When New York adopted its “grid plan” in 1811, there wasn’t much the city could do about Broadway. It was a highway linking the city to points distant, and to straighten Broadway so it would line up with the grid probably would have brought chaos to the whole economy of the northeastern United States.

At 14th Street, Broadway and Fourth Avenue don’t cross (as Broadway and Fifth Avenue do at 23rd Street), but rather do a kind of dance — a tango, it reminds me of — forming the first “union” of major north-south thoroughfares and suggesting that the resultant demarcation be made into a public square, something elite 19th-century New Yorkers, especially the lawyer and economist Samuel B.Ruggles, had been clamoring for.

In the 1830s, Union Square became a fashionable faubourg. Precious little remains to remind us of that long-ago time, but look hard at some of the buildings to the west on 14th Street and you can just about make out that they were once row houses.

When the rich families moved away, Union Square became the fancy shopping district. Such stores as Tiffany’s and Brentano’s Literary Emporium, and restaurants like Delmonico’s and Lüchow’s, were there. In Edith Wharton’s “The Age of Innocence,” which is set in the 1870s, society ritually congregates at the Academy of Music, on the northeast corner of 14th Street and Irving Place. (It was torn down for the Consolidated Edison Building, and shouldn’t be confused with the much later movie theater called the Academy of Music on the other side of 14th Street.) Theaters abounded around Union Square in the late 19th century, making it the forerunner to today’s Times Square theater district.

Nothing stayed put for long in old New York. Delmonico’s moved to 26th Street. Tiffany’s went to 37th Street. The Metropolitan Opera House (at 39th Street) put the Academy of Music out of business.When wealthy society moved uptown, loft buildings began to rise around Union Square — a classic New York pattern. After 1900, the labor movement grew vigorously. Like everything else in New York, the unions found it advantageous to bunch together in one part of the city. (It may be hard to believe, but it’s pure coincidence that the unions clustered at Union Square.) New Yorkers of a certain age remember the old Union Square of the unions, when Coffee Shop was actually a coffee shop where union business was routinely discussed by hard men whose daughters’ wedding receptions were held at the Marc Ballroom, which was named for the grandson of the founder of the Provision Salesmen and Distributors Union. (Did any union guy ever think that ballroom would one day be rented by Sony Entertainment, Goldman Sachs, or Wired magazine?) The most immediate reminder of the unions is the Amalgamated Bank on the east side of the square — on the site of Tiffany’s.

Not coincidentally, Tammany Hall, which had earlier been on 14th Street next to the Academy of Music, built its headquarters on the east side of the square in 1928 (in the building at Union Square East and 17th Street that is now the New York Film Academy). The amorphous north end of the square, where the Greenmarket sets up, was once the scene of political rallies, as well as of Hyde Park Corner-like oratory. Again not coincidentally, the union-era square became a discount shopping mecca. S. Klein on the Square was the Macy’s of discounters — before Century 21, before Target. (Klein’s was where Zeckendorf Towers now is.) This was the Union Square into which Andy Warhol inserted himself when he established his Factory in the old exotic Decker Piano Building on Union Square West. (Warhol moved his Factory around, but this was the one where Valerie Solanas shot him in 1968.)

Union Square changed yet again. Tawdriness set in. Drug dealers appeared. It’s hard to imagine now, but not long ago Union Square was a backwater. There were no fine restaurants. Lüchow’s even decided to move, to Times Square, where it died as it may not have, had its owners been superhumanly prescient and known that if they’d hired a good chef they could have cashed in on the booms in New York history and in Union Square as a restaurant mecca. They missed the boat by about two years. But that, too, is a standard bit of New York history.

The Greenmarket helped change everything. I refrain from saying, categorically, that it did change everything. At first, the notion of going to Union Square to buy your super-fine produce had a radical tinge to it. In 1978, John McPhee wrote a classic New Yorker essay, “Giving Good Weight,” about the city’s Greenmarkets. It’s now a classic period piece. He said that at Union Square, vendors “took along iron pipes for protection.” Whew!

Everything changed in the 1980s. The city shook off its fiscal torpor, and Wall Street roared as most people couldn’t remember it roaring before. Crime was still sky-high, and the city’s representative man was closer to Patrick Bateman than to any of the gang from “Seinfeld.” But the 1980s saw the Midtown office market hypertrophy, with the result that the loft buildings and office buildings of “Midtown South” benefited. At the same time, the Villages both West and East went into real estate overdrive. Union Square was peculiarly well-positioned to benefit from growth coming at it from both ends, had the right kind of buildings, and was a transit hub. That’s why I say, Greenmarket or not, Union Square would have boomed.

But the flavor, let us say, would have been different.The restaurant boom around the square may have happened anyway, as a result of loft businesses being replaced by office businesses as well as the gentrification of the residential population. That the restaurants feed off the Greenmarket is, however, undeniable. And because Union Square has become a foodie paradise, Whole Foods and Trader Joe’s have moved in. At first, the Greenmarket vendors were very apprehensive about this. And while I don’t know if the Greenmarket has lost money to the new gourmet emporiums, my own eyes tell me there’s business enough for everyone — indeed, that this clustering, so typical of New York, can work to the advantage of one and all.

It’s a truism that the only constant in cities is change. Cities occupy important places, which is why they can be sacked or depopulated or change ownership, yet always re-emerge. I think the same dynamic holds for some neighborhoods. Union Square occupies an important place, and it shouldn’t surprise us overmuch that it is the liveliest, trendiest spot on the island today.

fmorrone@nysun.com