Suspected of being nothing more than another invasive weed from Europe or Asia, the specimen was passed from one herbarium to another over the next few years. Finally, botanist Ihdsan Al-Shehbaz of the Missouri Botanical Garden identified the mystery plant, creating a stir when he announced that it was a new species native to California. Dubbed Twisselmannia californica Al-Shehbaz, the plant--which also, it turns out, represents a new genus--will be described in an upcoming issue of the scientific journal NOVON.

LaRue´s discovery, and those of other plant collectors across the country, are challenging a common assumption that dates back at least 100 years: that virtually all plant species in the United States already have been found and given scientific names. Indeed, because new species descriptions appear in scattered, obscure journals that are never compiled in one place, "even most professional botanists believe that the plant flora of North America north of Mexico has been well catalogued," says Barbara Ertter, curator of western North American flora at the University of California-Berkeley. These days, however, Ertter is one of a small band of botanists trying to dispel that myth. Noting that the only known population of Twisselmannia sits within close range of Interstate 5, she urges her colleagues to take a close look around them. At the dawn of the new millennium, new plant species are still, she says, "lurking in our own backyards."

The first hint that the long-accepted wisdom about new plant discoveries could be wrong came in 1987, when California botanists Dean Taylor and James Shevock pulled together published reports of new species found in their state between 1968 and 1986. They reported that an average of 10 plants had been described each year, and the number of new species was not going down. Based on his subsequent analysis, Taylor predicted that at least 300 more California plants remained undiscovered.

Shaking up the botanical world further, University of Wyoming botanists Ronald Hartman and B.E. Nelson published a national-scale analysis last year. Their report, Taxonomic Novelties from North America North of Mexico (published by the Missouri Botanical Garden Press), revealed that between 1975 and 1994, scientists described 1,197 new plants from North America--about 60 plants a year. Ranging from lilies, orchids and violets to shrubs and cacti, nearly all the discoveries are angiosperms, complex seed plants that bear both flowers and fruit. Hartman and Nelson also calculated that the rate of plant discovery has remained constant for the past 40 years--and shows no sign of slowing down. Says Ertter, "Even those of us involved in finding new species ourselves were not prepared for the magnitude of discoveries uncovered by the report."

Most new plants are discovered in remote, isolated habitats that differ in some way from the larger surrounding landscape. Geologically diverse, rugged and settled more recently than much of the rest of the nation, the western and southeastern United States contain more than their share of such unexplored habitats.  It´s not surprising, therefore, that the majority of new plants have turned up in these regions.

Exploring a limestone cliff in northern California, for example, botanists Glenn Clifton and Dean Taylor stumbled upon an unfamiliar shrub with white, star-burst flowers in 1992. Researchers later identified the shrub as a new species, Neviusia cliftonii, or the Shasta snow wreath. (The plant also is only the second representative of its genus; the first is a rare species known only in the southern Appalachian Mountains.)

California´s Shevock, now an assistant regional director for the National Park Service, discovered several new species when he became the first botanist to explore a newly opened stretch of the Pacific Crest Trail. Today seven plants are named after Shevock, and he has authored or coauthored descriptions of 13 new plants.

Yet plant discoveries can turn up virtually anywhere, even in sites that are developed or close to major cities and highways. As a 21-year-old botany student, James Morefield, for example, practiced by collecting plants around his neighborhood, which then was in Huntsville, Alabama. One specimen he picked up in a vacant lot, later named Morefield´s leather flower, or Clematis morefieldii, turned out to be a new plant now listed by the federal government as an endangered species. So far, it has not been spotted anywhere outside Huntsville city limits.

A Sacramento Valley sheep rancher, Lowell Ahart, sends Ertter "a couple boxes of excellent specimens every year," she says. That´s because Ertter had previously named two new species after Ahart, who had found the plants on his land. Another recent discovery, Lesquerella tuplashensis, as well as a new plant called Eriogonum codium, were found not long ago thriving on the grounds of Washington´s Hanford Nuclear Reservation. And a still-unclassified new buckwheat--host plant for the endangered Lange´s metal mark butterfly--was discovered in Antioch Dunes near San Francisco, "just an hour´s drive from where I sit," sighs Ertter.

Sadly, many newly discovered plants are more apt to be endangered than others, because most are both rare and endemic--that is, they inhabit only one specific location on Earth. Complaining that the effort to find new U.S. plants falls far short of the importance of the task, Ertter predicts that some undiscovered plants will disappear forever before scientists have even named them. "In a country as rich as the United States," she adds, "it´s inexcusable to allow a species to go extinct out of ignorance."

Ertter believes that about 5 percent of North American native plants north of Mexico remain undiscovered. Globally, the discoveries yet to come are magnified many times over. At a symposium last year at the Missouri Botanical Garden in St. Louis, Ghillean Prance, director of the Royal Botanic Gardens in Kew, England, reported that his institution´s research suggests that the world is home to anywhere from 300,000 to 320,000 flowering plant species, a major increase over the 250,000 that botanists had been citing for decades. Missouri Botanical Garden Director Peter Raven estimates that fully a sixth of Earth´s plants remain undiscovered, 90 percent of them in the Tropics.

Meanwhile, the number of qualified field botanists who can collect and describe new plants is declining rapidly. One reason it took so long to identify Twisselmannia, for instance, was that the first botanist LaRue sent the specimen to died before he could work on it. According to Ertter, of 56 experts who have described six or more new plants over the past two decades, the majority currently can be classified as either "emeritus" or "deceased."

Enter what Ertter calls "para professional botanists," who these days are racking up more than their share of new plant species. Take, for instance, Arnold Tiehm of Reno, Nevada. To pay his bills, Tiehm works full-time as a bellhop and limo driver for the Peppermill Hotel Casino. But on weekends, he does what he likes best: "I go botanizing," says Tiehm.

On one trip to nearby Mount Lewis, for example, Tiehm spotted a strange-looking plant with purple flowers clinging to rock. The plant, later dubbed Penstemon tiehmii, turned out to be a new species--Tiehm´s nineteenth new plant discovery, and the sixth that bears his name, all found on days off in his native Nevada "backyard."

Laura Tangley is a senior editor for U.S. News & World Report.