1911 Encyclopædia Britannica/Violet
|←Viola||1911 Encyclopædia Britannica, Volume 28
|See also Viola (plant) on Wikipedia; and our 1911 Encyclopædia Britannica disclaimer.|
VIOLET. The violets comprise a large botanical genus (Viola) — in which more than 200 species have been described — found principally in temperate or mountain regions of the northern hemisphere; they also occur in mountainous districts of South America and South and Tropical Africa, while a few are found in Australasia. The species are mostly low-growing herbs with alternate leaves provided with large leafy stipules (fig. 1). The flowers, which are solitary, or rarely in pairs, at the end of slender axillary flower-stalks, are very irregular in form, with five sepals prolonged at the base, and five petals, the lowest one larger than the others and with a spur, in which collects the honey secreted by the spurs of the two adjoining stamens. The five anthers are remarkable for the coloured processes which extend beyond the anther cells and form a sort of cone around the style (fig. 2). The ovary is superior and one-celled, with three parietal placentas and numerous ovules; it bears a single style, which ends in a dilated or hood-like stigma (fig. 3). The fruit is a capsule bursting loculicidally, i.e. through the centre of each of the three valves. By the contraction of the valves the small smooth seeds, which form a row down the centre, are shot out to some little distance from the parent plant. The irregular construction of the flower is connected with fertilization by insect agency. To reach the honey in the spur of the flower, the insect must thrust its proboscis into the flower close under the globular head of the stigma. This lies in the anterior part of a groove fringed with hairs on the inferior petal. The anthers shed their pollen into this groove, either of themselves or when the pistil is shaken by the insertion of the bee's proboscis. The proboscis, passing down this groove to the spur, becomes dusted with pollen; as it is drawn back, it presses up the lip-like valve of the stigma so that no pollen can enter the stigmatic chamber; but as it enters the next flower it leaves some pollen on the upper surface of the valve, and thus cross-fertilization is effected. In the sweet violet, V. odorata and other species, inconspicuous permanently closed or “cleistogamic” flowers (fig. 4) occur of a greenish colour, so that they offer no attractions to insect visitors and their form is correspondingly regular. The anthers are so situated that the pollen on escaping comes into contact with the stigma; in such flowers self-fertilization is compulsory and very effectual, as seeds in profusion are produced.
Several species of Viola are native to Great Britain. Viola canina (fig. 5) is the dog violet, many forms or subspecies of which are recognized; V. odorata, sweet violet, is highly prized for its fragrance, and in cultivation numerous varieties have originated. The Neapolitan or Parma violet (var. pallida plena) is a form with very sweet-scented, double, pale lavender flowers; var. sulphurea has shining deep green leaves and lemon-yellow flowers, deeper yellow in the centre, and with a pale-violet spur. Sweet violets like a rich, fairly heavy soil, with a north or north-west aspect if possible; they are readily increased by dividing the crowns after flowering. Other species known in gardens are: V. altaica, flowers yellow or violet with yellow eye; V. biflora, a pretty little species 3-4 in. high with small yellow flowers, the large petal being streaked with black; V. calcarata, flowers light blue or white, or yellow in var. flava; V. cornuta, flowers pale blue — there are a few good varieties of this, including one with white flowers; V. cucullata, a free-flowering American species with violet-blue or purple flowers; V. Munbyana, a native of Algeria, with large violet or yellow flowers; V. pedata, the bird's-foot violet, with pedately divided leaves and usually bright blue flowers; V. rothomagensis, a native of western Europe, with flowers bright blue striped with black, and sometimes called the Rouen violet; and V. suavis, a native of Asia Minor, the Russian violet, with pale-blue sweet-scented flowers. The garden pansies or heartseases are derivatives from V. tricolor, a cornfield weed, or V. altaica, a native of the Altai mountains. (See Pansy.) “Bedding violas,” which differ from pansies in some slight technical details, have been raised from V. cornuta and V. lutea by crossing with the show pansies. The application of an infusion of violet leaves was at one time believed to have the power of reducing the size of cancerous growths, but its use is now discredited.
Fig. 5. — Dog Violet (Viola canina), half nat. size.
1. Floral diagram of Viola, showing arrangement of parts in
horizontal plan, b, pair of bracteoles below the flower; s, sepals;
p, petals; st, stamens; o, ovary.
2. Fruit, split open.