2. Viola Linnaeus, Sp. Pl. 2: 933. 1753; Gen. Pl. ed. 5, 402. 1754.
[Classic Latin name derived from Greek ion, violet] [Classic Latin name derived from Greek ion, violet]
R. John Little
Landon E. McKinney
Chrysion Spach; Crocion Nieuwland & Kaczmarek; Lophion Spach
Herbs, annual or perennial, caulescent or acaulescent, homophyllous (heterophyllous in V. palmata, V. sagittata, and V. septemloba), hairs concentrated or scattered throughout. Stems usually deciduous and withering at end of season, 0–5(–10+), erect to ascending, decumbent, or prostrate, simple, [woody], leafy; from horizontal or vertical, thick, fleshy or subligneous, shallow or deep-seated rhizome (caudex); or from narrow or thick rhizomes; or spreading, thin stolons; or slender taproots in annual species and seedlings; in caulescent species, short, axillary branches are sometimes present on main stems. Leaves alternate on caulescent species, simple (compound in V. beckwithii, V. douglasii, V. hallii, V. sheltonii, and V. trinervata), petiolate; caulescent plants with 0–11(–22) basal leaves per caudex, acaulescent plants with 1–12(–18) leaves per rhizome, prostrate to erect; stipules adnate to petiole or not, not leaflike (sometimes leaflike in V. lobata), unlobed, shorter than leaves (except V. arvensis, V. bicolor, and V. tricolor); blade not overlapping basally (except occasionally in V. blanda and V. rotundifolia), ovate, reniform, deltate, orbiculate, lanceolate, spatulate, or linear, adaxial surface not mottled (except in V. hastata and V. hirsutula). Inflorescences axillary (rarely umbellate in V. sagittata forma umbelliflora) from distal and proximal stem nodes in caulescent species or scapose from rhizomes or stolons in acaulescent species, 1(–3)[–5]-flowered; peduncles not jointed; bracteoles present. Flowers: sepals entire, equal or subequal, margins ciliate or eciliate, auriculate, auricles prominent or not; upper 2 and lateral 2 petals showy, 5+ mm, lowest petal showy, not narrowed at middle of limb; lateral petals and sometimes others bearded proximally with variously shaped hairs; style bearded or beardless; spur gibbous or elongated; stamens connivent, but distinct, forming cone around ovary, not adherent to style, dehiscing introrsely, lower 2 filaments spurred with nectary that protrudes into petal spur; cleistogamous flowers absent or produced in summer, apetalous or petals 0 or 2(–3) and scarcely developed, stamens 2, usually adherent to style. Capsules ovoid, ellipsoid, oblong, spherical, or subglobose, glabrous, puberulent, or tomentose, sometimes muriculate. Seeds 6–75, spherical or ovoid, glabrous, often arillate with elaiosome. x = 6, 7.
Species 400–600 (73 in the flora): nearly worldwide, temperate regions, also South America, Pacific Islands (Hawaii, Philippines, Taiwan).
The taxonomy of Viola is often considered difficult partly because of hybridization; more than 100 named hybrids occur in the flora area. Hybrids among the blue-flowered, acaulescent species in eastern North America and among other species are well known (E. Brainerd 1924; N. H. Russell and M. Cooperrider 1955; G. L. Stebbins et al. 1963; T. S. Cooperrider 1986; L. E. McKinney 1992; H. E. Ballard 1993, 1994; A. Haines 2011).
Other factors contribute to phenotypic variation. The dimensions of leaves and stems often increase substantially between early spring and late summer (D. Klaber 1976). Stem, leaf, and flower size vary with environmental factors such as aspect, light, available soil moisture, and other edaphic conditions (Klaber; L. E. McKinney 1992).
The number of recognized taxa and the ranks attributed to taxa vary among authors (E. Brainerd 1921; N. H. Russell 1965; L. E. McKinney 1992; H. E. Ballard 1994; N. L. Gil-Ad 1997; D. B. Ward 2006).
