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
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
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!

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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