Many people like indoor plants, even people with large outdoor gardens. After all, they can talk to the plants without being interrupted. And think of the therapy the plants provide.

Plants appeal to all five senses, says Dr. Albert E. Griffiths, University of Rhode Island professor of plant and soil science, who has considered adding a course in garden therapy to the URI's curriculum. Griffiths contends that everyone, whether tired from a day's work, or physically, mentally or socially disabled, may find some degree of fulfillment through plants.

Longwood Gardens, in Kennett Square, Pa., recently made a substantial contribution for those who like to grow plants indoors. They prepared life-size exhibits of different balcony gardens designed specifically for apartment and townhouse dwellers. One exhibit is of a folding table and chairs surrounded by greenery, offering a pleasant place to eat. Plant hangers hold terra-cotta pots of ivy and Boston fern. Flue liners, cut to different heights, provide variable staging for other potted plants.

Another balcony garden features triangular planters filled with hardy groundcovers, with bulbs or annuals at the appropriate season, accented with a small dogwood. A bird feeder and bench complete the setting.

Another balcony resembles a porch or veranda. The planting is concentrated at the railings, leaving plenty of room for people. There are hanging baskets of flowering maple (Abutilon) and forget-me-not (Myosotis) and lush potted plants add richness of flowers and foliage.

Nearby windowsill gardens illustrated another aspect of apartment gardening. Two windows have sills lined with copper pebble trays filled with orchids and foliage plants. A second set of windows is equipped with adjustable shelves to hold potted vegetables and herbs. Other windows display collections of ivy and geraniums.

On the outdoor side of these windows is a series of colorful window boxes. Two are filled with hardy plant materials with year-around interest. Three boxes contain vegetables and herbs as well as primroses and pansies. The leafy vegetables were chosen for shape and color as well as for taste.

To help apartment gardeners learn more about balcony gardens, window gardening and indoor light gardening, a 23-page booklet is available.

Longwood Gardens is open all year, outdoors 9 a.m.-5 p.m., and the conservatories 10 a.m.-5 p.m. Admission is $2 for adults, $1 for children 6 to 14, and free for children under 6.

The idea for the balcony gardens came to Dr. Russell J. Seibert,, Longwood's director, when he and Mrs. Seibert toured Italy about a year ago. There they were impressed with the planted balconies that add delightful color to Mediterranean cityscapes.