If you have recently bought a dracaena, a philodendron, a Chinese evergreen, or some other favorite foliage plant, chances are that it came from Florida. According to the crop reporting service of th U.S. Department of Agriculture, most of the foliage plants grown in America originate in Florida, and in wholesale sales Florida ranks way ahead of California and Texas, two other leaders in production of green plants.

During the busy months of February, March and April more than a million plants leave Florida daily, mostly by truck, some by air. The indoor foliage plantsmen grow more than 450 different species of plants. Commercial growers in the vicinity of Apopka, Fla., devote most of their energy to small plants, such as philodendron, dracaena and schefflera. South Florida nurseries grow mostly larger plants like palms, Ficus and cane varieties.

A houseplant laboratory was established near Apopka in 1969. It is the only one in the world exclusively researching tropical foliage. Such work as soil composition, shade factors necessary to make plants thrive under home conditions, interactions between light intensities, developments in watering and fertilizers, and disease and pest control studies are a continuing program.

This research when applied to production of plants in specialized nurseries, makes it possible for the indoor gardener to obtain better plants in greater variety. The U.S. Departmetn of Agriculture estimates that the average American household now has eight to 10 plants. Some of the most confirmed indoor gardeners probably exceed this average.

Propagation of ornamental plants from small clusters of cells in test tubes is being used by a number of nurserymen and flower growers in the United States. In the production of ferns, for example, one small runner tip can produce 2,000 to 3,000 plants and saleable four-inch plants can be grown in eight to nine months. With conventional propagation techniques, 12 to 15 months may be required. Researchers refer to this technique as meristem or tissue culture. It is an extremely complicated technique performed under rigidly controlled conditions. Presently there are 20 commercial laboratories in California producing plants by this method. Some of the plants you have been buying, especially orchids and ferns, may have been propagated in this way. READERS' QUESTIONS

Mrs. L. Leibowitz, 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 clear water.