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Sweet Pea (Lathyrus odoratus) is a flowering plant in the genus Lathyrus in the family Fabaceae (legumes), native to the eastern Mediterranean region from Sicily east to Crete.

It is an annual climbing plant, growing to a height of 1-2 m where suitable support is available. The leaves are pinnate with two leaflets and a terminal tendril, which twines around supporting plants to help it climb. The flowers are purple, 2-3.5 cm broad, in the wild plant, larger and very variable in color in the many cultivars. Sweet peas have been cultivated since the 17th century and a vast number of cultivars are commercially available. Gardeners often grow them for their bright colors and the sweet fragrance that gives them their name.

Horticultural Development
The earliest recorded sweet pea in cultivation was a purple and maroon bicolor native to Sicily. In 1699 Sicilian monk Francisco Cupani sent seeds of this local plant to Dutch botanist Caspar Commelijn in Amsterdam, who published the first sweet pea illustration. Early sources believed sweet peas arrived in England simultaneously when Cupani also sent seed to a schoolmaster in Enfield; English botanist Leonard Plukenet collected flowers and seeds from the Enfield garden for his herbarium, which includes the oldest sweet pea specimens on record and is currently stored in the Natural History Museum in London.

Early writings about wild sweet peas also referred to a pink and white bi-colored variety, which was believed to originate in Ceylon. The Swedish botanist Carl Linneaus cited Ceylon as a possible origin, naming the species after it ("Lathyrus zeylandicus") in his Species Plantarum (1753). Twentieth century horticulture research, however, has discovered the purple and maroon flower from Sicily and southern Italy to be the most primitive and closest to the wild species. It is commercially available under the names "Cupani" or "Cupani Original". Until 1850, there were less than ten available colors of sweet peas, most likely sports of the original.

In the late 19th century, Scottish nurseryman Henry Eckford went to work on breeding new colors and forms of sweet peas, transforming them into the some of the most popular garden flowers of the Victorian era. His initial success and recognition came while serving as head gardener for the Earl of Radnor, raising new cultivars of pelargoniums and dahlias. After introducing the sweet pea cultivar "Bronze Prince", he was awarded a First Class Certificate by the Royal Horticultural Society, marking the start of his long association with the flower. In 1888 he set up his development and trial fields for Sweet Peas in the North Shropshire market town of Wem. By 1901 Eckford had introduced a total of 115 cultivars, out of total 264 cultivars grown at the time. Eckford's sweet peas and others in the same style were called "Grandifloras" for their relatively larger and more numerous flowers compared to the earliest types.

So great was their popularity that sweet peas became known as the "Queen of the Annuals" and in 1901 a Sweet Pea Bicentennial was held in London, during which the National Sweet Pea Society (UK) was born. The following year, a new sweet pea caused a sensation at the Society's first show. Silas Cole, head gardener of the Spencer estate, exhibited a sweet pea he called "Countess Spencer" with the dramatic characteristics of larger flowers and a waved petal form. The enthusiasm for flowers in this style eventually caused Spencer sweet peas to surpass the Grandifloras for most of the 20th century. In the last 20 years, however, breeders have revived interest in preserving old-fashioned Eckford types as well as developing new Grandifloras with the fragrance of early sweet peas. Other contemporary developments include continued efforts to produce a yellow sweet pea, work on dwarf and shorter non-vining types, and heat-resistant plants which can be grown in the hotter climates of the U.S.

Genetics
Gregor Mendel is today recognized as the "Father of Modern Genetics" for his work with the cross breeding of pea plants (Pisum sativum) with different characteristics, and sweet pea has been used in a similar way. The sweet pea is thus a model organism being used in early experimentations in genetics, particularly by the pioneer geneticist Reginald Punnett. It is highly suitable as a genetic subject because of its ability to self-pollinate and its easily observed Mendelian traits such as color, height and petal form. Many genetic principles were discovered or confirmed in sweet peas. It was used by Punnett in early studies of genetic linkage. Complementary factor inheritance was also elucidated in sweet pea, from the cross of two pure-breeding white strains which gave rise to a blue hybrid, the blue color requiring two genes, derived independently from the two white parents.

Like the blue rose, the yellow sweet pea remains elusive. In 1983, a hybrid between the yellow Lathyrus chloranthus and a Spencer sweet pea produced a mutation which breeders are continuing to cross with other Spencer sweet peas in hopes of producing a true yellow. The more recently discovered Lathyrus belinensis has also been used for the purposes of a yellow, and is still being used to produce new colors in sweet peas. If these efforts do not succeed, breeders are considering genetic engineering as a method of producing a true yellow.

Uses and Possible Diseases
Unlike most peas, the seeds of the sweet pea are poisonous as they contain several toxins that prevent the cross linking of collagen by inhibiting lysyl oxidase. Because of these toxins, ingestion of sweet peas by humans or livestock may lead to a disease known as odoratism, or sweet pea lathyrism. Symptoms are similar to those of scurvy and copper deficiency, which share the common feature of inhibiting proper formation of collagen fibrils.

Sweet peas are subject to a number of common garden diseases, such as powdery mildew, downy mildew, Anthracnose (U.S.A. only), and mosaic virus, among others.

Related Lathyrus species
Lathyrus odoratus is frequently confused with the related garden flower, but perennial species Lathyrus latifolius. The latter often goes by the common name "Sweet Pea", despite its unscented flowers. Although the two share similar climbing habits and size of flowers, Lathyrus latifolius (also known as Lathyrus grandiflorus and "Everlasting Pea") is a perennial species and is mostly unscented. Its pinnate leaves are narrower than the annual species, and the keel (the lower petals) is proportionately smaller than the standard (upper petal). It has naturalized outside of garden cultivation in some regions of the United States.