A terrarium is a miniature world, an indoor garden in a sealed, transparent container. Here's how one works: The plants' leaves release water vapor, which condenses on the glass, trickles down into the soil and re-waters the plants - all in a cycle that seldom needs to be supplemented if the container is airtight.

When you have all the right conditions - a good soil mixture, the correct amounts of light and moisture and the appropriate plants - a terrarium is a self-contained ecosystem.A London surgeon named Nathaniel Ward invented the terrarium by accident in 1829. Trying to hatch a moth chrysalis, he placed a cocoon and some soil into a glass jar with a lid. Much to his surprise, a fern and some grass sprang up in the jar, even though he had added no water or fresh air.

Ward was transfixed by his discovery. He went on to experiment with hundreds of fern species, and he developed the principles behind what became known as the Wardian case. Wardian cases became quite fashionable in Victorian times as showcases for exotic plant species gathered from all over the world.

Building a terrarium

A terrarium makes a lush, beautiful addition to a room. You can make one at home from a few simple materials.

First, choose your container. Aquariums, fishbowls, candy jars, laboratory jars and bottles are all acceptable, though beginners should choose wide-necked containers to make the terrarium easier to plant and maintain. A clear glass container is best since colored glass will not admit as much light.

You'll need a tight-fitting clear glass lid (not Plexiglas, which can warp); have one cut for you by a glazier if necessary. You'll also need gravel, such as aquarium gravel or small river stones, for drainage at the bottom of the container. Add a 1/2 inch to 1 inch layer, depending on the size of your container.

Next, it's good to include a light layer of finely ground charcoal to keep bacteria at bay. Now add soil mix that's two parts good, dark potting soil, two parts peat and one part builder's sand. The soil layer should be 2 to 3 inches deep; if you like, mound up soil higher in the center to make your terrarium seem more like a natural landscape.

When the drainage and soil layers are in place, add your plants (see below for advice on choosing them). Shake their roots free of as much excess soil as you can, place each plant in an indentation in the soil, and firm the soil around it.

The number of plants depends on the size of your container. Don't overcrowd, and make sure you leave ample space for the plants to grow. Place larger or faster-growing plants in the center of your container if your terrarium will be viewed from all sides, in the back if it will be set against a wall. If you plan to put the terrarium against a wall, consider backing the container with a mirror to reflect the plants inside.

Maintaining it

Once you've created your terrarium, you'll find its maintenance to be simple. Water it lightly to begin with - overwatering is the most common mistake people make at the start. If you ever notice a great deal of condensation clouding the sides of your container, remove the lid for a day to allow the excess moisture to evaporate. On the other hand, if the soil appears parched, the water may be evap-o-rating from your container. Add a bit more water (a spray mister is a useful tool for this), and give the terrarium a new, airtight lid.

Keep your terrarium in a spot with diffused light since direct sunlight can cause temperatures inside the enclosure to rise too high and can scorch the plants. Artificial lighting, such as a balance of cool and warm florescent lights, can also suffice. Prune away leaves that are touching the sides of the container or that are dead or turning yellow. Remove plants that aren't doing well or have outgrown their space.

Most common houseplants are tropical in origin and will work in a terrarium; avoid cactuses and other plants that prefer low humidity. Miniature begonias and palms, ferns, philodendrons, prayer plants and moss are all good candidates.

You might choose plants and objects that lend your terrarium a theme. Create a Japanese garden with stones, moss and miniature trees such as Serrissa foetida "Flore pleno" and Malpigha coccigera (miniature holly).

Or collect mosses, ferns, sticks and stones from nature (be sure to get permission if the land is not your own) and plant them in fresh soil for a landscape that resembles a forest floor. Often, mosses will contain seeds from other plants that will sprout into wonderful surprises.