During the early twentieth century, sweet peas briefly held centre stage in the unfolding drama of plant genetics. Some of the founding fathers of genetics, e.g., William Bateson and Reginald Punnett, became seduced by sweet peas, whilst professional breeders saw money to be made.
Sweet peas, native to Sicily and Sardinia, were first mentioned by the Franciscan monk Francisco Cupani in the Hortus Catholicus (1696). By the early eighteenth century, Robert Uvedale was growing them in Enfield, Jacob Bobart was growing them in Oxford and Jan Commelin had published an illustration and description of the plants growing in Amsterdam; all apparently from seed provided by Cupani.
Cupani's plants were scramblers with small, short-stalked, sweet-scented, bicoloured, paired flowers. Whilst the peas were floriferous, they attracted little attention from eighteenth-century gardeners but nurserymen, such as Robert Furber, were offering sweet peas for sale as early as 1730. Indeed, until the mid-1800s there were only five variants cultivated; Cupani's wild type and types with white, 'black', red or pink and white flowers. As the century progressed, gardeners began to cross the naturally selfed flowers and discovered it was possible to reveal numerous other variants, such as dwarves, cretins, hoods and picotees.
Finally, in 1900, sweet pea breeding stopped marking time and a variant emerged, on at least four separate occasions, from crosses involving the large, pink-flowered mutant 'Prima Donna'. The new variant had large flowers with frilly standard petals, which eventually produced the Spencer types. Despite the Spencer types being less scented than the wild type they transformed the sweet pea world. In 1911, the craze was such that the Daily Mail offered an extraordinary prize of £1000 for the best bunch of sweet peas. There were approximately 38,000 entries and third and first prizes went to a Scottish minister and his wife, respectively. Of the thousands of sweet pea cultivars bred during the twentieth century, the fickleness of the market place operated to ensure only the most popular survived; the rest went extinct.
Two important genetic phenomena were first demonstrated in sweet peas: genetic complementation and linkage among genes. Within 20 years of sweet peas being introduced into cultivation, white-flowered plants appeared. Punnett observed that if certain white-flowered types were crossed with each other they produced purple-flowered offspring. When these offspring were selfed, the progeny comprised white-flowered and purple-flowered plants. The explanation was that two complementary genes were needed for pigment production.
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Rice G (2002) The sweet pea book. B.T. Batsford.