The sweet little, dear little shamrock of Ireland is a sham, a figment of our collective imagination. It turns out there is no such thing as shamrock.
Or perhaps it would be more correct to say there are many shamrocks? For, in truth, any small, green three-leafed or trefoil plant will do. At this time of year, when people are inclined to wear the green, at least five different species fit the bill as shamrock. They include various clovers, wood sorrel and a herb called black medick, and all have been used as shamrock.
Strangely, although the shamrock is an internationally-recognised symbol for all things Hibernian, none of these plants is uniquely Irish. Indeed, until 10 years ago, almost all the shamrock grown commercially in Ireland came from seed imported from France and New Zealand.
The word shamrock derives from the Irish seamair og, pronounced shamirogue, meaning young clover. The association with St Patrick comes from the popular belief that Patrick used a three-leafed plant to explain the concept of the Holy Trinity.
The wearing of the green, or at least a sprig of shamrock on St Patrick's Day, was mentioned as early as 1726, when the Rev Caleb Threlkeld, an English minister serving in Dublin, wrote about Irish plants and reported that the trefoil was "worn by the people in their hats on the 17th of March".
Over the next century the shamrock became increasingly popular among the Irish at home and abroad. Coins issued in 1813 by the Bank of Ireland, for example, were festooned with sprigs of shamrock, as were Irish ballad books and sheet music, fabric designs, and even plasterwork and architectural carvings.
Although Ireland's official national emblem is the harp, the shamrock became our unofficial one; as it did, gardeners, botanists and the Victorian equivalent of the cultural studies expert all debated the plant's origins and identity.
In 1827, for example, Miss Louisa Beaufort, from Co Meath, a sister of Admiral Francis Beaufort, inventor of the Beaufort wind scale, argued at the Royal Irish Academy that the word shamrock was of ancient Persian origin. Others believed that wearing shamrock was a pagan tradition.
The first scientific survey of shamrock was in the 1890s. Nathaniel Colgan, a keen amateur Irish naturalist, asked people around the country to send him samples of shamrock picked from the wild. He received dozens of specimens, which he planted at the National Botanic Gardens until they flowered in summer, when they could be identified.
In this way Colgan discovered that five plants were used as shamrock. Most common was the slender yellow clover or lesser trefoil (seamair bhui, Trifolium dubium); followed by coarser white clover (seamair bhan, T. repens); then red clover (seamair dhearg, T. pratense); black medick (dum heidic, Medicago lupulina); and wood sorrel (seamsog, Oxalis acetosella).
In 1988 Colgan's experiment was repeated at the Botanic Gardens by Dr Charles Nelson when, following a national appeal, 230 shamrock plants were sent in from around the country. Despite the intervening century and the fact that few people now get their shamrock in the wild, Dr Nelson found that the same five species were still popular.
He noticed regional variations, however. Yellow clover was popular in the south and east, for example, white clover in the west and north, and red clover in Sligo.
Today, the species used by commercial growers, and licensed by the Department of Agriculture, is yellow clover. Thanks to work done in the 1980s by Dr John Seager, of Teagasc, all of it is now grown from Irish seed.
Another innovation is growing the plants in small vials of gel, a technique developed at UCD in 1993 by Ray O'Hare and Prof Joe Morgan. Irish Plants International (IPI) produces over 120,000 plant vials each year for the home and export markets.
Most shamrock in a gel is bought for posting and IPI's managing director, Mr James O'Leary, says that demand is growing at 10 per cent annually.
Traditional sprigs and potted shamrocks are still produced. Ms Rosemary Swan, of Swan Nurseries in Co Meath, for example, grew 5,000 shamrock pot plants this year. Also a director of IPI, she believes that the gel vials will replace pots eventually.
Strangely, the shamrock's identity has implications for the timing of St Patrick's Day. Some time back the government considered moving the holiday to summer to improve the chances of good weather. The idea was dropped - just as well, for there is plenty of clover in July, but no shamrock.
Ingenious Ireland, Mary Mul vihill's book on Ireland's scientific heritage, is due for publication later this year.