Violaceae - Violets and Pansies

Above: Common Dog-Violet, Viola riviniana.

The violets not only add attractive form and colour to woods, grasslands and other habitats and to gardens, but they have a complex structure and fascinating strategies to ensure pollination. Here we focus on the remarkable structure and life strategy of the violet flower. The structure of the flower is shown below:
Viola flower structure

Violas often produce two types of flower: the chasmogamous flowers, as shown above, meaning flowers that open to disperse pollen and then later in the season it may produce cleistogamous flowers, flowers which do not open but instead pollinate themselves. The cleistogamous flowers set the majority of seed, but the effort put into chasmogamy is not wasted as even a small amount of cross-pollination benefits the gene pool. The chasmogamous flowers rely on insect pollinators, the intention being for pollen collected from one plant to arrive at a receptive stigma on another plant, thereby affecting cross-pollination.

The chasmogamous flowers actually have a dual strategy. Five to six days before
anthesis (opening of the petals) the sepals open out and the enlarging petals take over their role in protecting the developing reproductive parts in the bud. There are five sepals: two anterior (upper) sepals, one posterior (lower) and two posteriolateral sepals. The corolla, or flower tube, consists of five distinct petals: two posterior (forming the roof and sides of the bud), two lateral (forming the floor of the bud) and one anterior petal (rolled-up like a scroll inside the bud). Pollinating bumblebees have good ultraviolet (UV) vision and see flowers quite differently to humans. The petals reflect UV light, accept at
their bases which absorb UV light, so the middle of the corolla probably appears as a black tunnel to the bumblebee. Anterior and lateral petals have an array of hairs which close the entrance to the pollen. The lateral hairs reflect UV and so appear bright. The large bumblebee can force its way past the hairs and can access a flower which is even in the early stages of anthesis.

spur is an extension of the petals at the back of the corolla which stores nectar. The intention is that insects pushing deep into the flower to reach the nectar brush past the anthers, picking up pollen, or past the stigma, depositing pollen. The adaxial surfaces of the petals (the upper surfaces nearest the floral axis) are covered in conical epithelial cells. This is a characteristic of many petals and the conical nature of the cells performs several functions. It gives the petals a matt texture and possibly makes it easier for landing insects to grip the surface. It also gives the petals anti-wetting properties -
rainwater rolls off, cleaning the petal without obscuring its visual display which is designed to attract pollinators (a property called
superhydrophobicity). In many flowers, the conical cells also secrete aromatics. The cuticle over the papillae is typically ridged and this pattern can cause diffraction of the light, giving the petals iridescence, enhancing their visual display.

The female organ, the carpel, consists of the ovary containing ovules, each ovule containing a developing egg cell, and the style and stigma. The
style is a stalk holding the stigma in position. The stigma is the receptive surface which must contact pollen if the flower is to be pollinated. In Viola, the style is hinged at the base, at a kink or 'knee' joint. The stigma in Viola is cup-shaped and contains an opening (the stigmatal aperture). This aperture opens into a stigma cavity which extends into the style where it is filled with mucilage. When an insect brushes past the style and deflects it, the canal is squeezed shut and a blob of mucilage protrudes from the stigma aperture. The knee joint ensures the style springs back into shape and the mucilage goes back inside. There are two hypothesised functions of this mechanism: it may touch the insect's back and make it sticky ready to pick-up pollen; alternatively it may collect pollen from the insect's back and then draw it into the stigma where pollination can commence. This first mechanism is a specialised mechanism intended for specialised pollinators, such as the large bumblebee.

The 5 stamens are essentially filamentless (or have very short filaments) and each contains two microsporangia each of which dehisces (splits open when ripe and dry) along its own suture from top to bottom. Each bears a distal appendage (on top of the anther) in the form of a triangular projection, coloured yellow or bright orange. These are sometimes called connective appendages, I shall call them
conal appendages, since the five close together in a cone around the style. The two anterior stamens bear am additional appendage at their base - a nectariferous appendage (filament
appendage) which projects into the spur into which they secrete nectar (which apparently oozes out of modified stomata on the ends of the nectariferous appendages).

