Dana Therrell, age 13, of Richmond, Va., for her question:

WHAT EXACTLY ARE AIR FERNS?

Maybe a few impossible pixie characters can survive on a diet of air. But surely all the ordinary plants and animals need much more to keep themselves going. Not so. The air ferns and a lot of other plants can do just this. This airy group includes an assortment of ferns and orchids, lichens, Spanish mosses and ordinary mosses.

Sometimes the instructions tell us that a certain fern or other plant can survive on air  with no soil or water. This may be true  with a few buts. Botanists class these specialists as epiphytes, meaning plants that live above the ground. They do indeed live on air, but they also need light and the air must be moist. It may also help when the air contains particles of dust or when dusty fragments gather around their roots.

Air ferns and other epiphytes establish themselves on boughs, roofs and even telephone poles  where a lofty perch provides air and light. Unlike parasitic plants, such as the pale green mistletoe, they do not probe in their roots to extract nourishing sap from a host plant. Epiphytes merely borrow a lofty perch, and they do no harm to a tree unless they become crowded enough to shut out its air and light.

When we visit the Deep South, we see festoons of Spanish moss trailing from the boughs of ancient trees. These are epiphytes, thriving on air and moisture, plus a few chemicals dissolved from specks of dust. In temperate climates certain mosses, lichens and small ferns live above the ground, surviving mainly on air, light and moisture.

All sorts of showy epiphytes nest in the tall trees of the tropical jungles. The ground level is shrouded in gloomy shade, and a high perch is necessary to find light and circulating air. Orchids and other jungle air plants often dangle thick porous roots to absorb moisture from the air. Many of these tropical types have gorgeous flowers.

Naturally the epiphytes have chlorophyll, which uses the energy of sunlight to manufacture basic plant food from moisture and carbon dioxide. Their leaves absorb moisture from the air, and certain tropical air plants have elaborate traps to store water. Some store several gallons and share their reservoirs with tree frogs and water insects.

The various epiphytes do not belong to a separate plant family. The Spanish moss belongs to the pineapple family. Others are special plants of the fern, moss or lichen groups. Some, such as certain orchids, can live either as air plants or thrive in peaty soil.