Viola (plant)

Viola, commonly called violets, is the largest genus in the flowering plant family Violaceae, with over 680 species. Most of these species grow in the temperate Northern Hemisphere, but some are also found in places as scattered as Hawaii, Australasia, and the Andes. The plants can be perennial or annual, and a few are small shrubs. Many species, varieties, and cultivars are raised in gardens for their showy flowers. In gardening, the term "pansy" usually refers to the large-flowered, multi-colored cultivars grown annually or biennially from seed and used widely in bedding.

The plants can be herbs, shrubs, or very rarely small trees. In stemless (acaulescent) types, the leaves and flowers seem to sprout directly from the ground; others have short stems with leaves and flowers growing from the leaf axils. Leaves are typically heart-shaped or kidney-shaped with scalloped edges, though some are linear or palmate. All leaves are arranged alternately, and stemless species form basal rosettes. Every leaf has stipules, which are often leaf-like.

Most Viola flowers are strongly zygomorphic (bilaterally symmetrical) and grow singly, though occasionally they form clusters (cymes). Each flower has five petals: four are upswept or fan-shaped (two per side), and one broad, lobed lower petal points downward. This lower petal may be slightly or much shorter than the others and is not strongly distinct. The shape and placement of petals help define species—for instance, some have a spur on each petal, while most have a spur only on the lower petal. The spur can be barely projecting or very long, as in *Viola rostrata*. Solitary flowers grow on long stalks with a pair of small bracts (bracteoles). The five sepals persist after blooming, and in some species they enlarge afterward. The corolla ranges from white to yellow, orange, or various shades of blue and violet, often multicolored with blue and yellow, and may or may not have a yellow throat.

The flowers have five free stamens with short, free filaments pressed against the ovary. Each stamen has a large, entire, oblong to ovate dorsal connective appendage. Only the lower two stamens have nectary spurs inserted into the lowest petal’s spur or pouch. The styles are threadlike or club-shaped, thickened at the tip, and can be globe-shaped or beaked. The stigmas are head-like, narrowed, or often beaked. The ovary is superior, with one cell containing three placentae and many ovules.

After flowering, thick-walled fruit capsules develop, with few to many seeds per carpel. These capsules split open along three valves. When dry, they can eject seeds with considerable force, sometimes several meters. The nut-like seeds are obovoid to globe-shaped, typically with a specialized outgrowth (aril), straight embryos, flat cotyledons, and soft, oily endosperm.

A notable feature of some *Viola* species is the elusive scent of their flowers. Along with terpenes, a major scent component is a ketone called ionone, which temporarily desensitizes the nose’s receptors, so no further scent is detected from the flower until the nerves recover.

The genus was first formally described by Carl Linnaeus in 1753 with 19 species, and it bears his botanical authority, L. When Jussieu established the hierarchical system of families in 1789, he placed *Viola* in the Cisti (rock roses), though by 1811 he suggested separating it. In 1802, Batsch had already created a separate family, Violariae, based on *Viola* as the type genus, with seven other genera. Although some authors (like Bentham and Hooker in 1862) continued using Violariae (as Violarieae), most adopted the name Violaceae, first proposed by de Lamarck and de Candolle in 1805, and later by Gingins (1823) and Saint-Hilaire (1824). De Candolle also used Violarieae in his 1824 Prodromus.

*Viola* is one of about 25 genera and roughly 600 species in the eudicot family Violaceae, which is divided into subfamilies and tribes. While most genera are monotypic, *Viola* is very large, with 500 to 600 species. It was historically placed in subfamily Violoideae, tribe Violeae, but these divisions have been shown to be artificial and not monophyletic. Molecular phylogenetic studies place *Viola* in Clade I of the family, along with *Schweiggeria*, *Noisettia*, and *Allexis*; *Schweiggeria* and *Noisettia* are monotypic and form a sister group to *Viola*.

The genus has traditionally been divided into sections. One early treatment by Gingins (1823) used stigma morphology to define five sections (Nomimium, Dischidium, Chamaemelanium, Melanium, Leptidium). Wilhelm Becker’s extensive studies, culminating in his 1925 conspectus, resulted in 14 sections and many infrasectional groups, the largest being section *Viola* with 17 subsections. Series were also described. Some authors preferred subgenera instead. Later treatments by Gershoy (1934) and Clausen (1964) used subsections and series, all based on morphology. However, molecular phylogenetic studies (e.g., Ballard et al., 1998) have shown that many of these traditional divisions are not monophyletic, largely due to a high degree of hybridization.