THIS IS THE leaf of the papaw plant," said Ranger Bill Rudolph, holding up a palm-size green leaf. "It has a smell like oil or kerosene." He passed the leaf to a slim blond fourth grader.

"Oaaugh!" said Mary MacKenzie Fyfe (Mary Mac to her friends), quickly thrusting the leaf along to another hesitant sniffer.

Mary Mac was one of 35 students from the Forcey Christian School in Silver Spring on a recent class outing to the Rock Creek Park Nature Center off Oregon Avenue near Military Road NW. She and her classmates, teachers and chaperones were learning about edible plants, a lesson that Rudolph began with the warning: "Never experiment! Do you know why?"

"Because there might be a bug on it?" replied one uncertain student.

"No," Rudolph said, "because you might be wrong. Dead wrong! The plants might be poisonous."

Rudolph started the lesson with a film on edible plants, roots and flowers before taking the class out into the woods surrounding the center. "Fall is the time of seeds and nuts," said Rudolph. "They're all developing right now."

A white pine was the centerpiece of an illustration on vitamin-bearing plants. Its needles have more vitamin C than a glass of orange juice, Rudolph said.

Patrick Taltaball, Mary Mac's classmate, picked up a prickly, green husk and asked the ranger to identify his find. "That's a chestnut," said Rudolph. "We don't have chestnuts in the park, so some animal must have brought it here."

Rudolph led the group along a wood-chip trail deeper into the woods, identifying a witch hazel tree and explaining how its inner bark was used by Indians to make a germ-killing liquid, how to identify poison ivy and other plant wisdom.

The edible-plant outing is one of more than 900 programs the National Park Service presents each year to organized groups and individuals at the Nature Center. These programs are just a small portion of the recreational possibilities in a park that attracts more than 2 million people a year, a figure that doesn't even include the thousands of commuters who use the park as an eye-pleasing and speedy drive into the center city.

On one recent morning the park was home to joggers, hikers, bikers and early picnickers.

Warren James, a self-described "intermittent student" from Northwest Washington, was taking a bike ride on Glover Road near the equitation field on the park's western ridge. "I ride here several mornings a week," he said. "Nothing too far or too strenuous. It helps me think."

At Picnic Grove No. 2 near Pierce Mill, Jim and Marie Wright from Brookland were having an early picnic. "I'm off on Friday, and we like to come to the park for a quiet cookout. It's nice; no people now," Jim explained.

"Sometimes we walk to the mill," said Marie. "Mostly we just enjoy the scenery and the quiet."

The park Mary Mac, James and the Wrights are sharing is 100 years old this week. On Sunday Rock Creek Park celebrates its centennial with music, activities and educational exhibits. In the park are jogging or biking paths, hiking trails, forests in which to ride horses, a golf course and a nature preserve filled with numerous species of flora and fauna. The park also offers places to fish, picnic and get a suntan, see a concert, play tennis, or explore a winter wonderland on cross-country skis. And in all seasons, the park is a classroom, offering up lessons in nature, conservation and history for students of all ages. FROM EYESORE TO PARK

Little more than a century ago, the valley created by the modest stream that meanders more than 20 miles from the Maryland uplands down to the Potomac River, was a nuisance, a polluted eyesore that slashed through a still young capital city.

Though sea-going ships once anchored in its mouth, silting from development and pollution from the port called Georgetown had fouled and filled the narrow stream. The stream valley was too rugged and too rocky to attract developers, who saw easier money on the gentler land nearby. The rugged valley did have its admirers. The military saw strength and purpose in the rock-laden bluffs and built a fort on its northern reaches. And millers -- at one time there were eight -- saw power in the stream and its tributaries and built their grain mills, mill races and homes along the banks of the stream.

It wasn't until two years after the Civil War that Rock Creek's potential as a city park was envisioned. Maj. Nathaniel Michler of the Army Corps of Engineeers, directed by the Senate to find a new site for a presidential mansion and park, ignored the mandate to find a new presidential home and instead proposed the creation of an urban park that would encompass most of the Rock Creek valley inside the federal city.

Michler saw his park as "a wild and romantic part of the country," a place to celebrate nature and to promote open space and exercise. His vision ignored the reality of the valley at the time, for industrialization, pollution and silting had transformed the once-pristine creek into an ugly scar on the District.

Michler's proposal was ill-timed for a nation recovering from a destructive and expensive war. Congress, as it often does, decided to put off action.

It wasn't until 1890 that Congress approved a Rock Creek Park of 1,754 acres, an area that contained most of the valley from the Maryland line to where the National Zoo is today.