Anthesis proceeds over several days. The petals curl apart throughout. Initially the only structures visible from outside the flower are the style apex and the conal appendages. One day after anthesis the posterior anther dehisces releasing pollen. On the second day, the two posterio-lateral anthers dehisce. The two remaining anterior anthers dehisce around day 4 to 6. The stigma is receptive throughout. The cone of conal appendages serves to trap pollen ready for a specialist pollinator to brush past. After day 4, however, the conal appendages separate and pollen falls onto the anterior petal where it can be reached by a wider variety of insects. Thus,
Viola adopts a three-part strategy:
first its chasmogamous flowers are primed for specialist pollinators, then later they open up for more general pollinators, thus increasing their odds of pollination, and thirdly they produce cleistogamous flowers which self-pollinate.

The petals continue to open-out wider throughout anthesis and the distal part of the
pedicel (flower stalk) slowly curves over into a hook-shape. This ensures that the opening petals present the maximum surface facing forwards to attract pollinators. Additionally, it positions the flower by day four in such a way that pollen will fall out onto the anterior petal when the anther cone opens out. Eventually the flower senesces. This is not triggered by pollination in Viola, and usually occurs after day 7 post-anthesis (sometimes day 14 to 15 in bright sunlight).

On the various floral parts there may be various types of hairs and papillae, which help to characterise the various species, but which have largely unknown functions. For example, a lower lip of papillae is generally present on the stigma beneath the aperture. Clearly there is scope for further research in understanding these remarkable flowers!

Common Dog-violet

Above and below: Common Dog-Violet (Viola riviniana) is perennial and can be distinguished by  its spur, which is curved upward, blunt tipped and notched at the tip and paler than the petals. it is found in woods, hedge-banks, grasslands and among rocks. each of he 5 sepals of Viola bears an appendage or lobe beneath its point of insertion. These squarish appendages can be seen in the photograph below and their length can aid species identification. The main blade of the sepal is pointed. This is one of the early Spring flowers, flowering from March to May in the British Isles (and may flower again from August to October, but often without petals).

Common Dog-violet

Another useful taxonomic feature concerns the stipules. In V. riviniana they have hair-like teeth along their margins. In V. reichenbachiana the stipules are narrower, as are the leaves. Below: leaves of V. riviniana.

Common Dog-violet, leaves

Below: Early Dog-violet (Viola reichenbachiana). This specimen occurred in the same wood as the Common Dog-Violet above, which is not unusual. However, this species prefers dry more chalky soils in woodlands and on hedge-banks. The upper petals tend to resemble 'rabbit ears' and the spur is straighter, darker than the petals and not notched. The nectar-guide veins on the lower petal are also much less branched than in Viola riviniana. The sepal appendages of V. reichenbachiana are shorter (< 1.5 mm) than those of V. riviniana (> 1.5 mm) and are generally very short, as in the specimen below. V. rivinana is found across Eurasia and nw Africa and  has also become established on the pacific coast of the USA and has also been introduced in Australia.

Early Dog-violet, side-view

Early Dog-violet, side-view

The specimen below is somewhat inconsistent.The spur is darker than the petals, straight but notched at the tip. The nectar-guide veins are not very branched, but the broad spreading upper petals suggest V. riviniana. This could quite possibly be the hybrid V. x bavarica = V. riviniana x V. reichenbachiana which is largely sterile. The sepal appendages of the hybrid are said to be intermediate in length between the two parents (i.e. about 1.5 mm long). The hybrid is, however, not common despite the frequent cohabitation of both parents (See Stace: New Flora of the British Isles) and one has to be cautious when determining it. Since the hybrid is partially fertile, I would not rule out back-crosses with either parent.

Dog-violet, side-view

Dog-violet, side-view

Further Reading

Beattie, A.J. 1969. The floral biology of three species of Viola. New Phytol. 68: 1187-1201.

Kuta E.,  J. Bohdanowicz, A. Słomka, M. Pilarska and H. BotheFloral 2012. Structure and pollen morphology of two zinc violets (
Viola lutea ssp. calaminaria and V. lutea ssp. westfalica) indicate their taxonomic affinity to Viola lutea. Plant Syst Evol 298: 445–455.

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