It's Friday afternoon and you're stuck in town. The temperature hovers near 95 degrees, the humidity seems the same, and everybody else in the world is heading for the Eastern Shore. You've seen more than your share of movies and museums this humid summer, so you've planned a quiet weekend at home, curled up with your air- conditioner. Or worse, you're expecting guests from out of town who can't wait to see the sights you've seen all too often. Indoors or out, you plan to be hot and miserable.

But take heart. Every summer day, Washington's trees -- nature's great air-conditioners -- are keeping you a lot cooler than you may know.

Washingtonians of the 19th century weren't so lucky. By mid-century, the haphazard felling of trees -- combined with a casual attitude toward sewage disposal -- had rendered summer in Washington a life-threatening proposition. Come summer, presidents were whisked out of the White House for fear they would fall victim to the malaria emanating from the swamps along the Potomac. With much of the native timber gone, Washingtonians who couldn't evacuate suffered the dog days bereft of even the mercy of shade.

In the 1870s a man named Alexander ("Boss") Shepherd changed all that. The second and last governor of D.C., Shepherd planted 60,000 trees along the streets of Washington before being hounded out of office for running the District into debt. But his trees flourished, and within a few years Washington came to be known as the "City of Trees."

In those pre-air-conditioning days, Washington's trees were viewed as veritable temples of comfort and worshipped accordingly. Planting trees became a city-wide passion and a major source of civic pride. Trees were brought to the city from every corner of the earth and, miraculously, many survived. So, while the capital's human transplants wilted in the heat, their proteges from the plant world thrived on it, and Washington evolved into the arboreal showcase it is today.

So why not turn a picnic into an education? Combine a tree tour with a day of traditional sightseeing. Or send your guests indoors to view exhibits and memorials while you explore the trees. You'll find the National Arboretum a joy minus the spring throngs. Many trees there and around town are already sporting fall fruit, making them easier to identify.

Here's a sampling of the botanical wonders awaiting your summertime rambles:

THE CAPITOL GROUNDS -- A good place to start a summer swing through the Capitol grounds is at the red-brick grotto tucked into the northwestern slope of the Hill. Overhead you'll see the shaggy-barked branches of a rare Chinese cedrela adding charm to the grotto setting created by landscape architect Frederick Law Olmsted. As you walk east up the hill, you'll come upon a little Christmas-tree- shaped evergreen with cinammon-colored bark -- a "giant" sequoia from California. Although spring is Washington's peak flowering season, two gorgeous Asian tree species bloom on the Capitol grounds from mid to late summer: The Japanese pagoda tree (or Chinese scholar tree) puts forth spreading clusters of tiny yellow-green or cream-colored blossoms during July and August. In late summer or early fall, these blossoms will be replaced by pendulous, sausage-shaped pods. The most magnificent pagoda grove stands just to the east of the giant sequoia. The pagoda tree is a member of the pea family, known for its showy flowers and pod-shaped fruit. Other members of the pea family planted around the Capitol include the spring-flowering redbud and the magnificent Kentucky coffee tree. The other Asian tree to put forth showy summer flowers on the Hill is the delicately proportioned crape myrtle. Popular in gardens throughout the city, the crape myrtle is best known for its tall clusters of crinkly pink, red, lavender or (rarely) white blossoms. But it's also valued for its smooth, ornamental bark. Several different kinds of magnolias are planted on the Hill, some native to the southeastern U.S. and some to Asia. Although you're not likely to find more than an occasional southern magnolia in bloom this late, two less- known native species are noteworthy for their foliage. The big-leaf magnolia is the largest-leafed tree in the temperate eastern U.S. It's a native of the southern Appalachian region, and in spring it bears giant white blossoms. The tropical-looking leaves often reach lengths of three feet. Look for the big-leaf magnolia, with its distinctive orange-yellow bark, near the pagoda grove and, on the south side of the Capitol, for a similarly leafed tree nearby. This is the umbrella magnolia, with slightly smaller leaves, which is an Appalachian native. You'll undoubtedly notice that most of the trees on the Hill are labeled. While the labels are helpful, don't believe everything you read: Even seasoned botanists have been misled by the well-meant but occasionally misplaced label on the Hill. Some trees, you'll notice, have mustard-colored labels, signifying commemorative plantings, long popular with members of Congress. One of the Capitol's commemorative trees, a white oak in the southeastern section of the grounds, was planted by Speaker of the House Sam Rayburn in 1949. The speaker -- or a member of his staff -- was frequently spotted outside the House chamber, tape measure in hand, keeping track of the tree's growing circumference. On hot days you'll find plenty of shade on the Capitol grounds. The giant willow oaks, white ashes and other large trees in the southern section of the grounds provide a cool canopy for picnicking, daydreaming and resting tired feet.

THE BOTANIC GARDEN -- The summertime lures of the Botanic Garden, just down the Hill from the Capitol, are many. Flowering plants of every description surround the tree-filled conservatory. Outdoor tables are nestled near this profusion of flowers, creating a perfect place to recover from museum and monument burn-out. The Botanic Gardens extend from the Capitol reflecting pool, across Independence Avenue to the Bartholdi Fountain, which is ringed with flowering plants throughout the summer.

THE SMITHSONIAN AND THE MALL -- The Smithsonian museums are all handsomely landscaped with interesting collections of trees, shrubs and herbaceous borders. A meander around any of the museums flanking the Mall will be rewarding. The Mall has had an eventful arboreal history. During the last century, it became an arboretum, filled with trees and circular pathways. Around the turn of the century, a commission studying the landscape architecture of the capital recommended that the trees be cut down to make way for the mall we know and enjoy today. This, the commission argued, would be in keeping with the original city plan of Pierre L'Enfant. Controversy ensued, with tree-loving Washingtonians decrying the proposed "heartless" felling of trees. In the end, the commission triumphed, and most of the trees were removed, opening up the dramatic vista from the Capitol to the Washington Monument. Uniform rows of American elms replaced the 19th century groves. However, a few of the Mall's old trees were spared, including a massive bur oak near the Hirshhorn Sculpture Garden. This magnificent tree, with its delicately lobed leaves, is a proud reminder of bygone days. Other trees that survived the Mall's relandscaping include a small grove of bald cypresses in front of the National Museum of Natural History and a cluster of botanically noteworthy trees just west of the Capitol reflecting pool. Among the reflecting pool survivors is an extremely rare Asian zelkova -- or Caucasian elm, as it is often called. Standing near the the southwestern corner of the pool, this tree is easy to pick out with its pale gray-and- orange bark and many ascending limbs. Finally, a tree you shouldn't miss is the giant copper beech located on the southeastern corner of the Natural History Museum grounds. Its smooth gray limbs and reddish leaves reach out toward the scorched sidewalk, as if to draw you into its welcome shade. Stop to rest under this magnificent tree, indulge in an ice cream, and in 20 minutes you'll feel ready to conquer another exhibit.

Melanie Choukas-Bradley is co-author of City of Trees: The Complete Botanical and Historical Guide to the Trees of Washington, D.C. (Acropolis Books Ltd.).