In the decades since its creation, the park has grown to 2,800 acres, stretching from the Potomac River up to the District border and encompassing such disparate parks and sites as the Old Stone House in Georgetown and Montrose, Glover-Archbold and Dumbarton Oaks parks. And the Maryland portion of the creek's valley has spawned another series of parks, reaching north to the headwaters and covering almost another 4,200 acres.

Today's Rock Creek Park is an urban wilderness, a place where you can park your car and then walk a few minutes into the forest and feel as isolated as if you were in a national forest miles from any city.

The valley's first visitors weren't seeking recreation, however. About 4,000 years ago, more or less, unidentified prehistoric Indians quarried the Piney Branch valley and other areas of the park for quartzite and soapstone. The quartzite would be flaked into tools and weapons, the soapstone carved into bowls. Later, sometime around the 2nd to 5th century AD, the Algonquin Indians began using the valley as a hunting ground.

Centuries later, early colonists built mills and military leaders built a fort. Some of these early constructions remain even today. The earthen embankments of Fort DeRussey, one of 68 forts that encircled Washington during the Civil War, are almost lost in the thick tangle of vines and trees east of Oregon Avenue and north of Military Road NW.

The fort played a supporting role in the July 1864 battle at Fort Stevens, says Barry McIntosh, historian for the park. "Its role was related to that battle, when Jubal Early {and his Confederate troops} tried to invade Washington from the north. Fort Stevens turned back the attack, and Fort DeRussey helped with flanking fire."

The mills, save one, have vanished. The survivor is the most complete example of life along the creek more than a century ago.

In the years after the Revolution, Isaac Pierce, a Quaker from Pennsylvania, bought 150 acres of land just west of the creek where Tilden Street is today. When Pierce bought the land, a mill was standing on the edge of the stream, using the water power to grind grain for local farmers.

In the 1820s, Pierce's holdings reached from where the zoo is now up to the present Chevy Chase. He and his family built a new, three-story mill, using the blue granite rock from the Broad Branch tributary area a short walk upstream from their plantation.

The mill is a marvel of machinery that transforms the power of a tiny stream of water into motion that moves the great stone wheel inside. The water carried down the millrace to power the wheel is not creek water, though. Modern-day limitations force the mill to use city water, an advantage the Pierces didn't have 180 years ago.

Pierce's mill ground corn and grain, and flourished until the Industrial Revolution's steam engines put the water-powered mills out of business. It operated sporadically until finally closing when the park was created.

"It was used as a tea room in the early 1900s," McIntosh says. "The mill was restored in the 1930s as a working gristmill, and ground flour to make bread served in the government cafeterias. There have been times when it was out of operation. The wheel had to be replaced, and there was damage by floods."

Today, Pierce Mill grinds away five days a week, offering fresh stone-ground corn meal and flour for sale.

The mill isn't the only Pierce building still standing. His carriage house, made of the same Broad Branch granite, is now the Art Barn, where the works of local artists are displayed and classes are offered. The spring house, built in 1801 to keep milk and butter chilled, is in the median on Tilden Street about 100 yards uphill from the mill.CLASSROOM IN THE WOODS

The gristmill and fort are aberrations in the park, relics of the valley's human-made past. Nature is the real attraction here. Eighty-five percent of the park is woodland, with all but 23 acres of the rest taken up by lawns, buildings, roads and parking lots. Those 23 acres are undisturbed meadows, broad fields of tall grass and wildflowers where small mammals, birds, insects and more than 400 species of plants thrive. The meadows occupy 12 areas in Rock Creek Park, and you will need a map from park rangers to visit them.

You can get the map at the Nature Center, a low-rise building hidden in the woods just off Oregon Avenue south of Military Road. The center has rooms full of stuffed animals (foxes, squirrels, eagles, hawks and other birds), a working beehive, a cabinet filled with now-still butterflies and moths, a few live reptiles, and, on the tree roost in the back of the exhibit hall, Max the Barred Owl, a 17-year resident of the center. Max cannot fly; he lost his left wing when he was hit by a car in circumstances unknown, so the rangers at the center have, well, taken him under their wing.

Max's left eye is a bit damaged, too. "Max has glaucoma, and he gets eye drops each day," explained Ranger Mona Phelps. "We tell the children that Max is just like the president; they have the same eye problem. It helps reassure them. He injured his eye from a fall from the tree. He's just a ball of fluff and doesn't get hurt when he falls."

Max is the star of the center, appearing in a number of the monthly programs that cover everything from owls to insects, spiders, gypsy moths, beavers, seeds and nuts. Regular educational hikes along the creek and in the forest are also offered.

"We're more a classroom than a nature center," said Ranger Phelps. Recent programs offered by the center included a walk to examine edible plants, films and talks on tracking animals and how bees and spiders live, and storytelling.

