Portrait of the American panorama - its enduring diversity, its secrets, its glories.

Some people sit around and wait for the world to poke them. Right here in this old curiosity shop of a world, they say, ''Poke me, world.'' Well, you have to keep the challenges coming. Make them up if necessary.

- Alice Middleton, Smith Island, Md.From William Least Heat Moon's ''Blue Highways.''

It undoubtedly had a lot to do with my parents, who insisted, as parents will , that every year we all take our places in the black Ford station wagon and swing off on a family vacation across the American West.

Needless to say, back when teens' hearts longed for the not-so-instructive allure of Fort Lauderdale, yet another trip across the plains states was about as welcome as spending Easter in the ''snow belt.'' I recall that my sister and I sulked all across Kansas. Such were the joys of seeing the USA, almost in a Chevrolet, 20 years ago.

It was not until later, much later, that I realized that even among the well-traveled I was unique in having visited not only the Grand Canyon but the lesser-known and more mysterious Black Canyon of the Gunnison, Mesa Verde, and the eerie redness of Sedona, Ariz. A litany of the underappreciated - I was proud to have seen them.

In his best-selling book, ''Blue Highways,'' William Least Heat Moon writes that ''there are two kinds of adventurers: those who go truly hoping they find adventure and those who go secretly hoping they won't.'' He neglects to mention those who find adventure, like greatness, thrust upon them. Such is the traveler that some of us have become here in our own land - backing our way into new horizons.

America has always been prone to self-criticism. Never mind that we are the birthplace of the cheeseburger and the Mason jar. We find our culture inferior and our society undifferentiated by comparison with our European cousins. Conversely, our sheer geographic spaciousness has fostered not only a some-where-over-the-rainbow restlessness but also a belief in infinite possibilities. As travelers, perhaps, we are caught in Janus-like concepts of our country: the fear of ''What if you lived here! The Middle of Nowhere,'' as Least Heat Moon describes it, and the availability of flame-broiled hamburgers from sea to shining sea. Our rambunctious birthright, derived from equal parts of V-8 engines and Alexis de Tocqueville, seems somehow diminished in the face of Japanese imports and franchised food.

But listen to three writers-cum-travelers, Joel Garreau, Least Heat Moon, and John McPhee, who in their own way and own prose strike a blow for that By Jingo legacy that runs from de Tocqueville to Jack Kerouac. Garreau argues that sociological and regional differences are alive and well and reach from Portland , Maine, to Portland, Ore. Least Heat Moon puts a human face on those differences. And McPhee confronts that last American frontier - Alaska.

A longtime Washington Post writer and editor, Garreau takes the bull by the proverbial horns and insists that ''travel is our great national pastime because of our enduring diversity.'' Impossibly Panglossian? Garreau, who turned his thesis into the best-selling ''The Nine Nations of North America,'' hastens to chastise the unbelievers. ''The existence of Interstate highways, dense air connections, cheap long-distance rates, ubiquitous television, and the celebrated franchised hamburger has lulled many, incorrectly, into some sense that North America has become utterly homogenized, if not bland.

''What's been happening for the past few decades,'' he continues, ''is that North America has been maturing. Houston, Kansas City, and Atlanta, for example, only twenty years ago were crossroads not even their Chambers of Commerce could love. Now they're world-class cities.''

Take, for example, Iowa - Sioux City, to be exact. ''This was the land,'' writes Garreau, ''I'd been told for which the 1957 Chevrolet and warm summer nights were invented.'' It is entirely different from the Pacific Northwest (Garreau calls it ''Ectopia''), where people ''become irate over a Twinkie.'' Or New England, where he says ''old-fashioned intellectual snobbery'' and a ''faded industrial legacy'' have caused poverty to become ''rather chic.'' Or the West, where ''development is a religion.''

Clearly this is a man who recognizes our country's assets wherever they may be. Or, as he insists, ''Get off the Interstates and out of the airport.''

He elaborates in an interview. ''I don't particularly like Holiday Inns,'' he says, ''although I understand the reason they exist. That need for a common imperial culture in order to communicate across the country. That's fine and dandy if you think that makes us all the same, but you won't realize that what's driving the guy at the other end of the (long distance) phone is entirely different from what's driving you.''

This is hardly talk of figurative beauty and literal abundance, of purple mountain majesties and ripe wheat. No, Garreau is urging us toward a self-definition more ineffable but ultimately more significant: ''power, money, influence, and attitudes.''

