledge of rhododendrons. He knows their structure. He knows the soil, the climate, the position they favor. Fine distinctions obviously delight him, and these are impossible without concentration. The details of roots, stems, bark, leaves, buds, and flowers offer him an astonishing range of variations on one theme. ''They range from alpine creepers an inch or so high to 80- or 90-foot trees,'' he explains. ''It is hard to think of any other genus of such range and size: over 1,000 species. Many people could do a whole life's work with rhododendrons without overlapping.''

Such things are intertwined for Peter Cox with two primary interests: breeding new hybrids and collecting.

He has studied and collected rhododendrons in India, Turkey, Europe, the United States, and in the Cangshan mountains in Yunnan, China.

There is a boyish quality to this slim man with his ready grin and quick wit. His love of rhododendrons was fostered early, in precisely the place he still lives. His father planted the garden with over 100 species of rhododendron. At school one day the young Peter was told to write an essay on a subject of his choice. He was baffled. So he settled expediently for the near-at-hand. It sparked his interest in rhododendrons. Now he is an internationally respected authority with two standard books and a host of new hybrids to his credit.

On one level Mr. Cox is the full-time, practical gardener. On another, he is an intellectual fascinated by the problems of classification. He has a close interest in the reorganization of a genus notorious for awesome complexity. He favors more natural divisions, aiming at a cease-fire between warring factions of botanists.

''Some are 'lumpers,' '' he says with a chuckle, ''and some are 'splitters.' I'd like to feel I'm in the middle, trying to merge it all together. . . . We try to fit what nature has produced into neat cubbyholes, and it just doesn't work! But,'' he adds, ''that's the fascination of it.''

His business is selling rhododendrons. Experience has taught him what demand to expect for a given variety: He will sell only five of one to 100 of another. He manages to cater to both the popular market and the specialists, a rather rare achievement at a time when plant cloning is dictating a mass production of plants for an ever-growing public.

But it's when he talks about hybridizing that his eyes light most brightly. This is an occupation that involves a creativity that brings horticulture close to an art form. Crossing plants to produce new colors and qualities means endless potential, secret skills, thrilling rewards.

''I think I've been fairly successful,'' he says with actual modesty, ''but I could go on for many, many years!'' The prospect by no means depresses him.

Like Peter Cox, Peter Moore of Iden Green in Kent, England, owes his lifelong affection for one plant family - in his case cyclamen - to a boyhood beginning. This was not the large windowsill cyclamen everyone gives as Christmas presents. What he encountered unforgettably at the age of 14 was the tiny wild cyclamen, with its delicate white or pink shuttlecock flowers and small leaves patterned with silver: a miniature plant of extraordinary enchantment.

Now about to take early retirement from a city job, he plans to make his already successful nursery into a second career - selling nothing but cyclamen.

Might he ever be tempted to branch out? He admits he finds it hard to resist other small bulbs. But a friend encouraged him by saying that if there were a single kind of plant out of which a business could be built, it would have to be the wild cyclamen. It is the only plant, when in flower, that everyone stops to look at, it is so strikingly beautiful. ''There must be a large market for it,'' he maintains.

Would he consider growing the big ''florist'' forms of cyclamen? ''I hope not ,'' he replies. ''As with most flowers, the wild species are far more appealing, much daintier. The large hybrid cyclamen, I think, are . . . blowzy.'' He makes one feel vulgar ever to have had one in the house.

His devotion to wild cyclamen alone is all the more remarkable because, as he points out, ''there are only 27 known species.''

What exactly is the cyclamen's unique attraction?

Mr. Moore answers by saying: ''Out of 100 cyclamen of precisely the same species, no two are identical. I can show you real differences in every one of them. . . . It's just like a shepherd with his flock.''

Richard Cawthorne, who also has a nursery in Kent, may not know each of his plants personally. But he claims that his collection ''is the only one left in the world of old and new named varieties of viola and violetta.''

His overriding motivation in concentrating on this exclusive group of garden plants is conservation. With its butterfly flowers, its long flowering period, its neat habit, its ability to live for many years with scant attention, it seems amazing that the garden viola ever fell on bad times. The Victorians and Edwardians adored it. But World War I saw its virtual extinction.

Then, in the 1950s, Mr. Cawthorne appeared on the scene. He has been collecting and hybridizing violas ever since. Now he has as many as 250 different varieties. If the plant survives for future generations, he will be the man to thank. The old varieties he has ''rescued'' from old gardens up and down Britain; now he is conserving them by supplying some of the nation's most famous and cared-for gardens, preserved by the National Trust. He is also duplicating his entire collection at Leicester University, with perpetuity in mind.

But it is his careful methods of marketing his violas that are most original - perhaps even slightly eccentric. Nothing could be further from the supermarket. He is choosy about his customers. He refuses to sell either to casual gardeners or to ''the trade.''

''These are rare plants,'' he argues, ''and I won't sell them to people who are going to neglect them.''

Money is not his priority. He grows violas for the love of them. When he produced a completely black-flowered viola, a man came up to his stand at the Chelsea Show in London and offered him (STR)3,000 outright for it. ''I couldn't believe it. I refused of course.''

So what motivates an individual to give all he's got to a single plant family? The beauty of a chosen kind of plant is, it seems, only one concern among many. These three specialists share certain aims. All are waging a small rear-guard action against the monotonizing, monopolizing tendencies of today's supermarketing of garden plants.

Then there's the fascination that comes from breeding new varieties, along with the demand for years of undistracted work. The opportunity to develop a deep knowledge in a limited field is doubtless as attractive for plant lovers as for any specialist. But perhaps, in the end, the phenomenon of a man giving everything to one kind of plant defies analysis.

Or perhaps it is an intriguing ecological technique by which humans become the willing but unwitting tool of plants determined to survive and multiply. Who knows?