Some structures change the way you look at architecture. Others make you see the world in a new way.

A recent example of the latter is New York's High Line park. A large part of the attraction of the newly landscaped railroad viaduct is that it offers us a fresh vantage for viewing the urban jungle. You're above the fray, but not by much. Closer to home, the Morris Arboretum of the University of Pennsylvania does something similar for our experience of the forest with its new "Out on a Limb" exhibit.

Instead of hiking along a dirt path where you're hemmed in by tree trunks, undergrowth and the semi-darkness of the forest floor, the lofty elevated structure thrusts visitors into the thick of things. You're immersed in the forest's leafy camouflage and treated to a bird's-eye panorama of the surrounding terrain. So this, you marvel, is what it's like to perch among the trees.

But the creators of "Out on a Limb," led by Philadelphia's Metcalfe Architecture & Design, do more than simply bring us up to the level of the treetops. They apply time-honored architectural tricks to enhance nature's drama - opening up a wide vista here, compressing the focus over there, while frequently testing our fear of heights. The result is a space that exerts an irresistible allure, to young and old alike.

Unlike conventional structures, there is no real roof or walls, except for the small teahouse section. While the High Line pretty much hews to the straight lines of Manhattan's grid, "Out on a Limb" behaves like a tree, sending its steel-and-wood limbs out in multiple directions.

The arboretum's director, Paul W. Meyer, came up with the idea for the "Out on a Limb" exhibit after visiting a treetop walkway at London's Kew Gardens. Judging from pictures of Kew's straitlaced, classically inspired walkway, the Philadelphia architects were able to infuse their $3.5 million version with a lot more imagination, whimsy and style. The frank use of galvanized steel gives it a surprisingly modern feel, and is a nice departure from the typical, all-wood nature center fare. It will be a relief, however, when some of that steel dulls down a bit.

The designers signal the walkway's architectural qualities right from the start, where a lattice of bentwood ribs makes an openwork canopy that hints at more solid and familiar entryways and awnings. Just as you set forth on your journey, though, you find your path blocked by the grizzled trunk of a 250-year-old chestnut oak that rises right through the walkway's black locust floor.

Part of the exhibit's mission is to celebrate trees and teach people about their role in the ecosystem. So the confrontation serves both an educational and a design purpose.

At the chestnut oak, the path splits and visitors are forced to choose directions. The branching walkways lead to a variety of roomlike enclosures - an Asian-style teahouse, a human-sized oriole's nest and an expansive deck featuring a pair of sunken hammocks. The materials and textures vary in each space, as does the density of the tree canopy. The differences further distinguish the rooms from one another.

The exhibit's indirect pathways took their inspiration from the traditional Chinese garden. Set on small scraps of land, those green oases rely on a complex, zigzagging network to make the space feel bigger than it really is. The fun is discovering what lies just down the road.

As an arboretum attraction, "Out on a Limb" is clearly trading on the growing popularity of treehouses. The designers, who had a big assist from the engineers at CVM in Wayne, gave the structure an almost intentionally rickety look, evoking the homemade shelters of children's imaginations.

The most distinctive feature is the bird's nest, woven from grapevines. It appears to dangle precariously from a pair of alarmingly angled steel columns, nicknamed "The Chopsticks."

Meanwhile, the architects modeled the walkway that leads to the nest on that summer camp favorite, the rope bridge. Although safety codes dictated a solid wood floor for the bridge, it has just enough sway built in to remind you that you're 50 feet off the ground. "Out on a Limb" manages to be fully handicap-accessible, thanks, in large measure, to the Forever Young Treehouse Foundation, which specializes in such exhibits.

Material choices play a big part in the exhibit's sophistication. The designers included plenty of wood, but in ways that make it count. Except for the teahouse floor, built from Alaskan cedar, all the wood comes from Northeastern forests, and much of it was sustainably produced.

In using steel for the columns and railings, the designers upend our expectations of what a nature center exhibit should look like. But then they confound us again by topping the steel with handrails made from smooth ironwood, gathered from the forest floor and stripped of its bark. It's a subtle contrast between the industrial and the handmade, the designed and the found.

"Out on a Limb" is intentionally deferential to all things woody. Because the area where it is located is a remnant of the region's virgin forest, the architects were instructed to work around the big specimens. That's another reason for the path's twists and turns.

When the exhibit veers toward its open deck area, called the Squirrel Scramble, the architects had to deal with two other significant trees, a tulip poplar and a black gum. These become the centerpieces for two hammocks, which resemble swimming pools made from rope. Because they're sunken, the triple-reinforced nets invite sociability. People can't help diving in for a snooze or sitting around the rim.

The brave ones might even try looking down through the openings. What they see 50 feet below might alter their perceptions of nature forever.

"Out on a Limb" is at the Morris Arboretum of the University of Pennsylvania, 100 E. Northwestern Ave. Information: 215-247-5777, http://www.business-services.upenn.edu/arboretum.

Contact architecture critic Inga Saffron at isaffron@phillynews.com or 215-854-2213.