Wednesday, 12 April 2017

Salix caprea (the goat willow) Introducing the willow family (Salicaceae)

In a previous entry I described dog’s mercury which is a dioecious plant.  The willows are the same with the trees being either male (bearing the “pussy willow” catkins) or female.  These are a conspicuous feature of the English countryside in spring.

The genus name Salix is the old Latin name for willow; the species name caprea is the Latin for goat, its foliage browsed by them on the Continent.  The earliest reference to this is in the herbal by Hieronymus Bock which shows the species being eaten by a goat. 

The leaves taper to the base and the tip and may be broadest at half their length (middle photo - such leaves are described as the elliptic series) or broader beyond half way (left photo - the obovate series).  The ratio of length to breadth is about 2:1 in the obovate leaf illustrated and this means the leaf is described as narrowly obovate.  The elliptic leaf is between the descriptors for elliptic (ratio 2:1) and narrowly elliptic (ratio 3:1).  The margin of the leaf is crisped so it won’t lie flat.  The base of the leaf tapers onto the petiole and this is called cuneate (wedge-shaped).  The underside is more grey-green due to a dense covering of curly hairs.  Each of the veins curves round at its tip to meet the next vein forward as shown on the right hand photo.  This arrangement is termed brochidodromous.  

The male and female flowers are on separate trees.  The flowers are very densely arranged in elongate inflorescences. The male inflorescences have a fluffier appearance and the female inflorescences are narrower.  The individual flowers do not have stalks and are attached very closely together on a central axis (rhachis). This arrangement is called a spike.  The flowers are also very simple.  This type of inflorescence is called a catkin (i.e. a dense spike of simplified flowers).
The male flowers consist of two stamens comprising a long filament and small yellow anther.  These give the catkins their fluffy appearance.  At the base of the stamens is a bract which is white towards the base and brown at the tip.  The bract is covered with long white downy hair.  The photo on the left shows the flower taken as it were from the axis.  At the base is a small golden yellow tube which is a nectary.  Although the pollen is said to be wind-dispersed, the presence of a nectary implies that insects are also involved in pollen transfer.  The flowers of willows do attract a number of species of flies (Diptera).

The female flowers are similarly simple.  They too have a hairy bract at the base and a small nectary on the inside surface.  The ovary is has a short stalk (i.e. the ovary is stipitate).  It is hairy and asymmetrical at the base and contains two cavities, each containing several ovules.  At the tip of the ovary are four stigmas which are shrivelled and brown in the photos.

The floral formula is

This shows that the sexes are separate.  There is a single bract and the calyx and corolla are absent.  The male flowers have two anthers and the female flowers two carpels which are fused together into a single ovary.  The ovary is superior.

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.