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.

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.

Wednesday, 22 March 2017

Forsythia x intermedia - introducing Oleaceae (the olive family)

The yellow flowers of Forsythia are a familiar sight in gardens in the spring.  The garden form is a result of crossing the Japanese species Forsythia suspensa and Forsythia viridissima repeatedly over the last 120 years or so.  Hybrids are given the genus name followed by a letter x to represent it is a cross between two or more species and are given a hybrid name. 

Forsythia is named after William Forsyth, a Scottish botanist who managed the gardens at Kensington and St James’, London from 1779.

The flower has four green sepals which are joined towards the base, forming a cup.  These make up the calyx which is said to be gamosepalous as the sepals are joined.  The corolla is bright yellow and is divided beyond half way into four lobes. The lobes of the corolla are joined at the base so the flower is called gamopetalous. The united part of the corolla forms a cup-shaped structure.  The veins running into the corolla lobes are coloured towards the base. 

In the centre of the flower are two short stamens.  The filaments are attached to the corolla near the base, in line with a cleft between two of the lobes - they are termed epipetalous. The filaments are attached to the base of the anthers - they are basifixed.  The anthers open to expose the pollen outwards (extrorse).

There is a single ovary which sits adjacent to the base of the filaments and the corolla, so the ovary is superior and the flower is hypogynous. Attached to the top of the ovary is a single style.  At the top of the style is a stigma which is divided into two rounded lobes.  If the ovary is cut across with a blade it is found to be formed of two hollows (bilocular) each containing a number of ovules which are attached to the top of the ovary - apical pendulous ovules.  

Also in family Oleaceae in Britain are the native ash tree (Fraxinus) and privet (Ligustrum) along with cultivated species of privet, jasmine (Jasminum) and lilac (Syringa).  All of these have only two stamens, four sepals, fused towards the base and leaves that are opposite one another.

Introducing the floral formula

The floral formula is a shorthand way of indicating the structure of a flower using simple notation.  

In the formula above for Oleaceae, the sex sign on the left indicates that the flowers have both male and female parts.  Ca with an encircled 4 shows the calyx is made of 4 lobes, joined at the base.  Co with an encircled 4 shows the corolla is also made of 4 lobes joined at the base.  The G with an encircled two shows the gynoecium is formed of two locules that are joined together into a single structure with one style and stigma.  The A with the two shows there are two anthers that are separate from one another.  This is underlined and placed above the corolla showing they are attached to the corolla.  Finally the calyx, corolla and gynoecium are all underlined showing they are all attached at the same level, indicating the ovary is superior. 

In botanical speak this would be written: Flowers hermaphrodite.  Calyx gamosepalous, four-lobed.  Corolla gamopetalous, four-lobed.  Anthers two, epipetalous.  Gynoecium with two united carpels, superior.

Tuesday, 14 March 2017

Prunus spinosa - a member of family Rosaceae

When the blackthorn is in bloom, spring is definitely heralded, preceding by a couple of weeks the more showy blossoms of the cultivated cherries (also genus Prunus) in towns.  My previous two studies have been of families of monocotyledons and so a change to a dicotyledonous family is due as I explain more botanical terms in a straightforward way.

Prunus is the Latin word for plum tree, while spinosa refers to the spines that are present.  These spines are often quite difficult to find, but you can find shoots that have long spines extending from the dark brown to black woody shoots. 

The flowers open before the leaves in the spring, but the flowering period is quite long so in a few weeks there will still be flowers along with leaves on the shoots. 

The flowers of the blackthorn are produced on the previous years’ shoots and arise very close to the main stems on short side shoots.  These shoots have a bud at the tip which opens to reveal leaves (vegetative bud) and the flowers arise from the base of the bud.  There may be one, two or three arise from the same level.

There are five white sepals at the end of the pedicel (flower stalk) and five white petals. These are called sepals and petals because they differ greatly in structure and colour.  The five sepals together are collectively called the calyx and the five petals together are the corolla.  The sepals are not attached to one another and are described as being free. The same is true for the petals.  There are 20-25 stamens and these consist of long white filaments and small yellow anthers, turning orange and then brown.  In the centre of the flower is a single green style with a single yellow stigma.  The stigma is rounded (termed capitate). In between the base of the filaments and the style is a glistening area which is an indication of nectar.

If the flower is cut in half with a blade it becomes clear that the sepals, petals and stamens are all attached to the top of a cup-shaped structure which has the ovary attached at its base.  This structure is called the receptacle. In other flowers this may be flat or of other forms but this cup-shape is quite common in the rose family.  The colour of this cup shape is golden and glistening and has glands on the surface producing nectar; it is described as being a nectariferous disc.  The stamens, petals and sepals aren’t attached to the top of the ovary or below the ovary, but rather on the rim around it.  This arrangement is called perigynous (peri = around; gynous = female parts).  The ovary however is described as being superior as it is itself attached to the base of the receptacle.  If it is cut across, it can be seen there are two ovules in the ovary.  They are attached to the top of the ovary and are said to be pendulous.

After fertilisation one of the ovules aborts and the ovary swells into a spherical fleshy fruit containing a single stony seed.  A fleshy fruit of this type with a hard stone in the middle is called a drupe. The fruit matures from green to dark blue with a whitish “bloom” that comes off when rubbed with a finger. These are the familiar sloes, collected for jam and gin.

Sunday, 12 March 2017

Crocus verna - an example of a species of family Iridaceae

The flowers of crocuses appears to come straight out of the ground and this is indeed the case. The ovary of the flower is actually below ground level at flowering time and unless you pull the flower up to include at least some of the below-ground structure you will probably miss the ovary altogether - ensure you only do this on crocuses from your own garden or if you have permission to do so.  Crocus verna is probably the commonest crocus grown in Britain, abundant in gardens.  It comes in a variety of colours, mostly purples and whites.

