Capsella bursa-pastoris - an example of family Brassicaceae

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.