''The last time there was a real interest in regionalism was in the '30s, and it was cultural,'' he says. ''You know, there's Cajun cooking in Louisiana, they talk funny in Georgia, and they eat lobsters in Maine. But then (technological advances) broke down those idiosyncrasies and now what's left are the real differences.'' Differences that Garreau maintains are increasing, not decreasing.

''The computer revolution is starting to customize why you are where you are, '' he says. ''A steelworker has to be site-specific, but with computers as a basis, you increasingly hear people say, 'Where do we want to be and how do we get there?' '' It's a premise, he says, based upon ''a fundamental difference between San Diego and Buffalo. We're the most mobile civilization in the history of man, and a lot of what people are looking for is home. Whether that's a small town in Vermont, an Iowa farm, or a houseboat off Key Largo.'' As for the traveler, he adds, ''I presume that's why people travel - the differences - even when they can't articulate it.''

When William Least Heat Moon ran into Madison Wheeler, a former farmer and storekeeper in Nameless, Tenn., Mr. Wheeler didn't hesitate to tell him right off that ''with these hard roads now, everybody gets out of the hollers to shop or work. This tar road done my business in, and it's likely to do Nameless in.''

But Least Heat Moon, a week or two into what would turn out to be a three-month journey around the United States, had already determined that ''life doesn't happen along the Interstates.'' Interstates, he wrote, ''hid the people.'' It was one of the reasons he had chosen the ''blue highways,'' or back roads, and the small towns ''that hadn't seen the neon light,'' as the backbone of his trip. Towns with names like Scratch Ankle, Ala., Gnawbone, Ind., Humptulips, Wash., and Dime Box, Texas, were bait to Least Heat Moon. He bit on the last one - Dime Box. As he did with Brooklyn Bridge, Ky. And Frenchman, Nev. And Moscow, Idaho - to name just few towns whose names and residents caught Least Heat Moon's attention and usually his affection.

''I was attempting to stay away from the areas I knew well,'' he says when asked about his epochal drive around the country. ''But besides that it was serendipitous except that (a) I stayed off the Interstate and (b) I went where it was warm when it was cool and cool when it was warm.'' Least Heat Moon barely needs to tell us that ''the traditional way for Americans to travel is to take long trips. We've got a good highway system, and big cars can move over the country quickly.'' But then he adds: ''We get the notion that we know what someone is like because the long distances in this country have encouraged us to be superficial. People drive across Kansas and say, 'I never want to go back there.' But they probably never even got out of the car.''

In his book he talks about the temptation of the American highway as ''taking a trip instead of making one.'' ''There are other ways to travel,'' he says, ''and very significant travel occurs . . . while you're on your feet. You've got to be willing to trade the banalities (with strangers). You can't just cross the street and ask someone what their job means to them. If the travel is intelligent and of the right kind, you will get the understanding (of one's fellow countryman) that is so crucial to democracy.''

It is the kind of understanding that allows men to shape their future. As Least Heat Moon writes, ''A human being is . . . at liberty to use the inclinations of the past as he sees fit . . . . By seeing both the futility in trying to relive the old life and the danger in trying to obliterate it, man can gain the capacity to make anew.''

Or one can simply discover that the idea for the Mason jar was born in Othello, N.J. Or that the Adirondacks is a wilderness second only to the north woods of Maine. That the Natchez Trace is a highway that does not ''outrage the landscape.'' That the Palouse area of Idaho is the most ''visually striking topography'' in the US. That St. Martinsville, La., is ''distinctive and memorable in a tattered way.'' If you ask Least Heat Moon, he will give you not only his philosophical reasons for travel, but also some travel tips - especially appropriate for the hardy, but helpful to all:

* Never eat or stay in any place that has the words ''Flamingo'' or ''El Rancho'' or ''Thunderbird'' in the title. ''This works,'' he says.

* Never stay in a motel that has a pink or aquamarine color scheme.

* Any cafe whose name you cannot distinguish from the outside is probably pretty good.

* Stop and read highway markers. ''People ought to do this,'' he says. ''Some are excellent. Massachusetts has a good system; so does Texas.''

* Visit local libraries along the way for local history and tips on good places to eat.

* On the other hand, avoid asking advice from filling stations, or anyplace where travelers are frequent. ''I wonder how many people's impressions of this country are based on experiences had at filling stations,'' he asks.

* Read old WPA guides. ''Published during the '30s,these are magnificent books,'' he says. ''Excellent for having kept writers employed then and for information now.''

* University and college dining halls are good places to get a good meal inexpensively.

