corn now grown in the United States and Europe is started annually from hybrid seed. Hybrid breeding has expanded in recent years, and hybrid varieties are now common in grain crops, vegetables, and many flower species. MutationOccasionally an individual plant shows an important change in one or more traits arising from a spontaneous mutatation. Usually a change in a single gene is involved. Most mutations are deleterious, bt occasionally one has a distinct advantage. The plant showing the mutation may be used directly as a variety, a common practice in apples and other fruits, or the new trait may be added to exisiting varieties through hybridization and backcrossing. Plant mutations caused by single-gene changes have found wide use in ornamentals, resulting in double-flowered forms, weeping stems, dwarfism or unusual. Mutations can be induced artifically by X rays or ultraviolet light.Doubling the number of chromosomes is another plant-breeding technique that has been useful in improving some flower and crop plants, sometimes producing forms with increased vigor and with larger leaves, flowers, and fruits....
