In each plant species, certain individual plants have more desirable qualities than others, from the viewpoint of human needs. These qualities include size and flavor of edible parts and attractive foliage. Plants are able to pass these qualities on, through their seeds, to their descendants.

Humans learned long ago, by trial and error, to select the seeds of desirable plants and thus improve, year after year, the quality of the species they cultivated. Today there are thousands of varieties of agricultural and garden plants.

Since the late 19th century, when the laws of heredity were discovered, plant improvement has become a science. Plant breeders have produced varieties that resist diseases, such as rot, smut, and wilt. Experimenters also are able to develop new varieties to fulfill special requirements. Examples are double-blossomed petunias, seedless grape-fruit, and early-maturing wheat. Desirable traits can be introduced into the genes of crop plants using the techniques of genetic engineering. .)