My herb garden is coming along nicely, and I'm having fun using the different herbs in the kitchen as the whim strikes me. But I've been wondering about these wonderful plants -- mint, parsley, basil and so on -- and about herbs in general. What makes them so aromatic? What's special about all those leaves and petals that make them edible and flavorful?

What makes them edible is the fact that people have been eating them for thousands of years. What makes them aromatic and flavorful are their essential oils.

The word "herb" comes from the Latin herba, meaning grass or green blades. To a botanist, herb is the term for all the soft, non-woody, non-seedy parts of a plant. In common usage, though, an herb (or in Britain, a herb) is any leafy plant material used for its flavor and aroma.

Herbs historically have been used not only in cooking but in mystical ceremonies and for healing. Today the market is flooded with "herbal remedies." For some reason, many people choose to believe that anything that is herbal is also "natural," and therefore healthful, but that is not always the case. ("Here, Mr. Socrates, drink this cup of hemlock tea. It's an all-natural herbal supplement.")

Two families of plants provide the lion's share of our culinary herbs. The mint family (Lamiaceae) provides us with basil, catnip, thyme, marjoram and rosemary, while the parsley family (Apiaceae) gives us anise, caraway, coriander cumin, dill and, yes, hemlock.

Nature's reason for offering us these fragrant plants is that most plants must reproduce by attracting insects to pollinate them. They therefore broadcast molecules of highly aromatic chemicals that act as air-traffic control beacons for bees and other flying insects. These molecules are small and light enough (molecular weights of less than 300-400) to float easily through the air.

Those same airborne chemicals are what we value so highly in the plants' leaves, because, depending on whom you ask, our sense of smell constitutes anywhere from 70 percent to 85 percent of what we perceive as flavor. Molecules of the aromatic chemicals enter our noses directly or through the backs of our mouths, where they are released as we eat.

These aromatic chemicals, so attractive to both bee and thee, are components of what are known as essential oils. Essential oils are quite volatile, meaning that they evaporate easily and waft into the air. They can be obtained in pure form by steam distillation: boiling the crushed plant material in water and condensing the mixed vapors of oil and water. Most essential oils can also be extracted into volatile organic solvents, which are then evaporated away.

"Essential oil" is an unfortunate name. An essential oil is not necessarily an oil in the chemical sense and may not even feel oily at all. Nor is it "essential" in the sense of being indispensable in some way. (Aromatherapy and cosmetic flacks take advantage of this misunderstanding by touting the "essential oils" in their products as if they were somehow imperatives for health and beauty.) The adjective "essential" in "essential oil" simply means that it represents the essence, the concentrated spirit, if you will, of the plant.

The Farm Surplus

Herb gardeners are often the victims of overproduction. Not being able to use more than a few gallons of pesto, for example, you might not be able to keep up with your crop of basil. Unfortunately, there is no federal price support program for window-box farmers, so as the growing season progresses you may be faced with the necessity of preserving your harvest by freezing or drying. Or, maybe you inexcusably forgot to plant borage or comfrey this year and must resort to buying dried versions of these essentials. In either case, you will want to know how to substitute the dried herb for the fresh one in cooking.

There can be no hard-and-fast rule, because herbs differ so much from one another. But the following considerations may give you a few clues. Remember that it's not the amounts of fresh or dried vegetable matter that count in flavoring, but the amounts of essential oils they contain.

The leaves of herbaceous plants are 80 percent to 90 percent water. At 80 percent water, 100 grams of whole leaf will contain 20 grams of dry matter, so that the dried herb is five times more potent. So in using dried herb, you would use one-fifth the amount of the fresh herb by weight. At 90 percent water, the fresh-to-dry factor is 10, so you would use one-tenth as much dried herb as fresh. All of this assumes, however, that only water, and no volatile oils, are lost in the drying process.

But the ratio of their volumes (teaspoons or tablespoons) -- and that's how we generally measure them in the kitchen -- will depend mostly on the forms of the fresh and dried herbs: whole leaves, withered leaves, minced pieces, powder, etc., and, of course, on the relative potencies of the individual samples.

Thus, short of taking your herbs to a laboratory and having them analyzed for their relative contents of essential oils, there can be no general rule of thumb for how much volume of dried herb to use instead of the fresh form.

Nevertheless, if you have to make the substitution of dried herb for fresh and can't wait 30 days for the essential oil analyses to come back, try using somewhere between one-quarter and one-half as much dried herb as the fresh kind. Of course, if you're substituting in the other direction, use between twice as much and four times as much fresh herb as dried herb. In most cases, you won't stray disastrously from the Yellow Brick Road.

Okay, so I lied. That's a rule of thumb.

Labelingo: Perspicacious reader Matt Williams of Centreville writes that the wrapper on a two-tub package of Yucatan Guacamole states that the entire package contains a total of 10 avocados. But a small graphic on each tub indicates that it contains 3.5 avocados. Will anyone who finds three homeless avocados please turn them in to the Saint Michael's Home for Abandoned Fruits? (Saint Michael is the patron saint of grocers.)

(Have you noticed any silly things on food labels? Send your Labelingo contributions, along with your name and town, to Food 101, Food Section, The Washington Post, 1150 15th Street, N.W., Washington, D.C., 20071 or to the e-mail address below.)

Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Cook: Kitchen Science Explained" (W.W. Norton, hardcover, $25.95). He can be reached at wolke@pitt.edu.