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Viola (plant)

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Viola
Viola reichenbachiana
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Rosids
Order: Malpighiales
Family: Violaceae
Subfamily: Violoideae
Tribe: Violeae
Genus: Viola
L.
Species

List of Viola species

Viola tricolor

Viola is a genus of flowering plants in the violet family Violaceae. It is the largest genus in the family, containing between 525 and 600 species.[1][2] Most species are found in the temperate Northern Hemisphere; however, some are also found in widely divergent areas such as Hawaii, Australasia, and the Andes.

Some Viola species are perennial plants, some are annual plants, and a few are small shrubs. Many species, varieties and cultivars are grown in gardens for their ornamental flowers. In horticulture the term pansy is normally used for those multi-colored, large-flowered cultivars which are raised annually or biennially from seed and used extensively in bedding. The terms viola and violet are normally reserved for small-flowered annuals or perennials, including the wild species.[3][4]

Description

Viola typically have heart-shaped, scalloped leaves, though a number have palmate leaves or other shapes. The vast majority of Viola species are herbaceous, and a substantial number are acaulescent in habit - meaning they lack any noticeable stems and the foliage and flowers appear to rise from the ground; the remaining species have short stems with foliage and flowers produced in the axils of the leaves. The simple leaves of plants with either habit are arranged alternately; the acaulescent species produce basal rosettes. Plants always have leaves with stipules that are often leaf-like.

Opened seed capsule of Field pansy, showing the seeds
Viola cultivar showing the large round flowers and the novel coloration that has been achieved through breeding.

The flowers of the vast majority of the species are zygomorphic with bilateral symmetry. The flowers are formed from five petals; four are upswept or fan-shaped petals with two per side, and there is one broad, lobed lower petal pointing downward. The shape of the petals and placement defines many species, for example, some species have a "spur" on the end of each petal while most have a spur on the lower petal.

Solitary flowers end long stalks with a pair of bracteoles. The flowers have five sepals that persist after blooming, and in some species the sepals enlarge after blooming. The flowers have five free stamens with short filaments that are oppressed against the ovary, only the lower two stamens have nectary spurs that are inserted on the lowest petal into the spur or a pouch. The flower styles are thickened near the top and the stigmas are head-like, narrowed or often beaked. The flowers have a superior ovary with one cell, which has three placentae, containing many ovules.

Viola are most often spring blooming with chasmogamous flowers with well-developed petals pollinated by insects. Many species also produce self-pollinated cleistogamous flowers in summer and autumn that do not open and lack petals.[5] In some species the showy chasmogamous flowers are infertile (e.g.,Viola papilionacea).[6]

After flowering, fruit capsules are produced that split open by way of three valves.[7] On drying, the capsules may eject seeds with considerable force to distances of several meters.[8] The nutlike seeds have straight embryos, flat cotyledons, and soft fleshy endosperm that is oily.[9] The seeds of some species have elaiosomes and are dispersed by ants.[10]

Flower colors vary in the genus, ranging from violet, through various shades of blue, yellow, white, and cream, whilst some types are bicolored, often blue and yellow. Flowering is often profuse, and may last for much of the spring and summer.

One quirk of some Viola is the elusive scent of their flowers; along with terpenes, a major component of the scent is a ketone compound called ionone, which temporarily desensitizes the receptors of the nose, thus preventing any further scent being detected from the flower until the nerves recover.[citation needed]

Selected species

For a more comprehensive list, see List of Viola species.
White-and-purple form of Viola sororia

Note: Neither Saintpaulia ("African violets") nor Erythronium dens-canis ("dogtooth violets") are related to the true Viola.

The genus includes dog violets, a group of scentless species which are the most common Viola in many areas, sweet violet (Viola odorata) (named from its sweet scent), and many other species whose common name includes the word "violet". Several species are known as pansies, including the yellow pansy (Viola pedunculata) of the Pacific coast.

Common blue violet Viola sororia is the state flower of Wisconsin, Rhode Island, Illinois, and New Jersey.

Australia is home to a number of Viola species, including Viola hederacea, Viola betonicifolia and Viola banksii, first collected by Joseph Banks and Daniel Solander on the Cook voyage to Botany Bay.

