Monday, 3 April 2017

Viola odorata - in introduction to family Violaceae

The genus name Viola is derived from the Greek word ion, meaning purple, which became the Latin viola.  The species name odorata refers to the fact that this species is fragrant. 

The leaves all arise from the top of the root (in a basal whorl), along with a cluster of bracts (stipules).  The stipules are mostly transparent with a few green patches. 

The sides of the stipules have some teeth along one side which have got a round swelling a

t the tip - this is a gland and thus they are termed gland-tipped teeth.  The stipules taper into a narrow point (= acuminate) and the tip is curved to the side (= falcate).

The leaves are described as broadly ovate (this has a mathematical definition - the length divided by the width is about 1.2).  The base of the leaf is notched (= cordate) and the tip is bluntly pointed (= obtuse).  The margin of the leaf is roundly toothed (= crenate).  There are several main veins that start at the end of the leaf stalk; these curve towards the tip of the leaf and then repeatedly divide (= actinodromous-reticulate venation).  The upper and lower surfaces are sparsely covered with short transparent bristle-like hairs which are denser on the veins and on the margin and become denser towards the base.  The leaf stalk (petiole) is quite densely bristly hairy, concave on the top surface.  

The flowers are borne on long stalks (pedicels) which have two small pointed bracts part way along.  The pedicels are bristly hairy.  There are five sepals which are dark green with a narrow transparent edge.  They are attached just below the ovary and extend back as a rounded lobe from this point of attachment.  They are similar in size and are more or less free from one another.  The five petals are rather different in shape from one another.  If this is the case the flower is said to be zygomorphic.  The lower petal is extended backwards forming a rounded purple swelling (spur).  There are two side petals and then the two upper petals are curved around backwards.  The
petals are all free from one another at the base.  In the centre of the flower by eye there appears to an orange triangular projection.  With a lens this can be seen as five anthers which are all closely attached and point to the centre (they are said to be connivent).  The stigma can be seen in the centre of the anthers and it is hooked.  There are several varieties of Viola odorata that occur in Britain.  This variety is dumetorum characterised by the violet-purple spur, white petals with the side petals with a cluster of hairs at the base next to the stamens (probably visible on this photograph).

If the petals are removed it is found that three of the stamens are on very short stalks while the other two have a large green appendage which points back into the spur of the lower petal.  This appendage contains nectar.  The anthers split open towards the centre of the flower and this means they are introrse

If all the stamens are removed along with the petals and most of the sepals, the ovary can be seen.  This has three green ridges which are hairy with purple lower areas in between.  The style is narrow at its point of contact with the ovary.  It then broadens and ends with the hooked stigma.  If the ovary is sectioned it will be found to contain a single hollow containing many ovules.  There are ridges on the inner surface which match the hairy ridges on the outside.  The ovules are attached to these ridges.  The ridges on the inside are called placentas.  The term given to this arrangement where the ovules are attached to the outer wall of the ovary is parietal placentation.

The floral formula indicates that the calyx is formed of five sepals (Ca), five petals form the corolla (Co) and these are dissimilar (Z for zygomorphic), five anthers and a superior gynoecium with of three placentas.

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