THE seeds of Duncan McDougall's horticultural success were sown long before he left the police force - and what started as a hobby has taken over his retirement.

He was always interested in gardening and visits to St Andrews' Botanic Gardens nurtured his interest in unusual plants. His innovative idea for marketing the seeds of the world's rarest plants grew from there.

Gradually McDougall built up a computer index of the world's rarest plants and four years ago began trying to find places where their seeds could be obtained. He wrote hundreds of letters to botanic gardens all over the world asking about the availability of surplus seeds.

He received few replies but these were enough to encourage him to start his Seed Guild, which offers a selection of rare seeds to subscribers.

Today, he has more than 1000 members in 26 countries who can select from the 4400 varieties of seeds he has to offer.

One of the more unusual requests McDougall received was for the seeds of plants that can absorb toxic materials from the soil. Scientists have discovered that there are plants which can take up elements such as cadmium and zinc from the ground and absorb them into their foliage.

This has tremendous potential for the clearing of old industrial sites and research is ongoing into which plants best perform the cleansing function.

Other interesting work is being conducted into which plants activate enzymes in material so that it absorbs dye better.

''Letters flood in every day from all over the world and my postman always comments on the exotic character of my mail,'' said McDougall. ''Recently I had a letter from Kazakhstan asking for some seeds.''

Now, because of a surplus of sees, McDougall has launched an offshoot operation - Compass Seeds which will supply to selected garden centres in Scotland.

The sort of seeds McDougall sells will appeal to dedicated gardeners who are prepared to grow them on, often for years, and his list offers all shapes and sizes - from tiny alpines only two centimetres high to 300ft-tall North American pines.

The range includes many unusual species - 500 types of gunnera, for example; 20 alliums, 20 campanulas, 15 gentians and dozens of primulas. Among the most rare are a Paeonia Suffruticosa, subspecies Rockii, which is an unusual flowered species of Chinese paeony, that grows two-foot high and produces flesh pink or silvery white semi-double flowers. McDougall receives this seed from China and it is being launched in Britain at the Strathclyde show.