A reader in Allentown, Pa., wrote to ask questions about "air fern." Is it really a live plant? Does it come from the ocean; if so, why doesn't it have to be in water? If it is a plant, how tall does it get, how long will it live, and why doesn't it need water or fertilizer?

The "air fern" is not a plant. The filmy green plume sold as air fern or neptune plant is the dried and dyed skeleton of a filamentous aquatic animal indentifed by soologists as an ocean-dwelling "moss animal," Bugula. It is found growing under water on rocks or floating pieces of wood off the English or Norman coast. It is not a fern; it is not a plant. It will not grow.

A reader asks when can she plant the spadix of her Peperomia.

Peperomias are usually propagated by leaf or stem cuttings. Unless she is sure that there are fertile seeds, which she can remove from the spadix (flower stalk) for planting, she will have better luck propagating her plant from cuttings. Seed of one species, Peperomia maculosa, is listed in the catalog of Geo. W. Park Seed Co., Greenwood, S.C. 29647.

Mrs. John Edwards of Springfield, Va., writes: What are the-chances of successfuly air-layering a young, tall, skinny Ming Aralia? Also, what can be done about minute flying insects that live in baskets of ferns lined with damp sphagnum moss?

According to James Underwood Crocket, author of Foliage House Plants (Time-Life Encyclopedia, 1972, $7.95), the propagation of Aralias is neither easy nor impossible. Crockett suggests growing Aralias from stem cuttings set in coarse damp sand. Cover the cutting with a plastic bag or glass jar, place it our of the sun and mist frequently. When the cutting is rooted, pot it in any good growing mix.

If you are experienced with air layering there is no reason not to try it, but cuttings are easier.

The small black flying insects are fungus gnats, frequently a sign of overwatering. Allowing the soil surface to dry out between waterings is probably the best control. Where drying out is not feasible, plants can be watered with a solution of one teaspoon chlorine bleach in one quart of water. Another way of coping with these pests is Black Leaf 40 at the rate of one teaspoon to two quarts of water with a little soap added. Pour a little of this solution on the soil.

If you or your plants are not averse to chemical pesticides, a soil drench of Diazinon may be used. When using pesticides, always be careful to follow directions given on the container. Pesticides are poisonous.