Sunday mornings, rain or shine, Melvin Thompson and Paul Cornwell stroll in Virginia's Great Falls Park. Self-appointed guardians of the wild things, they feed the geese and keep track of the skunk cabbage, and for the past year they've been taking park visitors along with them, on weekly wildflower walks.

Thompson says Great Falls is the best place in the world for wildflowers, and recalls the days before endangered species. "People in Washington would come out in the streetcars, with little baskets, and gather the trailing arbutus and ferns. You have never smelled anything like the trailing arbutus in bloom. Now it's an endangered species and you'll only find a few patches of it in the park. They couldn't hurt the ferns.

"They'd take the flowers, with spring water to keep them fresh, and sell them in the market downtown. This is God's country then. The only way you could get into the park was by streetcar."

Times have changed, but still the search goes on in spring, for "Dutchman's breeches," whose lobed flowers look like starched pants hanging on a line: for "shadbush," which blooms when shad are running; for "squirrel corn," with root-nodules that look just like corn; for "wakerobin" or "stinking trillium," that attracts insects with its smell. These are flowers that visit our woodlands briefly in spring, before the canopy of the forest leafs over and takes the sunlight away. Peak blooming time for early spring wildflowers is mid-April.

The fun of looking for wildflowers eludes some people. "It's a much more subtle thing than doffodils," says Robert Haehle, director of Brookside Gardens. "A lot of people, because wildflowers are subtle, may just miss them: 'Well, I came here - where are they?' I think the average American looks for flowers all big and splash. But wildflowers aren't going to be something that's going to knock you eyeballs out. You may have to get on your hands and knees to see them and appreciate them. They're subtle, they're delicate, they're fleeting."

What they give in size, they make up for in numbers. Stanwyn Shetler, an associate curator of botany with the Smithsonian, says that there are upwards of 3,000 species of flowering wild plants in the Washington are - "ten times the order of magnitude of bird species."

The river is basically a gardner," says Shelter. "It's always uprooting things and carrying them downriver and planting them somewhere else. The river is a highway for plantlife and mingles things from higher and lower elevations, so over a period of time we get quite a variety."

And in the diverse habitat along the Potomac and the canal, you see a lot of different flowers, ones that grow best in bottomlands or near rock outcroppings, on rich slopes or on dry uplands. Asides from Great Falls, wildflowers are abundant in Turkey Run Park and Riverland Park. There are good areas near the river in Maryland: on the towpath above Sycamore Island (accessible where Walhonding Road meets MacArthur Boulevard); and farther up, along the Billy Goat Trail on Bear Island (between Carderock and the Great Falls); near the towpath above Seneca, and around Harpers Ferry.

For those who don't want to scrabble around in the woods there's Fern Valley, planted with wildflowers, at the National Arboretum, 24th and R Streets NE, and a wildflower rock garden at Long Branch Nature Center, 625 South Carlin Springs Road, Arlington.

Other good places for early spring wildflowers are farther afield. Some suggested by Philip Stone, a leader of wildflower walks for Potomac Appalachian Trail Club and the Audubon Society, are, in Maryland, Patapsco State Park (west of Baltimore, best hunting on trails in the McKeldin section), Catoctin Mountain State Park trails, and Cedarville State Forest on U.S. 301 toward Walforf. In Virginia, there's Lake Accotink Park and Prince William Forest Park. Bull Run Park is especially noted not for great variety, but for abundance - thick mats of springs beauties and bluebells in mid-Arpil.

And so they go out, equipped only with Peterson's Field Guide to Wildflowers" and sometimes a magnifying glass, in search of the wild. Like birdwatchers, wildflower anthusiasts often keep lists, but not the birder's scrupulous lifelist. Leader of walks keep a checklist and compare it with last year's flower sightings at the same place.

"The exciting experience," says Stone, "is when you find somthing that's rare. Every once in a while you do. But I'm not inclined to tell people where I find the best things.

"I found Jeffersonia - and It's rare in the Washington area - and I found it on Jefferson's birthday, April 13. But that wouldn't be exciting to people unless they were devoted to wildflowers."

To Shetler, a rare find is "a pink mayapple that showed up in Pennsylvania. I'd never heard of a pink mayapple and I was rather excited to see that there was one.

"But from a botanical standpoint it's a genetic variation that turns up. The lay public doesn't understand how much variation there is in nature. Like they will come up with a five-leaf or four-leaf clover and get excited about it, and it's sometimes difficult to explain gracefully that it's just an expectable biological variant."

By the way, it's against the law to pick wildflowers in parks, though scientist can get special permits to take flowers for study. As for an endangered species, Shetler says it's best not to draw attention to it."Well meaning professionals will go and wipe it out. A few feel it's more important to have a specimen in a museum than for the species to continue.

"It's very hard to explain why a furbish lousewort is so special. But you concentrate on it as a symbol of declining diversity in nature.

"There's no better season to make some points about keeping wildlands," says Shetler. "Everthing is new and rejuvenated, people are eager to go out. Spring wildflowers are a manageable number, people can learn them. By summer we're overwhelmed."

Ah yes, summer - a time for Joe-Pye-weed, toadflax and purple loosestrife, for spotted touch-me-nots and pipsissewa. CAPTION: Picture, YELLOW DOGTOOTH VIOLETS GLISTEN WITH DEW. By Harry S. C. Yen-The National Geographic.