Matthiola incana is one of several species of stock, and is sometimes called hoary stock. It is a common garden flower, available in a variety of colours.
Other varieties take longer to develop and are treated as biennials. These are often referred to as "Brompton stocks". In cool temperate regions they are generally sown in summer (June and July) to flower in the following spring. The extra trouble of overwintering the plants is compensated by the showy spring floral display. In hard winters there may be some mortality and a well-drained sheltered site suits them best.
Intermediate varieties (sometimes called "East Lothian" stocks as they originated in southern Scotland) may be treated either as annuals or biennials. If treated as annuals they give a fine late summer and autumn display.
Double-flowered stocks are prized by gardeners for their floral display but are sterile. They therefore have to be produced from the seed of single-flowered plants. The double-flowered form is caused by a recessive gene variant (allele) in the homozygous condition. Therefore, according to the Mendelian laws of genetics, heterozygous single-flowered stocks should "throw" one quarter doubles in their offspring and one third of the singles should be pure breeding singles incapable of throwing doubles.
Selection over the centuries has greatly improved these ratios, resulting in the so-called "ever-sporting" stocks, in which pure-breeding singles are absent and the proportion of doubles is one half or greater. The reason was first worked out by the Danish geneticist Øjvind Winge. In these varieties, the singleness allele is closely linked to a pollen-lethal gene. Thus the pollen (male) contribution to seed is always a doubleness allele, while the female contribution is either a doubleness or a singleness allele. The result of this linkage is that doubles and singles are produced in 50:50 ratios and there are no pure-breeding singles.
Furthermore, many modern strains produce doubles in even higher proportions: 60% or even 80%. This is due to generations of selection for further linked viability effects, producing higher mortality of heterozygous singles, relative to homozygous doubles.