Saturday, 25 March 2017

Buxus sempervirens introducing Buxaceae (the box family)

Buxus sempervirens is a rare native plant in southern England growing in scrub and woodland on chalk.  It is a shrub or small tree.  However it has been very widely planted throughout Britain being useful as a short hedge and it responds very well to topiary.  It is in flower in the spring.  The genus name Buxus is of Classical origin.  The species name sempervirens is a combination of the Latin semper = always and virens = flourishing, referring to the perennial nature of the plant.

The leaves are small and are much darker in colour on the upper and lower surfaces.  Leaves are narrowly to broadly elliptic, with a cuneate base and an obtuse or emarginate tip.  The margins are evenly rounded = entire.  The midrib is the only vein visible from above and stands slightly proud from the surface.  The same is true for the lower surface where the midrib shows up paler and under a lens it is marked by tiny white elongated dots.  The flowers are produced in the spring. 

There are three main sexual arrangements for flowers:

a) Flowers have both male and female parts and both these are functioning, the male parts producing viable pollen and the female parts ovules which can develop into seeds.  This condition is called hermaphrodite.  That has been the case in the previous four species considered in this blog series.

b) Flowers male or female with male flowers and female flowers occurring on the same plant.  This is called monoecious.

c) Flowers male or female with male flowers and female flowers occurring on different individual plants.  This is called dioecious.

There may in some cases be a mix of hermaphrodite flowers and single sexed flowers on the same plant or other combinations.

The flowers in box are monoecious.  They are in tight groups in the axils of the leaves. Groups of flowers are called inflorescences. The flowers do not have stalks so this arrangement is called a spike.  Because the flowers are so close together they are in a contracted spike.  The flowers at the base of each group are male and more conspicuous due to the yellow anthers.  Some spikes (particularly those towards the tip of the flowering shoots) have a female flower, which is green and has a much chunkier appearance. 

This photograph shows what you see if all the male flowers from a spike are removed except for one.  There are four stamens with white filaments and yellow anthers.  The anthers open towards the centre of the flower thus they show introrse dehiscence.  There are four perianth segments around the base of the stamens and they are free from one another.  In addition there is a small yellow domed structure in the centre of the flower which is “pretending” to be the female part of the flower.  Because it is not functional it is termed a pistillode.

A dissected out female flower is photographed here.  It has six perianth segments which are separated from one another around the base and which lie against the surface of the ovary.  They are green with pale margins.  The ovary is globular with three styles.  The stigmas are flat to concave surfaces on top of the styles.  This arrangement implies that there are three carpels united together each with its own stigma.  This is called a syncarpous ovary.  If the ovary is cut through with a blade it will be found to consist of three sections.  

Each section contains two ovules side by side which are attached at the top of the ovary.  The surface between the styles is slightly domed and has a sheen - it produces nectar and is called an interstylar nectary.  After fertilisation the ovary swells and becomes woody, retaining the styles which then appear as horns.  These are seen in the top photograph.

The floral formula of the flowers is

In shorthand this would be written.  Flowers unisexual.  Male flowers with four free perianth segments and four anthers.  Female flowers with six free perianth segments and three fused carpels.

The other species of family Buxaceae you are likely to find is Pachysandra terminalis which is widely planted as a ground cover plant.  It also flowers in the spring.

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