OUBLE Matthiola flowers are completely sterile, since they consist of D masses of indefinitely replicated petals. Double-flowered plants can be obtained only from seed of singles that are heterozygous ( S s ) for doubleness, which is recessive to singleness. Any ordinary (diploid) floricultural variety consists of slightly more than 50 percent sterile doubles and slightly fewer fertile singles ( SAUNDERS 191 1 ) . The single line is kept permanently heterozygous for doubleness (eversporting or double-thrower) by a pollen lethal, 1, which sterilizes the single-carrying or SI pollen of Sl/sL plants and slightly reduces the viability of the S1 embryo sacs. This lethal is occasionally lost, in some races at least (GOLD-
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