Viola riviniana Rchb., Common Dog-violet
Account Summary
Native, common, widespread and locally abundant. European temperate.
1881; Stewart, S.A.; Co Fermanagh.
Throughout the year, but mainly April to October.
Growth form and preferred habitats
This rosette-forming, wintergreen perennial is very variable in form, but when in flower is usually recognised by its broad, overlapping violet-coloured, upper petals decorated with much-branched, spreading, purple veins on the broad, lowest petal. The flower also has lanceolate sepals, 1.5 mm or more and enlarging when in fruit, and a stout, pale, yellowish-green spur that is conspicuously notched at the rear. In comparison with V. reichenbachiana (Early Dog-violet), with which it is most likely to be confused, the leaf stipules offer a useful distinction: in V. riviniana they are narrow (but broader than those of V. reichenbachiana), and they are fringed with coarse fimbriae (marginal teeth) that are distinctly shorter than those of Early Dog-violet.
V. riviniana tolerates a very wide range of soil type and nutrient status, though it is most frequent and abundant on moist, semi-shaded, infertile, unproductive habitats, especially in lowland deciduous woods and margins of scrub. V. riviniana is also occasionally found in disturbed habitats on roadside banks and verges, beside paths, on waste ground and in gardens. It is also frequent on calcareous screes and on coastal sand hills. It is really only absent from situations of extreme wetness, dryness, disruption, or acidity below about pH 4.0. In pasture situations, it is little grazed (Grime et al. 1988).
The establishment strategy of V. riviniana in England was described by Grime et al. (1988) as intermediate between a stress-tolerator and a more general C-S-R (a midway balance of Competitor, Stress-tolerator and Ruderal species). The exceptionally wide habitat range it displays in Fermanagh helps to confirm this view of its general, intermediate ecology and competitive, colonising and surviving ability.
Under deciduous woodland canopy, V. riviniana is not sufficiently vigorous to compete successfully with dense patch-forming vernal species such as Hyacinthoides non-scripta (Bluebell), Ranunculus ficaria (Lesser Celandine) and Anemone nemorosa (Wood Anemone). It can however become plentiful and conspicuous on more shallow, rather drier soils on banks or near rocks, and also in ground where relatively large amounts of tree litter accumulates, a situation that excludes most other herbaceous species.
An experimental study in England of the effect of leaf litter on garden transplants of five ground flora species including V. riviniana, found that it was slow to emerge from the covering layer, but shoots did eventually emerge from all of the transplants, even in plots where the largest experimental amounts of litter persisted. Thanks in part to the orientation and storage reserves of their short, stout, erect stock, the violet plants were able to respond to varying litter depths by elongation of the leaf petioles, and there was no significant effect of the litter depth or persistence on plant yield compared to exposed control plants (Sydes & Grime 1981).
In woodland, on heavy, often base-rich soils, V. riviniana regularly occurs along with V. reichenbachiana, although the latter is somewhat more shade-tolerant, flowers a few weeks earlier and produces fewer seeds (Grime et al. 1988). The two species occasionally hybridise (see below).
Variation
V. riviniana is not just the most common and widespread British dog-violet by a long way; it is also the most variable and 'plastic' in its response to environmental conditions (Valentine 1949). For example, some plants can form adventitious buds and shoots on the roots whilst others do not. This allows plants with the necessary gene(s) to reproduce vegetatively, forming clonal patches.
Although two forms of the plant have been described within this species in British Floras for over 70 years (Valentine 1941, 1950), and they have been variously rated as subspecies, ecotypes or varieties (Clapham et al. 1962, 1987; Stace 1991, 1997), I will not follow Prof. Valentine into the labyrinthian obscurities of g-Ecospecies, a-Ecospecies, or cytotypes on this one! (Valentine 1950). His split of subsp. minor and subsp. riviniana has not had very much attention in an Irish context − not even getting a mention in our main field Flora, An Irish Flora (8th ed., Parnell & Curtis 2012). Indeed, the general consensus throughout B & I now appears to be that Valentine's subsp. minor or var. minor, which is dwarfed to about half size in both flower and all of its vegetative characters (Porter & Foley 2018), is merely a phenotypic reaction to exposed, dry or coastal ecological situations and, at most, it is a habitat induced ecotype (Stace 1991, 2019). Porter & Foley (2018) mention and illustrate three colour forms or cultivars that are in garden cultivation: pure white, purple and pink.
Flowering and pollination
Like several other Viola species in these islands, V. riviniana plants often flower twice in a season. In spring, flowers are abundantly produced from mid-March to May, and although they are functionally self-compatible, pollination is only achieved in these flowers after insect visitation. A general lack of seed set by the spring flowers is probably due to a scarcity of insect visitors early in the year, even though the flowers do provide copious nectar, which is reserved in the spur specifically for longer-tongued insects (Beattie 1969a). It is interesting to note that the flowers do not appear to possess any scent to attract visitors (Press et al. 1994).
A great deal of extremely detailed research by Beattie into the pollination of V. riviniana and its near relatives (Beattie 1969a & b, 1971, 1972) has shown that the mechanism of flowering in the spring follows two distinct phases with respect to pollen presentation: an initial very precise pollination syndrome involving small amounts of pollen and "brief and irregular visits" by a few specialised long- or medium-tongued insects, chiefly hover-flies (Diptera: Syrphidae), bee-flies (Diptera: Bombyliidae) and bees (Hymenoptera: Apoidea) (Beattie 1969b). This is followed after four to six days by a second phase, which may last between two and nine days depending upon weather conditions, when the pollen is released from the anthers onto the surfaces of the petals. During this latter stage the pollination mechanism becomes completely unspecialized, and many more insect species (up to 40 or more) may become involved. During the second stage, selfing or chance crossing are much more readily achieved than before, although selfing is probably still predominant. In reality, however, although out-crossing can and does operate in the normal 'open pollinated', chasmogamous spring flowers of this species, viable seed production in this type of flower is frequently almost negligible (Beattie 1969a).
