Monday, 27 March 2017

Mercurialis perennis - Dog’s Mercury, introducing Euphorbiaceae (the spurge family)

Euphorbiaceae (the spurge family) contains the large genus Euphorbia which includes Euphorbia pulcherrima (poinsettia), Euphorbia characias (widely grown in parks) and other spurges.  These all produce a milky liquid when a leaf is broken off.  In addition to these there are a large group of other genera which are completely different.  In Britain these include the two species of Mercurialis and the castor oil plant (Ricinus communis).  Four further genera occur in the rest of Europe and the diversity increases further south - in tropical Africa there are over fifty. 

The common features are
  • the flowers are either male or female
  • female flowers have two to three locules with one ovule in each and with two to three stigmas
  • stigmas branched or with a lumpy surface

Dog’s mercury forms carpets on the chalk woodlands of central southern and south east England.  The plants are either male or female.  In late March and early April it is the male plants that are conspicuous with their yellowish stamens.  They form extensive patches spreading by underground stems (rhizomes).  You have to search carefully for clumps of female plants which have their flowers the same colour as the leaves; their leaves are very subtly different from those of the males.  Under a lens however, these flowers are very distinctive.  Later in the year it is the female plants that are obvious with their developing green fruits.

The genus name comes from the Latin Mercurius, the Roman mythological deity, and -alis, belonging to.  One interpretation has been that the plant was discovered by him.   The species name perennis refers to the fact that it is perennial (lives for many years).  The common name was given in the Middle Ages as the plant was considered good for nothing as far as medical use was concerned, being fit only therefore for dogs.

The arrangement where male and female flowers occur on different plants is called dioecious.
The leaves have short stalks (shortly petiolate) with a narrowly triangular stipule each side at the base.  The leaves are elliptic, crenate (with rounded teeth), with the tip bluntly acute and the base tapering to the petiole (cuneate).  The secondary veins curve towards the tip of the leaf and then repeatedly divide.

The male flowers are called staminate flowers because they just bear fertile stamens and no functional female parts.  They are attached to long stalks that arise from between the petiole and the stem (leaf axil).  The bottom one is sometimes single and the upper ones are paired.  The flowers are arranged in tight clusters of 3-4 flowers with gaps in between the clusters.  These clusters have the central flower opening first and the other two or three opening afterwards - such an arrangement is termed a cyme.  An inflorescence with cymes separated by gaps is called a thyrse.  The flowers do not have stalks (pedicels) and so the whole inflorescence is described as opposite pairs of spike-like thyrses.  Each cyme has a small green bract. 

The male flowers have three green perianth segments, joined at the base.  The number of stamens is variable, between 8 and 10.  The filaments all arise from the same point in the centre of the fused perianth segments.  There appear to be two anthers side-by-side at the tip of each filament; they are rounded with a rim before dehiscence.  The pollen is probably carried by wind.

The female flowers are called pistillate flowers.  They are also borne on shoots arising from the leaf axils.  They are much fewer in number, two or three.  The bottom flower has a bract.  The flowers have clear stalks (pedicels).  There are three green, triangular, perianth segments which are separate to the base.  These are topped by the ovary which is made of two globular sections (locules) side by side.  If they are sectioned using a blade each is found to contain a single ovule.  The ovary is covered with bristly hairs.  There are two narrow structures, one each side which are non-functional stamens (staminodes).  There is a stigma on top of each locule which is green on the back and white on the front.  The white surface is covered with rounded projections (papillae).  These increase the surface area of the stigmas and are adhesive, to trap the wind-borne pollen. 

After flowering the pedicels elongate raising the enlarged ovaries (fruits) above the upper leaves.  The bristles also enlarge so that they catch on passing animals for dispersal.

Floral formula

Sexes separate.  Pistillate flowers with three perianth segments and a superior gynoecium of two united carpels.  Staminate flowers with three perianth segments and a variable number of anthers.

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