Fossil record

One fossil seed of †Viola rimosa has been extracted from borehole samples of the Middle Miocene fresh water deposits in Nowy Sacz Basin, West Carpathians, Poland.[11]

Genetics

Habitat fragmentation has been shown to be minimally impactful to the genetic diversity and gene flow of the North American woodland violet Viola pubescens.[12] This may be partially attributed to the ability of Viola pubescens to continue to persist within a largely agricultural matrix.[13] This trend of unexpectedly high genetic diversity is also observed in Viola palmensis, a Canary Island endemic known only from a 15 square kilometer range on La palma island.[14] High levels of genetic diversity within these species indicate that these plants are outcrossing, even though many violet species can produce many clonal offspring throughout the year via cleistogamous flowers. Plants that produce copious amounts of clonal seeds from cleistogmaous flowers often experience increased levels of inbreeding.[15] These reportedly high rates of outcrossing and genetic diversity indicate that these violets are strong competitors for pollinators during the early spring when they are in bloom and that those pollinators can travel considerable distances between often fragmented populations.

Ecology

Viola species are used as food plants by the larvae of some Lepidoptera species, including the giant leopard moth, large yellow underwing, lesser broad-bordered yellow underwing, high brown fritillary, small pearl-bordered fritillary, pearl-bordered fritillary, regal fritillary, cardinal, and Setaceous Hebrew character. The larvae of many fritilary butterfly species use violets as an obligate host plant, although these butterflies do not always ovaposit directly onto violets.[16] While the ecology of this genera is extremely diverse, violets are mainly pollinated by members within the orders diptera and hymenoptera. [17] Showy flowers are produced in early spring and clonal cleistogamous flowers are produced from late spring until the end of the growing season under favorable conditions. Cleistogamy allows plants to produce offspring year round and have more chances for establishment.[18] This system is especially important in violets, as these plants are often weak competitors for pollination due to their small size.

Many violet species exhibit two modes of seed dispersal. Once seed capsules have matured, seeds are dispelled around the plant through explosive dehiscence.[19] Viola pedata seeds have been reported being dispersed distances of up to 5 meters away from the parent plant.[19] Often, seeds are then further dispersed by ants through a process called myrmecochory.[19] Violets whose seeds are dispersed this way have specialized structures on the exterior of the seeds called elaiosomes. This interaction allows violet seed to germinate and establish in a protected, stable environment.[20]

Many violet seeds exhibit physiological dormancy and require some period of cold stratification to induce germination under ex situ conditions.[21] Rates of germination are often quite poor, especially when seeds are stored for extended periods of time.[22] In North American habitat restoration, native violets are in high demand due to their relationship with the aforementioned fritillary butterflies.[23]

Violet species occupy a diverse array of habitats, from bogs (Viola lanceolata) to dry hill prairies (Viola pedata) to woodland understories (Viola labradorica). While many of these species are indicators of high quality habitat, some violets are capable of thriving in a human altered landscape. Two species of zinc violet (Viola calaminaria and Viola guestphalica) are capable of living in soils severely contaminated with heavy metals.[24] Many violets form relationships with Arbuscular mycorrhizal fungi and in the case of the zinc violets, this relationship allows these plants to tolerate such highly contaminated soils.

Horticultural uses

Species and cultivars

Cultivars of Viola cornuta, Viola cucullata, and Viola odorata, are commonly grown from seed. Other species often grown include Viola labradorica, Viola pedata, and Viola rotundifolia.[25]

The modern garden pansy (V. × wittrockiana) is a plant of complex hybrid origin involving at least three species, V. tricolor (wild pansy or heartsease), V. altaica, and V. lutea (mountain pansy).[26] The hybrid horned pansy (V. × williamsii) originates from hybridization involving garden pansy and Viola cornuta.[27]

Bedding plants

In 2005 in the United States, Viola cultivars (including pansies) were one of the top three bedding plant crops and 111 million dollars worth of flats of Viola were produced for the bedding flower market.[28] Pansies and violas used for bedding are generally raised from seed, and F1 hybrid seed strains have been developed which produce compact plants of reasonably consistent flower coloring and appearance. Bedding plants are usually discarded after one growing season.

Perennial cultivars

Selection of cultivated violas at the BBC Gardeners' World show, in June 2011.

