The coffee plant and the gardenia are both members of the same large plant family, the Rubiaceae or Coffee family. And both are known and admired for their gleaming dark green foliage and fragrant flowers.

If your urge to grow food plants is strong, coffee is a fast-growing tree from which you may, after 4 or more years, harvest a few, very few, berries - not an economical supply for home consumption. But the coffee plant is beautiful in its own right as a houseplant.

You can buy a well-grown coffee Plant at many plant stores or you can grow your own from seed obtainable from flower and garden mail order seedsmen.

Coffea arabica is native to tropical Africa. Its leathery, dark-green, glossy leaves probably help it to endure the dry atmosphere as a houseplant. With careful pruning it will become an attractive decorative houseplant of four to six feet, or even to ceiling height.It must be 4 years old or older before it will bloom profusely. The small white flowers in spring and summer have the fragrance of jasmine: they are followed by green berries, which turn red, then ripen black. The berries must be cleaned, roasted and ground to make America's favorite brew.

Indoors give the coffee plant eastern or southern exposure in winter; shade it in summer. Feed it monthly at this time of year with any houseplant fertilizer. You can set it outdoors in full shade in the summer and feed it more often - every two weeks - for it will be growing vigorously.

It must have good soil drainage: the soil should never become soggy. Maintain the glossiness of the foliage by washing or wiping with a soft moist cloth or sponge.

The gardenia appears to have more appeal than coffee to most indoor gardeners, maybe because of its exotic blossoms. Gardenia jasminoides is native to China where it has been cultivated for thousands of years. As a houseplant it is generally difficult and unpredictable, requiring pampering, patience and preseverance. Some, however, might contradict that statement because it is true that some gardeners have succeeded with gardenias in their homes year after year with no apparent difficulty.

When a gardenai is brought home from the plant store, dark green and loaded with buds, it will probably drop buds and leaves because of the change from greenhouse growing conditions; this may even continue all through the first year. Discouraging, yes, but don't give up.

Bud drop is the chief gardenia problem, judging from questions of indoor gardeners. The causes of bud drop are sudden changes of temperature or humidity, cold drafts, soil too wet or soil allowed to dry out, a series of dark cloudy days in winter, too much fertilizer.

During the winter place the gardenia at the sunniest window available. Keep the soil barely moist, never soggy; never let it dry out. Mist the plant frequently and set it on a pebble tray to increase humidity around the plant.

The pebble tray should be deep enough to accommodate about an inch of pebbles or marble chips and half an ich of water.

Feed the plant monthly with any houseplant fertilizer to maintain its healthy growth.

The next summer rejuvenate the gardenia by setting it outdoors in light shade. First repot it in a container of about two inches greater diameter. Use a potting mix high in organic matter, such as peat; soil in almost never too acid for Gardenias. Plunge the pot to its rim in the soil to keep it cool and prevent turning over. Continue to feed it monthly and remember to water it.

Plants under 3 years old are the most prolific bloomers. As the plant ages, new plants can be grown by taking stem cuttings from it.

In order to form flower buds gardenias have very particular requirements. First is sunshine. Few flower buds will be formed in the shade. Second is night temperature. Buds will not form if night temperatures exceed 65 degrees. For a plant to blossom continuosly careful attention must be paid to a maintaining a night temperature in the 60-to 65-degree range with daytime temperatures 15 or more degrees higher. Even a few hot nights will stop formation of flower buds.

Two other popular indoor plants in the coffee family are Pentas lanceolata and Nertera granadensis, the bead plant. Besides coffee, another economic member of the family is Cinchona, a group of large South American trees. The bark of several species of Cinchona is the source of quinine used in the treatment of malaria. READERS' QUESTIONS

Mrs. L. Lefbowitz, Rockville, Md., writes: My Dieffenbachia produces new leaves which lean over and die before unfurling.

A. When leaves of Dieffenbachia droop and fall, examine the plant for mealy bugs. These sap-sucking pests appear as a white cottony mass at the point where a leaf joins the trunk of the plant. Use a cotton swab dipped in alcohol to remove the pests. Rinse with clear water.

When new growth seems to be hindered or existing leaves begin to curl, suspect Aphids. Aphids are minute insects, brown, black or greenish, which suck plant juices. Frequent washing with mild soapy water is the way to remove them from the plant. Rinse with clean water.

Either of these treatments may need to be repeated until no further generations of insects appear.