Late season flowering
In late-summer and autumn, the same or different plants often develop a second flowering period of mainly cleistogamous flowers which are automatically self-pollinated while still in the bud. Since these flowers do not require insect visitors to function, they are often very inconspicuous and may remain tucked away at the base of the plant, more or less concealed by the leaves of the characteristic non-flowering central rosette. Thus in any season the majority of violet seed is produced by selfing and probably most of it is produced by the obligately inbreeding cleistogamous flowers (Valentine 1941, 1962; Beattie 1969a, 1971).
Fruiting and primary seed dispersal
After fertilization of the ovules, the ripening fruit capsule of both types of flower (ie chasmogamous and cleistogamous ones) generally become elevated above the leaves by reorientation and elongation of the pedicel or flower stalk. Seed release is often described as 'explosive', but it is more the case that they skite out of their already opened capsule by a sudden release of lateral pressure, in a manner rather reminiscent of playing 'tiddlewinks' but with spheres involved instead of flat plastic counters. Perhaps it is better described as being like a marble being squeezed between the thumb and finger. To explain this matter, the Viola fruit is a three valved capsule which splits along very obvious sutures or lines of weakness, and then opens out to form boat-shaped valves containing a total of up to about 20 smooth, ovoid, hard-coated, pale yellow seeds (Butcher 1961). The three somewhat elastic valves dry and shrink shortly after they open and, as they do so, they exert lateral pressure on the contained seeds, so that one by one, or in small groups of individuals, they are forcefully ejected. The first seeds expelled are usually from the central row in each valve and, not being under direct pressure from the valve walls, they often fall closer to the parent plant than those which remain to be expelled by direct pressure (Leavitt 1902, quoted in Beattie & Lyons 1975). Violet seeds may fly distances up to 2 m or so, depending upon whether or not the flight path is clear of leaves or other obstructions.
Secondary seed dispersal by ants
The seed coat of many Viola species has an attached oil-rich food body or elaiosome which may be of various origins; for example, often it is a portion of the funicle or stalk of the ovule, but sometimes it is an outgrowth from the seed coat. Irrespective of its origin, the elaiosome always contains a high percentage of lipids, sometimes with starch and proteins also present. The size of the elaiosome on offer is relevant to its dispersal potential: larger ones being more attractive to ants than small ones, but often in violets the difference is not significant (Beattie & Lyons 1975; Mark & Olesen 1996). Viola seeds may thus undergo secondary dispersal, being located and transported by the ants, in many cases all the way back to their nest (Culver & Beattie 1980). In other situations, they are transported a minimal distance, but even if they are simply scarified and the food body removed, this enhances subsequent germination (Culver & Beattie 1980).
Almost irrespective of the distance the seeds are physically moved, this 'myrmecochory' (from the Greek, 'myrmeco' = ant and 'khorein' = to spread around), confers several benefits on species which undergo the process; these involve not just additional dispersal from the parent plant, but avoidance of post-dispersal seed predators (which frequently include mice, voles and a considerable variety of invertebrates) and competitors, as well as scarification assisting germination. If removed to the insect nest or to ant refuse heaps near the nest, then additional moisture plus nutrients such as nitrogen and phosphate may become more available to the germinating seed (Culver & Beattie 1980; Beattie 1983). In practice this process is unlikely to transport the seeds very far in terms of distance, ie often only a few cm, but there remains the potential for a few seeds to travel a moderately long-distance (perhaps 50-100 m) to the ant nest.
Seed germination and buried seed survival
V. riviniana seed has a pronounced chilling requirement for germination so that seedlings only appear in spring after winter frosts are past (Thompson & Grime 1979). The majority of studies suggest V. riviniana seed is transient, surviving overwinter and germinating more or less immediately under a wide variety of environmental conditions once the chilling requirement has been met. However, a handful of reports indicate longer persistence may occur, ie more than one year and, in two instances, for more than five years (Thompson et al. 1997).
Fermanagh occurrence
V. riviniana is by far the commonest violet in Fermanagh. It has been recorded in 448 tetrads, over 84.9% of those in the VC, growing in a very wide variety of habitats – chiefly (but not exclusively) shaded or semi-shaded. This includes almost all kinds of woody vegetation, plus established grassland at all altitudes, rocky slopes, cliffs and screes.
British and Irish occurrence
The New Altas shows the species is almost omnipresent throughout B & I, apparently absent only from the prairie-like arable agricultural landscape areas of South Lincolnshire, West Norfolk and Cambridgeshire (VCs 53, 28 & 29) (Preston et al. 2002).
European and world occurrence
Beyond the shores of B & I, V. riviniana is distributed widely in W, N, S. and C. Europe including Iceland and Madeira. In the Mediterranean, it stretches to S Greece, Crete and to coastal and mountainous parts of N Africa, but it is absent in the eastern Balkans, Cyprus and Asia Minor, where it is replaced by the closely related V. sieheana Beck. (Hultén & Fries 1986, Map 1326).
Names
The specific epithet 'riviniana' is the Latinised form of the name August Quirinus Rivinus (1652-1723), a Professor of Botany at Leipzig, in whose honour the plant is called (Gledhill 1985). The English common name 'Dog Violet' and variants of it, all refer to the fact that such species are scentless non-garden plants, and as such are regarded as inferior. The name was apparently first given by John Gerard, who simply translated the botanical name 'Viola canina'. Thus 'Dog Violet' is really a good example of a book name, ie one invented by a book writer, rather than originating from common parlance or folk usage (Grigson 1987).
Threats
None.