There are hundreds of perennial viola and violetta cultivars; many of these do not breed true from seed and therefore have to be propagated from cuttings. Violettas can be distinguished from violas by the lack of ray markings on their petals.[3] The following cultivars, of mixed or uncertain parentage, have gained the Royal Horticultural Society's Award of Garden Merit:-[29]

Other popular examples include:[35]

  • 'Ardross Gem' (viola)
  • 'Blackjack'
  • 'Buttercup' (violetta)
  • 'Columbine' (viola)
  • 'Dawn' (violetta)
  • 'Etain' (viola)
  • 'Irish Molly' (viola)
  • 'Jackanapes' (viola)
  • 'Maggie Mott' (viola)
  • 'Martin' (viola)
  • 'Molly Sanderson' (viola)
  • 'Rebecca' (violetta)
  • 'Vita' (viola)
  • 'Zoe' (violetta)

Other uses

Culinary

When newly opened, Viola flowers may be used to decorate salads or in stuffings for poultry or fish. Soufflés, cream, and similar desserts can be flavoured with essence of Viola flowers. The young leaves are edible raw or cooked as a somewhat bland leaf vegetable. The flowers and leaves of the cultivar 'Rebecca', one of the Violetta violets, have a distinct vanilla flavor with hints of wintergreen. The pungent perfume of some varieties of V. odorata adds inimitable sweetness to desserts, fruit salads, and teas while the mild pea flavor of V. tricolor combines equally well with sweet or savory foods, like grilled meats and steamed vegetables. The heart-shaped leaves of V. odorata provide a free source of greens throughout a long growing season.[36]

A candied violet or crystallized violet is a flower, usually of Viola odorata, preserved by a coating of egg white and crystallised sugar. Alternatively, hot syrup is poured over the fresh flower (or the flower is immersed in the syrup) and stirred until the sugar recrystallizes and has dried. This method is still used for rose petals and was applied to orange flowers in the past (when almonds or orange peel are treated this way they are called pralines). Candied violets are still made commercially in Toulouse, France, where they are known as violettes de Toulouse. They are used as decorating or included in aromatic desserts.

The French are also known for their violet syrup, most commonly made from an extract of violets. In the United States, this French violet syrup is used to make violet scones and marshmallows.

Viola essence flavours the liqueurs Creme Yvette, Creme de Violette, and Parfait d'Amour. It is also used in Parma Violets confectionery.

Medicinal

Many Viola species contain antioxidants called anthocyanins. Fourteen anthocyanins from V. yedoensis and V. prionantha have been identified. Some anthocyanins show strong antioxidant activities.[37] Most violas tested and many other plants of the family Violaceae contain cyclotides,[38][39][40] which have a diverse range of in vitro biological activities when isolated from the plant, including uterotonic, anti-HIV, antimicrobial, and insecticidal activities.[41] Viola canescens, a species from India, exhibited in vitro activity against Trypanosoma cruzi.[42]

Viola has been evaluated in different clinical indications in human studies. A double blind clinical trial showed that the adjuvant use of Viola odorata syrup with short-acting β-agonists can improve the cough suppression in children with asthma.[43][44][45][46] In another study intranasal administration of Viola odorata extract oil showed to be effective in patients with insomnia.[44][47] Topical use of an herbal formulation containing Viola tricolor extract also showed promising effects in patients with mild-to-moderate atopic dermatitis.[48]

Perfume

Viola odorata is used as a source for scents in the perfume industry. Violet is known to have a 'flirty' scent as its fragrance comes and goes. Ionone is present in the flowers, which turns off the ability for humans to smell the fragrant compound for moments at a time.[49]

Cultural associations

Birth

Violet is the traditional birth flower for February in English tradition.[50]

Geographical territories

In the United States, the violet is state flower of Illinois[51] and Rhode Island.[52] Viola sororia was declared the state flower of New Jersey [53] and Wisconsin.[54][55] In Canada, the Viola cucullata is the provincial flower of New Brunswick adopted in 1936[56] In the United Kingdom, Viola riviniana is the county flower of Lincolnshire.[57]

Lesbians

Violets became symbolically associated with lesbian love.[58][59] This connection originates from fragments of a poem by Sappho about a lost love, in which she describes her as "Close by my side you put around yourself [many wreaths] of violets and roses."[60] In another poem, Sappho describes her lost love as wearing "violet tiaras, braided rosebuds, dill and crocus twined around" her neck.[61] In 1926, one of the first plays to involve a lesbian relationship, La Prisonnière by Édouard Bourdet, used a bouquet of violets to signify lesbian love.[62][63]

Tributes

Violets, and badges depicting them,[64][65] were sold in fund-raising efforts in Australia and New Zealand on and around Violet Day[66] in commemoration of the lost soldiers of World War I.[67]

See also

References

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  2. ^ Zhou, J. S.; et al. (2008). "Viola nanlingensis (Violaceae), a new species from Guangdong, southern China" (PDF). Annales Botanici Fennici. 45 (3): 233–36. doi:10.5735/085.045.0312. Archived (PDF) from the original on 2014-02-20.
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