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.

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