Maybe it's the wide expanse of pink and purple wildflowers, growing where your eye expects to find a stretch of lawn, that makes the half-acre garden off North Fairfax Drive in Arlington seem somewhat peculiar.

Or maybe it's the plants--many of them budding for the first time since they were tucked neatly into the earth in late October--that set the natural scenery apart from the other commercial landscapes.

Here the trees and flowers have funky contours and berries and flowers with delicate petals. When the sun is shining, the understated expanse of leaf and green draws a crowd--lunchers and loungers and mothers pushing strollers. In the afternoon, schoolchildren arrive en masse, for a newly popular stop on the field trip circuit, to hear the unlikely stories of the nearly extinct plants and their tales of survival.

That's why the Nature Conservancy chose them, almost 100 varieties, to be planted in the back yard of its new international headquarters. The plants, each native to the United States, were selected by planners to create a garden that would stand out--both as a place where you can admire natural beauty, and if the mood strikes, feed your mind.

Not so strange an undertaking for the Nature Conservancy, an international organization charged with preserving plants, animals and natural communities by protecting the lands and waters they need to survive.

The Conservancy operates the largest private system of nature sanctuaries in the world with more than 1,500 preserves in the United States alone. Some are postage stamp sized; others cover thousands of acres.

While the group's Arlington garden, which is accessible to the public any time, doesn't have a global mission, officials say its leafy presence already has improved neighboring property values and is a popular draw among students and would-be gardeners.

"What's the difference between a weed and a wildflower?" Conservancy chief botanist Larry Morse asked a class of rapt fourth-graders who walked from Arlington Traditional School on Friday afternoon to visit the garden with teacher Judith Katherman. "A weed is something we don't want. A wildflower is something we do," Morse said, chuckling. "That's the best definition we've got."

Morse, the Conservancy's botany guru, maintains a national database of plants that are native to the United States and helps designate which will need help to survive in the wild. The database also tracks nonnative plants to monitor which are the most invasive and are the most harmful to native plants.

Strolling from plant to plant, 25 schoolchildren in tow, Morse explained the importance of growing plants in the region where they have evolved, and of avoiding the introduction of species that can overtake the natural ones already growing there.

"They tend to out-compete and dominate since they have no predators or pests or diseases to control them," Morse said, giving English ivy and Japanese honeysuckle as two common examples.

Morse led the children to the white-flowered franklinia, which has been extinct in the wild since about 1800.

"It's grown in nurseries, but it no longer occurs naturally," Morse told the crowd. "It was saved by gardening, but it's otherwise extinct."

It was the garden planners' desire to display unusual species that presented a formidable challenge to landscapers, who were tasked with finding many of the rare or hard-to-find plants.

"It was an education for them," said Nancy Benton, a research associate in the Conservancy's botany department. "They had some trouble."

In addition to using native plants, Morse said, they wanted to select plants that had conservation stories. At the same time, he said, they stayed away from varieties with poisonous berries or leaves, and from plants that attract rodents.

"We decided against poison ivy although it's very attractive in the fall," Morse said laughing. "But with 16,000 species to think about, we had plenty of choices."

Friday's tour was the second trip to the Conservancy garden for Katherman's students, who took their first tour a month ago before the plants and flowers had begun to bloom. The students, in concert with other fourth-graders at their school, have their own gardening project in the works.

Together they have created a Colonial garden using historically accurate plants that are native to Virginia. With help from gardening experts and parents, the children researched, selected and planted 1,200 flowers and plants that were donated for the project.

"It was a dream-vision I had," said Katherman, who talks lovingly about the science project and the impact it's had on the students. "They've retained all the facts about the plants and the period."

Indeed. The children were eager to share their gardening prowess with Morse and crowded around a visitor to describe their work in detail.

"Our garden is a lot smaller than this," said breathless 10-year-old Nora Trachtman. "But we have herbs and flowers and irises and columbine and lemon balm," she said, her voice drowned out by the shouts of other students itemizing the dozens of other species they've planted.

Most of the Conservancy's garden is complete, although Morse said he still wants to experiment with a few other species and hopes to plant some buffalo clover, which he said looks suspiciously like an everyday garden weed.

Maybe the biggest challenge now, he said, is making sure the gardeners know what to yank and what to leave be.

"It's a big challenge," Morse said smiling. "For a while, they didn't know what was a weed and what wasn't."

CAPTION: Botanist Larry Morse checks on wildflowers at the Nature Conservancy's headquarters in Arlington. The garden contains plants native to the United States.

CAPTION: Botanist Larry Morse shows fourth-graders from Arlington Traditional School the Nature Conservancy's garden.