The education programs focus on the outdoors, explains Ranger Dave Smith. "We have two trails: the Edge of the Woods trail in front of the center and the Explore the Woodland trail in back," Smith says. "What we see is determined by the seasons. We point out the difference in leaves and why they change color. Or we point out the different decomposers. The walks are mostly through a hardwood forest."

The October programs will focus on autumn. A walk at 2 on Oct. 7 explores how seeds travel, while on Oct. 13 at 10 rangers will lead a 1 1/2-hour walk to look at "autumn's harvest" of wild fruit, seeds and nuts. At 2 on Oct. 13, a guided walk will look at the changing color of leaves in the park and rangers will explain how it happens. Among other major programs in October is a five-mile hike along the park's Western Ridge, leaving at 9:30 Oct. 21 from the Boundary Bridge at the northern end of the park and Beach Drive.

Other programs explore the wildlife in the park, which remains quite high despite the thousands of visitors and commuters passing through each day. The animals include the usual squirrels, woodchucks, foxes and other small mammals, as well as a resurgence of the white-tailed deer which once flourished in the valley.

"The deer are coming back now," Smith explains, suggesting that construction up in the Maryland portion of the park is pushing them south.

The center also has a planetarium, the only one in the National Park Service. The programs offered on weekends introduce budding star-watchers to what they can see in the sky over Washington. Once a month, special night programs offer viewing of the planets through a telescope. The next such viewing is at 7 on Oct. 13 in Military Field, at Glover and Military roads.

Adjacent to the Nature Center is the Rock Creek Park Horse Center, where visitors can rent horses, take lessons or join in guided rides of the forest trails. Special lessons for disabled riders are also available.

The trails leading from the center are part of a 30-mile-long network of riding and hiking trails that range from easy to difficult. There are two main horse trails, both rated moderate and about 4.5 miles long.

The two main hiking trails are rated moderate and go by different routes -- one 4.3 miles long and the other 5.2 miles -- from the Maryland line to Bluff Bridge near where Piney Branch Parkway enters the park.

The biking trail is paved and stretches 7.5 miles from the Maryland border to the Memorial Bridge, and passes by a fitness course just south of the Connecticut Avenue bridge. On weekends, bicyclists, joggers and hikers also enjoy the use of Beach Drive between the District line and where Broad Branch and Beach interest. Automobile traffic is barred on that section of the park road weekends from 7 a.m Saturday to 7 p.m. Sunday and on holidays.

Another major recreational attraction in the park is the 18-hole golf course. The entrance to the Rock Creek Golf Course is at 16th and Rittenhouse streets NW.

At the District line, control of the park passes from the National Park Service to the Maryland National Capital Park and Planning Commission. For another 14 miles, the parks continue, following the creek north to its source at Lake Needwood.

At Boundary Bridge, just off Beach Drive at the northern limit of the District park, the suburban sprawl is mostly hidden by the thick tangle of vines, trees and undergrowth.

With so many so near and yet seemingly so far, the park lives up to the standard set by naturalist John Burroughs earlier in this century: "Rock Creek has an abundance of all the elements that make up not only pleasing, but wild and rugged scenery. There is, perhaps, not another city in the Union that has on its very threshhold so much natural beauty and grandeur." ROCK CREEK PARK

Rock Creek Park Day, the centennial celebration, takes place from noon to 5 Sunday with activities at Picnic Grove No. 1 just north of the dam at Tilden Street and Beach Drive.

Activities are free and include information booths by 28 civic and environmental organizations, musical entertainment by the U.S. Army Blues Band, Mahina and the Polynesians and folk singer Mike Mulvaney. Entertainment for children includes Gabby the Mime, "balloonologists," face-painting and other activities. Call 202/426-6832.

Parking will be limited, so drive to the Carter Barron parking lot at 16th and Kennedy streets NW and take the free shuttle buses from the lot to the festivities near Pierce Mill.

The park is open only during daylight hours. Camping, fires not in the grills and fireplaces and loud music in most sections are prohibited. For more information about Rock Creek Park, call the park service at 202/426-6829.

ART BARN --

Glover Road at Tilden Street NW. The former carriage house on Isaac Pierce's plantation has month-long exhibits of work by local artists and occasional art classes. Open 10 to 5 Wednesday through Sunday. Call 202/244-2482.

FORT DERUSSEY --

Off the bike path northeast of the intersection of Oregon Avenue and Military Road NW. Self-guided tours of one of the forts built to protect the Union capital from attack by the Confederacy.