An effort to deal with the difficulties encountered, specifically with subsect. Boreali-Americanae (W. Becker) Gil-Ad, was made by N. L. Gil-Ad (1997, 1998). Much of his species; we do not share his conclusions about some species, such as Viola hirsutula and V. subsinuata.
We do not present an infrageneric classification for Viola here. Some species complexes have been identified (for example, V. adunca, canadensis, nuttallii, palustris, and purpurea). W. Becker (1925) provided the first infrageneric classification, which was largely followed by G. K. Brizicky (1961b), a scheme largely based on style morphology. For the most part, this classification continues to be followed (T. Marcussen and T. Karlsson 2010); some of the names used to refer to species complexes (for example, Nuttallianae) were published without rank and have been incorrectly assumed to be subsections [for example, J. Clausen (1964); Marcussen and Karlsson]. Chen Y. S. et al. (2007) replaced sections with subgenera.
Branching of stems in perennial caulescent North American Viola species is uncommon and has been documented in ten species: V. adunca, V. canadensis, V. canina, V. douglasii, V. glabella, V. pedunculata, V. pinetorum, V. purpurea, V. quercetorum, and V. walteri. Branching in these species involves the development of one or more relatively short, leafy, axillary shoots on one or more stems.
The three annual species in the flora area, V. arvensis, V. bicolor, and V. tricolor, commonly branch from the base of the main stem near or at the crown and from nodes higher on the stem.
The leaves of acaulescent species develop from the rhizome. In homophyllous plants, all leaf blades are lobed from early season through late season; the depth of sinuses depends somewhat on the age of the plant. In heterophyllous plants, the earliest leaf blades are not lobed; later-season blades are lobed. E. Brainerd (1910, 1921) placed great emphasis on these morphological distinctions to differentiate Viola taxa. Like Brainerd, L. E. McKinney (1992) found these differences to be reliable taxonomic characters.
Here, the distal portion of the style is called the style head. The shape, size, position of the stigmatic surface, and degree of bearding vary among species. Differences among style heads have been used in Viola classification (W. Becker 1925; J. Clausen 1929) and in keys. Here, we report whether the style head is bearded or beardless based on reports in the literature and our observations. Of the 30 acaulescent species in the flora area, one has a bearded style; of the 43 caulescent species, 35 are always bearded, three are bearded or beardless, and five are always beardless.
Of the 73 species of Viola in the flora area, 60 are known to produce cleistogamous flowers, nine do not, and the condition in four is unknown. In some species of acaulescent A. Mayers and E. Lord (1983, 1983b) reported that pollen grains in cleistogamous flowers of V. odorata germinate in the undehisced anther sacs; the pollen tubes then penetrate the sac and grow towards the stigma. In other groups, observations suggest that pollen is released in proximity to the recurved style (T. Marcussen and T. Karlsson 2010).
Substantial research has been conducted on Viola pollination including cleistogamous flowers (A. J. Beattie 1969, 1969b, 1971, 1972, 1974, 1976, 1978; Beattie and D. C. Culver 1979; A. C. Cortés-Palomec and H. E. Ballard 2006; T. M. Culley 2000, 2002; G. Davidse 1976; L. Freitas and M. Sazima 2003; C. M. Herrera 1990; A. Mayers and E. Lord 1983, 1983b). Pollinator rewards available in most Viola flowers include nectar and pollen.
Pollinators of violets include bumblebees, honeybees, solitary bees, syrphid flies, butterflies, skippers, hawkmoths, moths, and beeflies. Thrips have been reported in Viola flowers (M. S. Baker 1935) and while these insects are often observed with pollen attached to their bodies, there is currently no evidence they play a role in pollination of Viola. Due to the high frequency that thrips, aphids, and other minute insects with pollen on their bodies have been observed in Viola flowers, it seems probable that these insects sometimes effect pollination.
The three valves of Viola capsules usually are thick in perennial species and thin in annual species. The capsules of at least some species open relatively slowly, exposing the seeds. As the valves dry, they contract and squeeze the seeds causing them to be ejected (R. J. Little and G. Leiper 2012). Capsules that disperse seeds ballistically are usually on erect peduncles; capsules that passively release their seeds usually point downward (A. J. Beattie and N. Lyons 1975).
