To consider the shamrock is to consider the significance of a leaf. Or, in this case, three—the number of leaves that defines the shamrock, a plant that has evolved into an icon, one that will festoon any St. Patrick's Day festivity. Of the shamrock's ubiquity, you can be sure, but of its origins and species, however, there is little certainty. The shamrock's story is one of dueling legends and taxonomic disputes. 

Photo by: Cobalt123/Flickr, Oxalis acetosella (L); Mutolisp/Flickr, Trifolium repens (R).

There are two theories of the shamrock's origins. The first one begins in the fifth century, when St. Patrick was asked by the king to explain the mystery of the Trinity. Plucking a sprig of shamrock from the ground, he offered the three-leaved plant as a visualization of the theology. And so the patron saint of Ireland, and his shamrock, traveled the country, teaching Christianity and cultivating a national emblem. The second story begins a bit earlier, with the Druids, whose Celtic tradition considered three a mystical number, and thus the shamrock a sacred plant. Both are considered loosely founded legends, so take your pick.

The second contentious detail is identifying the species. No one is really sure which is the "true shamrock"—many will say it's a clover (Trifolium repens or Trifolium dubium), while others will tell you it's a wood sorrel (Oxalis acetosella) or a medick (Medigo lupina). The early Irish countryside flourished with three-leaved clovers and three-leaved wood sorrels and medicks, so all were considered shamrocks until taxonomy demanded differentiation. And so it's hard to say which trifoliate specimen would have toured with St. Patrick or decorated early Celtic artwork—before the 17th century, plants were classified by their leaves, rather than their flowers. The shamrock's story is an amalgam of Irish history and religion, not a narrative of botanic discovery. For most of the shamrock's history, the species might not have mattered to those who wore it. 

And perhaps it doesn't matter today, whether the true shamrock was a clover or a wood sorrel. It does matter, however—as historians, botanists, and Irish purists will agree—that the shamrock's meaning is ascribed in its three leaves. Which brings us, of course, to the four-leaf clover. It is not a shamrock. The four-leaf clover might well be lucky, according to the ancient Druids, who wielded the four leaves-representing the four elements of alchemy: earth, fire, water, and air—as a charm against malevolent spirits. Four-leaf-clover-collecter record holder Edward Martin would also agree on its luck; he's found 160,000. And anyone who's knelt in a clover field to beat the 1-in-10,000 odds might also say it's a lucky specimen.

But it is not a shamrock, of course, because a shamrock has only three leaves, as the Obama campaign was once reminded. A line of St Patrick's Day-themed wares featuring "O'Bama" and a four-leafed clover caused a bit of a coup among Irish purists, who appreciated its effort, but lamented the inaccuracy. “That is my particular pebble in my shoe, when four-leaf clovers are interchanged with three-leafs,” a pub owner told the New York Times. “People do it all the time. But if they were going to go to the trouble to do a T-shirt with the O and the apostrophe—which I think is very clever—someone on the staff should have gone to the trouble of finding out how many leaves to put on the clover.”

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