Not since Amelia Bloomer sallied forth in her version of pantaloons back in the 19th century have slacks caused such a stir. Trousers have virtually inundated the apparel market.

Disgraceful? Shocking? Not ladylike? Nowadays, all the objections Amelia met when she wore her revolutionary bloomers seem incomprehensible. The 20th Century has vindicated this comfortable, practical style; it has seen trousers climb to the top of the hit parade. Indeed, numerous well-dressed ladies consider smart pants their "uniforms" - clothes that can be counted on to look right whether hems go up or down; whether skirts skim or hug the body.In the fickle world of fashion, many consumers have come to regard pants as the one sure thing. And the American garment industry, sensing these feelings, has provided dozens of options from which to choose this spring - slacks to suit every occasion; slacks in every price range.

Whether a woman's on a tight budget or has big bucks to spend, she'll be able to find handsome pant outfits in the marketplace. Same thing applies whether she's seeking slacks suitable for casual daytime occasions or festive evenings.

Pants come in many versions, colors, patterns, fabrications. Perhaps the newest looking pants coming into the stores right now feature wide, flowing legs and high-rise waistlines (we used to call them palazzos). Especially pretty for evening, they're being shown with sheer blouses, short evening jackets and/or sequinned tops, and they provide a perfect alternative to the formal gown.

It should be noted that the wide, long pants - the big newsmakers for spring - do come in versions for daytime. But somehow they seem a bit too dressy, a bit too difficult to get around in to make them right for office and street wear. Better, certainly, are the numerous styles that are less extreme in cut - classic pleated trousers - some with cuffs, some without; slacks that are cropped ankle-length or a bit above; traditional pegged pants.

No matter what silhouette you choose, however, there's a general trend that should be emphasized, a definite mood to pant dressing for spring '89. Designers are calling it a "swing to softness." In the past, some people have objected to pants for women because, they said, trousers were so masculine and tough looking. Well, that objection simply doesn't apply this season. The pants being offered this time around are the very essence of femininity. They're cut to flow and drape over the body in such supple materials as featherweight wool crepe and airy rayon. Even linen pants have a gentler and kinder appearance as opposed to customary crispness.

To add to the feminine appeal of pants this season, numerous designers are teaming them with sheer blouses, organza coats, whirling dusters, lacy camisoles, jackets that wrap and tie, flowing chiffon scarves either tossed Isadora Duncan style at the neckline or worn like a sash at the waist. The approach, rather than paying homage to menswear, exalts the feminine mystique.

Some desiigners - Ralph Lauren, for one - believe in feminine-looking pants outfits so strongly that their collections feature the style almost exclusively. Even the apparel kings who are noted for their sexy skirts and dresses - Oscar de la Renta and Bill Blass - are doing plenty of softly pleated slacks this year that look perfectly appropriate for garden parties.

Favorite colors are pristine white, midnight black and rich navy blue.

Brights - hot pink, luscious lime, sunny yellow, orange crush - also are surfacing and look terrific for warm weather. Eleanor Brenner, a New York designer who does high fashion clothes at reasonable prices, often combines brights: pink pants with yellow duster, for example.

Another favorite and affordable designer, Adrienne Vittadini, likes prints and her spring collections loaded with attractive print pants that are often teamed with complementary sweaters or matching camp shirts. Prints, it should be noted, are making a strong fashion statement in slacks for warm weather. Watch for stripes, florals, dots.

Also keep your eye on jumpsuits. Closely related to pants, they're appearing in almost all the designer collections and are being shown in styles right for the beach or ball.

Designer Geoffrey Beene, who has always included jumpsuits in his collections says he feels they're the modern way to dress. Women, in his opinion, want easy clothes when they have to pack for travel, and jumpsuits certainly fill the bill.

The same thing can be said for pants. There isn't anything that's easier to wear on a plane, on a train or hopping in and out of cabs. (Ask legendary actress Katharine Hepburn, who has always sworn by slacks.) There isn't anything better for rough, hard work. (Ask all those Rosies who rivited during World War II.) But Yves Saint Laurent, the famous Parisian designer, certainly didn't have such practical stuff in mind when he began endorsing pants in his collections about 20 years ago.

Saint Laurent said pants were great for women because the style's just plain sexy. Like a woman wrapped in a man's bathrobe. So he promoted the pantsuit for day. He popularized "le smoking" (patterned after the man's dinner jacket and tuxedo) for evening. Other designers on both sides of the Atlantic followed his lead. So did lower-price manufacturers of women's clothing.

Enthusiasm for slacks, in all their incarnations, has varied since. But through the mini, through the midi, through the maxi - through all fashion's ups and downs - the style has endured. And by now, pants have become as essential to female fashion as lacy lingerie and high-heeled shoes.