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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