Most Viola seeds possess an outgrowth (elaiosome), or food body, of variable size that is often attractive to ants. S. Lengyel et al. (2010) estimated that over 70% of Violaceae species are myrmecochorous. Studies have been conducted on various aspects of myrmecochory in Viola (A. J. Beattie and N. Lyons 1975; R. Y. Berg 1975; D. C. Culver and Beattie 1978, 1980; Beattie and Culver 1981; G. Matlack 1994).
Violets of horticultural importance include Viola arvensis (field or wild pansy), V. odorata (English or sweet violet), V. tricolor (Johnny-jump-up), and V. ×wittrockiana Gams ex Nauenburg & Buttler (garden pansy). Over 120 species of Viola are grown as ornamentals (L. Watson and M. J. Dallwitz, http://delta-intkey.com).
E. Brainerd (1908) reported that Viola chinensis G. Don (= V. patrinii de Candolle ex Gingins) was established at the New York Botanical Garden, at a residence in the District of Columbia, and in his garden in Massachusetts. It appears not to have persisted; it was not mentioned by N. Taylor (1915). A. Haines (2011) noted that reports of V. chinensis in New England were based on misidentifications of V. japonica.
Native Americans in the United States and Canada used Viola species for drugs, dyes, and food (D. E. Moerman 1998). Medicinal uses included pain relief and treatment of ailments including colds and coughs and for dermatological, gastrointestinal, eye, heart, and respiratory problems. Leaves and stems of some species were used as vegetables, usually cooked. Some tribes are reported to have soaked the seeds of corn in an infusion of violet roots before planting to repel insects.
Mature plants are often needed for identification of violets. In preparing specimens of violets, care should be taken to record data on petal and spur colors, and presence and distribution of beards on lateral and other petals, or note if lacking.
Measurements of the lowest petal in the descriptions here include the spur.
When this treatment was being finalized, Viola calcicola R. A. McCauley and H. E. Ballard was described as new. Time constraints prevented it from being incorporated. Viola calcicola is acaulescent, heterophyllous, has short, vertical rhizomes, nearly white to purple corollas, and occurs only on limestone substrates. It is endemic to the Guadalupe Mountains of Texas and New Mexico.
SELECTED REFERENCES Baird, V. B. 1942. Wild Violets of North America. Berkeley and Los Angeles. Ballard, H. E. 1992. Systematics of Viola Section Viola in North America North of Mexico. M.S. thesis. Central Michigan University. Ballard, H. E., K. J. Sytsma, and R. R. Kowal. 1998. Shrinking the violets: Phylogenetic relationships of infrageneric groups in Viola (Violaceae) based on internal transcribed spacer DNA sequences. Syst. Bot. 23: 439–458. Beattie, A. J. and N. Lyons. 1975. Seed dispersal in Viola (Violaceae): Adaptations and strategies. Amer. J. Bot. 62: 714–722. Fabijan, D. M., J. G. Packer, and K. E. Denford. 1987. The taxonomy of the Viola nuttallii complex. Canad. J. Bot. 65: 2562–2580. Gil-Ad, N. L. 1997. Systematics of Viola L. subsection Boreali-Americanae. Boissiera 53: 1–130. Gil-Ad, N. L. 1998. The micro-morphologies of seed coats and petal trichomes of the taxa of Viola subsect. Boreali-Americanae (Violaceae) and their utility in discerning orthospecies from hybrids. Brittonia 50: 91–121. Klaber, D. 1976. Violets of the United States. South Brunswick, N.J. Marcussen, T. et al. 2012. Inferring species networks from gene trees in high-polyploid North American and Hawaiian violets (Viola, Violaceae). Syst. Biol. 61: 107–126. McKinney, L. E. 1992. A Taxonomic Revision of the Acaulescent Blue Violets (Viola) of North America. Fort Worth. [Sida Bot. Misc. 7.] Russell, N. H. 1965. Violets (Viola) of the central and eastern United States: An introductory survey. Sida 2: 1–113.