PIERCE MILL --

Glover Road at Tilden Street NW. Open 8 to 4:30 Wednesday through Sunday, the mill grinds flour on the half-hour from noon to 2 Saturday and Sunday. At 11 and 3 weekends, a ranger guides tours of the mill and explains how it works. Wednesday through Friday meal-grinding demonstrations are at 10 and 11, with tours at 11:30, 1, 2 and 2:30, with films at 3 on request. Other special programs include weaving, films and living history exhibits. Call 202/426-6908.

ROCK CREEK NATURE CENTER AND PLANETARIUM --

Entrance off Oregon Avenue just south of Military Road NW. Exhibits, guided walks and talks, two self-guided nature trails, astronomy programs and other activities are regularly scheduled. Planetarium shows are at 1 (for ages 4 to 7) and 4 (for ages 7 and up) Saturdays and Sundays. The next night program is at 7 on Oct. 13, when Saturn and Mars will be viewed through telescopes at Military Field, at Oregon Avenue and Military Road NW. The center is open 9 to 5 Tuesday through Sunday. Call 202/426-6829.

ROCK CREEK PARK HORSE CENTER --

Next to the nature center off Oregon Avenue. Rents horses and offers lessons, including lessons for handicapped riders. One-hour guided trail rides are $15 a person. They are offered at 3 and 4:30 Tuesday-Friday and at noon, 1:30, 3 and 4:30 Saturday and Sunday. Holiday times vary. Group lessons are offered from 4:30 to 8:30 Tuesday-Friday, and from 8 to 2 weekends. Group lessons are one hour and cost $20; private one-hour lessons are $45. For information, call 202/362-0117.

ROCK CREEK GOLF COURSE --

Public course that can be reached from 16th and Rittenhouse streets NW. There is an 18-hole course and clubhouse. Greens fees: $5 for 9 holes, $8 for 18 holes weekdays, $6 and $10, respectively, weekends and holidays. Call 202/882-7332.

TENNIS --

There are 20 outdoor and five indoor tennis courts at Rock Creek's Carter Barron Park, 16th and Kennedy streets NW. Rates range from $3.25 to $7.50 an hour for the outdoor courts, $15 to $20 an hour for the indoor courts, with package rates available. Call 202/722-5949.

IF YOU FOLLOW Rock Creek north from the District you'll pass from Maj. Nathaniel Michler's wilderness to a park hemmed in by the "costly suburban villas" the Army engineer feared would destroy the beauty of the Rock Creek valley.

The Rock Creek parklands in Maryland, though, live up to the other part of Michler's vision of a park in the valley: "grounds for play and parade, and many other useful purposes."

In Maryland, these "useful purposes" include athletic fields, nature centers, miles of hiking and biking trails and a wealth of other activities.

Visitors to Candy Cane City, a recreational area on Beach Drive a quarter-mile north of the District line, can play soccer and softball at the athletic fields or basketball on the courts, or just play on the children's playground. Nearby are the Meadowbrook Riding Stables, where riding lessons can be had.

The Maryland park twists and turns, following Rock Creek north and passing through what the park bureaucracy calls SVPs -- stream valley parks. These little-developed stretches of woodland have paved trails, fitness stations and, occasionally, playgrounds and game fields. Fourteen miles north of the District is the source of the creek, Lake Needwood and Rock Creek Regional Park.

CANDY CANE CITY -- This recreation area on Beach Drive just north of the District boundary has playgrounds, basketball courts, athletic fields and occasional organized children's activities.

MEADOWBROOK RIDING STABLES -- The stables are on Meadowbrook Lane, off East-West Highway just east of Beach Drive. Lessons, riding and boarding services. Private riding lessons are $30 for a half-hour, $50 for an hour. Semiprivate lessons (up to three riders) are $20 and $30, respectively. The stables do not rent horses except for riding lessons. Call 301/587-9762.

MEADOWSIDE NATURE CENTER -- 5100 Meadowside Lane, Rockville. Exhibits, naturalist-led programs and activities, movies and workshops. Outside the center are miles of trails, a frog pond, the 54-acre Lake Frank, a covered bridge, ruins of Muncaster Mill and other historic sites. Call 301/924-4141.

NEEDWOOD GOLF COURSE -- 6724 Needwood Rd., Derwood. Needwood has both an 18-hole and 9-hole course. Greens fees are $7.50 for 9 holes, $13.50 for 18. Hours are 6 a.m. to sunset weekends, 7:30 to sunset weekdays. Call 301/948-1075.

ROCK CREEK REGIONAL PARK -- Muncaster Mill and Avery roads, Rockville. The park offers a variety of activities including boating and fishing on 75-acre Lake Needwood, rides on a pontoon boat, hiking, biking, archery on a range, picnicking and ice skating on the lake (at your own risk). Call 301/949-8010.