Capsella bursa-pastoris
This
species is probably the second most common weed worldwide, after the chickweed
(Stellaria media). It originated in Eurasia and has been
introduced into the Americas, Australia and Africa as well as various islands
in the Atlantic and Pacific. It is
commonly known as the shepherd’s purse from the shape of the fruit. William Coles (1657) wrote “it is called
Shepheard purse or Scrip from the likeness the seed hath with that kind of
leathearne bag, wherein shepherds carry their victuals into the field”. This type of bag is depicted in paintings of
the time. An extract of such a painting
is shown here by Pieter Breugel showing a peasant woman with one hanging from
her belt (c. 1567). When the ripe fruit
of the plant is pulled into its two halves the seeds spill out, rather like
coins from a bag. The Latin name bursa-pastoris also means shepherd’s (pastoris) purse (bursa). The genus name Capsella is derived from the Latin capsa, meaning a box or case, with the
suffix –ella, meaning small.
One reason why this species has been so successful is that
it can be found in flower in England every month of the year. A second reason is that the time from
germination to flowering is short. This
means if it germinates in cultivated land it can flower and fruit in between
hoeing. It has a single tap root with
fibrous roots extending from it, meaning that it can root into confined spaces
for example between paving slabs. The
fruits contain about 12 seeds in each half, each one about a millimetre in length. An average plant picked from a roadside in
early November had 35 developing fruits, giving the potential for over 800
seeds with further flowering occurring.
If
left to live longer, the plant forms a rosette of leaves around the base of the
stem but before this it produces upright stems with stem-leaves and
flowers. It is as if it can’t wait to
produce seed. The stem leaves have small
extensions at the base which pass the stem – this means the leaves clasp the
stem. The extensions may be rounded (auriculate) or pointed (sagittate).
The leaves are about four times longer
than wide with the sides more or less parallel – this would be described as
being narrowly oblong. The tip is pointed (acute). The surface of the leaf has two types of
hairs. There are narrow hairs which
extend at an angle to the surface and are made of two or three cells (simple multicellular hairs). In addition there are hairs that have 3-5
arms – these are called stellate hairs. The stems are also covered with these two
types of hair. The leaves towards the
base of the stem are very variable and have the general appearance of dandelion
leaves. The base of the leaves tapers,
becoming a very narrow strip alongside the central vein. The leaves become increasingly more lobed
further down towards the base of the plant.
The lobes are roughly triangular and may be opposite one another (as in
the right hand basal leaf) or alternate.
This arrangement may be described as pinnately
lobed. The lobes may be angled
slightly towards the tip of the leaf, straight out or angled slightly towards
the base (runcinate). If the lobes are ignored and one joins the
tip of one to the tip of the next one has traced the outline of the leaf. The leaf outline is broadest at or beyond the
middle of the leaf. The leaves are 4-5
times longer than wide meaning the leaves are approaching narrowly oblanceolate
– obovate means broadest beyond half way; oblanceolate is upside-down
lance-shaped; three times longer than broad is oblanceolate and six times
longer than broad is narrowly oblanceolate.
The
flowers are small, 1.5-2 mm long and have four sepals, four petals, six stamens
and a flattened ovary, topped by a short stigma. They do not open widely and little is gained
from photographing them from above. The
sepals are separate from one another except at the very base. They are green to purplish and have a broad
whitish margin. Their tips are concave
or hooded (cucullate).
If a sepal is removed the white petals can be seen to
consist of a broadly rounded apical section which tapers quickly to a narrow
base.
If the
petals and sepals are removed the stamens can be seen. There are six of these with one on each side
of the narrow edge of the ovary and two closer together on the mid-line of the
ovary. The two single ones are shorter
than the paired stamens. The filaments are white and are fixed to the middle of
the anther (medifixed). The anthers face and open inwards (introrse) and this suggests self-pollination as they
are virtually touching the stigma.
If two of the stamens are removed the surface of the ovary
can be seen. There is a seam running
from the stigma to the base across the broad side of the ovary, indicating
division into two locules. The style is
short and green with a small stigma on top which is covered with pollen. At this stage the ovary is not as
heart-shaped as the fruit but has some indication of this appearing to have
“shoulders” each side of the style.
If the
young fruit is sectioned it can be seen to be formed of two halves (valves) separated by a narrow whitish wall (septum) which is concertinaed. When the fruit is mature the two valves of
the fruit fall away sideways. Each seed
is attached by a stalk (the placenta) to the point
where the septum is attached to the ovary wall so there are two rows of seeds
in each half on each side of the septum.
The surface of the seeds has a very fine net-like surface
sculpture. The placentas of the seeds
towards the tip are straight and those on those towards the base are curved so
the seeds are held alongside the septum.
The style and stigma remain attached to the end of the septum after the
valves fall away.
Diagrammatic
cross section through the fruit
green
= septum
blue
= placenta
red
= seed
black
= fruit wall
This
species is a member of family Brassicaceae (formerly known as Cruciferae). This is based on genus Brassica which is the cabbage genus. In this family there are two types of fruits
produced. Shepherd’s Purse produces
short and broad fruits which are known as silicles
or silicules (length less than twice the
width). Others, including cabbage have
fruits over three times longer than wide; these are called siliquae or
siliques.
The
floral formula would be:
This indicates
that the flower has both male and female structures, four separate sepals (calyx)
and petals (corolla), six anthers and an ovary with two locules all attached to
the receptacle.