The genus name Crocus probably comes from ancient words for saffron.  One species is cultivated to produce saffron from the stigmas.  The species name verna is Latin for spring.

The leaves are long and narrow and the flower parts are in threes, indicating this is a monocotyledonous plant.

There are six perianth segments (not called petals because they are all similar and in two whorls) and these taper at their base and all combine to form a very long perianth tube which continues below the level of the soil. This situation where the perianth segments are united at the base is gamophyllous. The perianth tube is surrounded by a papery bract and there is a further bract below this which surrounds the perianth tube and the base of the leaves. Both of these bracts are attached to the stem below the level of the ovary.

The flowers open during the day and close at night. When open the division into three outer perianth segments and three inner ones is clear. Inside the bright orange stigma is divided into three main lobes which are themselves irregularly lobed.

Stripping back a couple of the perianth segments reveals some of the flower parts. There are three yellow anthers which are long and pointed, attached at the base to white filaments. The anthers are therefore basifixed. The anthers and the filaments together are called the stamens. Pulling back the perianth segments further reveals that the stamens are attached to the base of three of the perianth segments. In this case the stamens are said to be epiphyllous.



The anthers are divided into two sections, each one called a theca. To release the pollen they split from base to tip towards the centre of the flower. The splitting of the anthers is termed dehiscence and this type of splitting is called longitudinal dehiscence. Splitting on the side towards the centre of the flower is called extrorse dehiscence.



If the two bracts and several of the perianth segments are pulled back right down the perianth tube, the ovary is revealed. The stalk below the ovary is called the pedicel and this is the name given to the stalk of any individual flower.


If the ovary is cut using a blade it is found to be made of three hollow sections (locules) packed with ovules. If an ovary has three locules is it called trilocular. The ovules are attached to the central axis of the ovary by a stalk (the placenta). The point of attachment of the placentas is described as the placentation and in this case we have axile placentation.   There are tiny circles visible in the wall of the ovary and these are the vascular bundles.  There is one of these where the wall dividing the locules reaches the ovary wall and another around half way round the locule.

The following combination of characters will identify a plant to family Iridaceae:

  • long thin leaves
  • perianth segments all similar in two whorls of three
  • three stamens
  • style appearing single with three stigmas 

Friday, 10 March 2017

The snowdrop (Galanthus nivalis) an introduction to family Amaryllidaceae

Getting into botany requires learning a lot of new terminology and this can be daunting for the beginner.  It is best to learn by actually looking at examples of different flower families and that can be done using common species that can be found in the average garden.  All you need for this one is a hand lens and a sharp blade

The scientific name of the snowdrops (Galanthus) is derived from the Greek gala meaning milk and anthos meaning flower.  The species name (nivalis) is Latin for snow.

The flowering plants are divided into two large groups, the monocotyledons and the dicotyledons, referring to the number of first leaves that emerge from the seed when a plant germinates.  This isn’t a helpful distinction to identify flowers but there are a number of clues that can be seen in the snowdrop that indicate they are monocotyledons. 

  • the veins of the leaf are parallel to one another and the leaves are long and thin
  • the flower parts are in multiples of three

The snowdrop flower consists of six white sections that one might call petals - three of them longer and rounded at the tips and three shorter ones with a notch at the end outlined in green.  In this case the botanical term perianth segment is used because you can’t tell whether they are petals or sepals.  There are three longer outer perianth segments and three shorter inner perianth segments, forming two whorls.  The perianth segments are attached to the end of the ovary which is obvious and green.  This flower is described as having an inferior ovary (rather than superior or half-inferior).  Unfortunately botanists also use another term for the same thing, calling the flower epigynous (epi = on top of; gynous = female parts).  Both terms refer to the ovary being underneath the other parts of the flower. Inferior emphasises the position of the ovary below the other parts and epigynous emphasises that the other parts of the flower are on top of the ovary.  When in bud the flower is enclosed in a single leaf-like structure termed a spathe.

If the outer perianth segments are removed along with one of the inner perianth segments and one of the stamens, the other parts of the flower are visible. 

From the centre of the ovary there is a narrow green style with the tip hardly differing in structure, termed the stigma.  These three structures make up the female parts of the flower and are collectively called the gynoecium.

Around the style there are six stamens.  The stamens are made of a thin white filament which is attached to the base of an orange anther which tapers to a point.  The anthers contain the pollen which can be seen in the photograph, staining the inside of one of the perianth segments and the stigma.  The stamens (anthers + filaments) are the male parts of the flower and are collectively called the androecium.  The position on the anther where the filaments attach is important.  In this case they are attached to the base of the anther so they are termed basifixed.  How the anthers open is also important - the opening of the anthers to release the pollen is called dehiscence.  Here the anthers open by a slit which opens towards the centre of the flower and this condition is called introrse dehiscence

If you cut the ovary through lengthways using a blade the internal structure can be seen.  The outside wall encloses hollow sections which are called locules.  These are packed with white ovules which are attached to the central axis of the ovary.  If the ovary is cut across it can be seen there are three locules.  After the ovules are fertilised they become the seeds and the ovary becomes the fruit.  

The snowdrop is placed with other genera in family Amaryllidaceae which share the following features
  • perianth in two whorls of three rather similar segments, both coloured rather than green
  • flowers containing both male and female parts
  • ovary inferior / flower epigynous
  • six stamens
  • narrow leaves