By following these rules, Least Heat Moon says, he completed his entire trip (not including gas and lodging) for $5 a day. ''Of course that's not luxury travel. That's peanut butter sandwiches and sleeping in the back of my van,'' he says. ''I'm not sure I'd do it again that way. But if you can't travel on $5 a day, you don't know how to travel.'' Besides, he adds, ''you can get tremendous meals for under $5. The best one I had was at Swamp Guinea's Fish Lodge (Ga.). The whole table was covered with food for $4.50.'' Or the offer from Madison Wheeler, which apparently still stands. ''If you get back this way, stop in and see me,'' Wheeler suggested. ''Always got beans and taters and a little piece of meat.''

So on to the frontier. And the chief Westward Ho'er seems to be a resident of New Jersey who, when given the choice ''between hiking and peeling potatoes,'' would peel the potatoes. That such a man would go on to spend 2 1/2 years of his life scrambling across the frozen surface of Alaska and writing about it - well, it's testimony to the difficult mystery that is the 49th state.

As John McPhee described it in ''Coming Into the Country,'' his 1976 treatise about Alaska, ''the country is wild to the limits of the term.'' In other words, L.L. Bean boots were everywhere in use.

There are other tips: In Alaska, ''you can't drive, you can't walk, you can't swim.'' (Apparently everyone takes to the air on a chartered wing and a prayer that the bush pilot will know the way.) As for wildlife, mosquitoes ''fly in dense whirling vertical columns, dark as the trunks of trees.'' McPhee once slapped his arm and killed seven. And the weather - fronts ''as dank as oysters'' would move in and out with the serendipitous frequency of those in Scotland. ''This was not a dare with nature,'' he writes. ''This was nature.''

Yes, but what's it really like?

''It's as if you were in Siberia,'' he says in a precise, careful voice over the phone from New Jersey. ''The space is incredible. If you laid Alaska on the 48 states, the heart would be in Kansas and the rest would stretch from Georgia to San Diego. It's just a gigantic piece of real estate - 375 million acres with 400,000 people. In some ways its absurd that it is a single state.''

McPhee suggests that if you ''take away'' Anchorage, Juneau, and Fairbanks - half the population lives in Anchorage alone - ''there isn't much left.'' At least not in terms of people. ''Imagine if the stretch between New York and Chicago didn't have a house on it,'' he says to convey the emptiness that is bush Alaska. Meeting someone north of Fairbanks, he says, is ''like getting a magazine.'' But for McPhee, who kayaked, flew, and dog-sledded his way around, this is the real heart and meaning of the state. ''Downtown Anchorage is like anywhere else,'' he says. ''If a person wants to go there, do anything to get away and see the essential Alaska and not a Burger King.'' Beyond the road system, McPhee says, ''it's another world.'' But one that is literally not so easy to see.

His Alaska travels included separate trips to four areas: the north, where he canoed alongside the Brooks Range on the Kobuk River; the interior bush country, where he lived in a town of 19 people who raised their children on moose meat; down south into glacier country, where one glacier is the size of Rhode Island; and Mt. McKinley, ''one of the most amazing sites in North America.'' But the average traveler will have a tough time seeing all this. On foot it is a challenge. Chartered helicopter is ''unbelievably'' expensive. Jeep? ''Never. You'd be finished in the first 300 yards.'' But spend one day in a float plane - this is the way to see the real Alaska. ''You'd never forget it,'' McPhee says.

In conversation and in print, McPhee never makes Alaska seem threatening - just ornery, elemental, and powerful. ''You come to the place on its terms,'' he said in his book. ''You assume the risk.''

But more than that, McPhee shows us Alaska as that corner of the country where the ancient man-against-the-land confrontation - which so keenly defined America in its early history - still rages.

The question of preservation rears its head all over the state. Recording his reaction to spotting his first wild grizzly, he wrote that ''What mattered was not so much the bear himself as what the bear implied . . . . He implied a world. He was an affirmation to the rest of the world that his kind of place was extant.'' And McPhee quotes an employee of the Bureau of Outdoor Recreation: ''Throughout the history of this country its been possible to go to a place where no one has camped before, and now that kind of opportunity is running out. We must protect it, even if artificially.''

McPhee confesses that much of Alaska reminds him of only one other state - Maine. ''I love Maine,'' he says, and then asks, ''Why go to Alaska? If you have to ask that question, you don't understand.'' There is something about both Maine and Alaska, he says, that suggests a threshold: ''They are at the opposite ends of the great north woods - the boreal forest that runs from Maine to Alaska and across Canada. They are the land of the spruce and loon and balsam and mink and martin and the whole business.'' And not one mention of a V-8.