John A. Beglin, Alexandria: "I have an enormous spider plant which has produced little plantlets profusely for over a year. About five or six inches above one of the plantlets there is what appears to be a seed pod. Is it in fact a seed pod? Is it possible to produce additional plants from seeds."

A. The spider plant does sometimes set seed along the stem on which the small white flowers have bloomed. Collect the seed and sow in a shallow container of growing mix; keep it warm (70 degrees) and slightly moist until seeds sprout. Then give seedlings bright light and feed them monthly with balanced houseplant fertilizer diluted to one-fourth the strength recommended by the manufacturer. When the seedlings are large enough to handle, transplant them to individual pots or several to a pot as you prefer.

Hope W. Piper, Oxon Hill, Md.: "Why does a white cottony "stuff" appear over the soil under my plants? It happens even though I have just repotted with new, supposedly sterile soil. It affects some plant soil and not others.

A. The growth you have described is the common mold which grows on the soil surface, not on the plant itself. It usually does not harm the plant, but may draw nutrients from the soil. Scrape as much of the mold off the soil as you can mix any remainder thoroughly into the soil. A plastic picnic fork is a useful tool, or use a pointed plant stake or toothpick.

The mold is a sign that you have been over-watering and that air circulation is inadequate.

Provide some ventilation or air movement and reduce excess humidity; spacing plants so as not to be crowded will help.

This White cottony growth which appears on the soil is a fungus which is form of plant life without chlorophyll. Fungi are parasites, taking their food from organic matter. Fungio reproduce by spores carried in the air. When spores settle in cool, moist situations where air is stagnant, they grow rapidly. Fungi in various forms are a serious threat to plants as they are the single tr most important source of plant disease. Horticultural Short Courses

Care and propagation of foliage houseplants will be the subjects of the U.S. Botanic Garden. Dates and Saturday through Jan. 14 - Ferns, palms, Norfolk Island pine, banana, spider plant, Schefflera, and Ficus species.

Jan. 21 through Jan. 28 - Rex begonias, grape ivy, croton, coffee, velvet plant, wandering jew, Swedish ivy, German ivy, Peperomia.

Feb. 4 through Feb. 11 - Chinese evergreen, Marantar species (Prayer Plant), coleus, Dieffenbachia, Aralia, Dracaena species, Philodendron species, Phothos.

All courses are free and open to the public. Advance registration is not requried. Each class lasts about an hour and will be repeated at 10 a.m., noon and 2 p.m. during the weeks indicated.

Classes are at the Botanic Garden Consevatory, First St. and Maryland Ave. S.W. For additional information, call 225-7099.