The bar-code scanner beeps.

The groceries glide toward the bagger, not to mention a new eco-enemy: the bag itself.

If it's made of plastic, all the worse.

Plastic bags are handy, to be sure. They carry lunches, wet swimsuits and pet waste.

Yet they last for centuries in landfills. Thrown away, they are often blown away, urban tumbleweeds that wind up draped in trees, plastered to fences, clogged in sewer drains.

In the open sea, they kill turtles.

So the ubiquitous "free" plastic grocery bag - that small, pale piece of processed petrol so flimsy that good baggers double up - is beginning to be targeted by lawmakers and others who want them restricted or banned.

If the trend spreads, it could alter the shopping experience, bringing fundamental change to the convenience that Americans have come to expect.

On Wednesday, New York Mayor Michael Bloomberg signed a law requiring large stores to collect and recycle plastic bags.

The day before, Whole Foods vowed to use its last plastic, probably by Earth Day.

China banned the bags, known there as "white pollution," this month, part of its cleanup for the Olympics.

Ireland imposes a tax.

South Jersey legislators introduced a bill to require big stores to halve plastic-bag giveaways by the end of 2009 and eliminate them in 2010.

The nation's strictest big-city bag ban took effect two months ago in San Francisco. Stores now can offer only paper or a relative newcomer to the ever-more-complex world of bagdom, compostables.

"Much of our plastic-bag usage is so frivolous," said Christine Knapp of Citizens for Pennsylvania's Future. "You don't need a plastic bag to carry a can of soda."

Or a paper bag. Increasingly, the question is not "paper or plastic?" but "did you bring your own?"

Some stores pay you to do it. "Want our two cents?" reads a ShopRite promotion. "Save the planet and reuse our grocery bags."

Nevertheless, on any given day it's still plastic, often with a paper bag inside, that's headed out the supermarket doors onto Roosevelt Boulevard.

The Northeast Philadelphia ShopRite goes through 120,000 plastic bags a week, plus 15,000 paper bags. Baggers give about 1,000 of the discounts, $20 worth.

Some shoppers don't reuse their bags because the containers rip. Mona Blatstein, 81, doesn't think it's "sanitary."

"Here's what they should do," said Lucia Rossillo, 39, as she headed out with her plastic-bagged items on Friday. Instead of giving away a turkey after you spend a certain amount of money, she said, "they should give you a free bag."

They do sell them for 99 cents, but still . . .

Then there was Herman Schneid, a retired engineer who hates waste, with items tucked into a Save-A-Tree canvas bag.

Indeed, reusables are becoming chic.

Chicago-based www.reusablebags.com offers dozens of styles, ample data about the evils of disposables, and a plea:

"Do the right thing and BYOB - bring your own bag!"

Plastic grocery bags were introduced in 1977. Within two decades, four of five bags used were plastic, according to the Society of the Plastics Industry.

Americans now use an estimated 100 billion disposable plastic grocery bags a year, about 20 per household per week.

Environmentalists say that producing those bags requires 12 million barrels of oil, the equivalent, counters the industry, to just half of one day's oil consumption in the U.S.

Indeed, manufacturers are vigorously fighting proposed bans.

Naysayers outnumbered supporters nearly 4-1 at an October hearing on Philadelphia City Councilman Frank DiCicco's proposal to ban noncompostable or nonreusable plastic bags.

"This city is a dirty city," DiCicco said.

But Managing Director Loree Jones said that without a composting facility, most biodegradable bags would end up in landfills anyway.

Denise Earley of the Greater Philadelphia Chamber of Commerce argued that alternative materials would increase costs. (Plastic bags cost merchants one to two cents apiece, paper three to eight cents, compostables a bit more.)

The bill died in committee, but a task force will study the issue.

Citing cradle-to-grave studies that take into account manufacturing emissions, transportation fuel and other factors, manufacturers say plastic is more eco-friendly than paper.

Environmentalists generally concede that point. Or avoid it.

"They both have bad environmental impacts," said Christine Flowers-Ewing of Keep California Beautiful.

It's plastic's hellish afterlife that's so worrisome.

Biologists know from necropsies that the bags, which can look like jellyfish, choke or block the digestive tracts of marine species.

Volunteers for Clean Ocean Action's annual trash pickup collected 6,349 plastic bags along 130 miles of New Jersey coastline in 2006, about one every 30 or so steps.

Of those that make it into the waste stream - including plastic wrapping, 4.45 million tons in 2005, according to the U.S. Environmental Protection Agency - just 5 percent are recycled.

But manufacturers prefer recycling to bans.

Most supermarkets now place recycling bins by the entrance. Most of the collected bags are carted off to a Trex Co. plant in Virginia, where they are mixed half and half with reclaimed wood and resold as plastic decking.

"We're hopeful this trend will increase," said Keith Christman, with the American Chemistry Council and Progressive Bag Affiliates. "Demand certainly outpaces supply."

San Francisco was not keen on that idea when it first considered options for dealing with the bags a few years ago. Only about one percent were making it back into store recycling bins. Curbside pickup and processing was estimated to cost $3,000 a ton and to earn $30 in resale.

A tax sufficient to cover the city's $8 million in costs, including litter clean-up, would come to 17 cents a bag; the state nixed that idea.

So the city decided to limit stores to paper or compostable plastic bags.

Many shoppers seem to be embracing BYOB instead.

"People definitely bring their own bags," Thea Stein, 29, said.

So is she running short on trash-can liners? "When you become responsible about recycling, you actually have very little trash at all," Stein said.

Among those watching all this is Robert Bateman, president of the Oroville, Calif., plastic bag-maker Roplast Industries.

The company responded to San Francisco's ban with the B4, the Bio Bring Back Bag. Bateman has watched it decompose like oak leaves in his backyard bin. (The B4 is made of corn starch.)

Bateman predicts that the thin plastic bag will be gone in a decade, as a movement against all sorts of disposable products gains steam.

Meanwhile, he has a brand-new bag. It's still plastic. It's just thick enough to be considered reusable.