Corylus avellana - a typical species of family Betulaceae

The name for each higher group in the classification of plants up to the order level is derived from one of the genera it contains.  This genus is termed the type genus.  Corylus and a few related genera used to be classified in family Corylaceae but have now been transferred to Betulaceae as subfamily Coryloideae.  Corylus avellana (hazel) was first named as such by Linnaeus in 1753.  Corylus is the classical Latin name for the hazel which has a distribution right across Europe and western Asia, except for the far north and some islands.  The English names for the species include hazel, hazelnut and avelline. The first recorded use of the species name was by Pliny the Elder in his Naturalis Historia, from Avella, a town in Italy.  It was later used by Leonhart Fuchs in his De historia stirpium commentarii insignes (1542) in which he uses the name Avellana nux sylvestris (the wild nut of Avella). 

The text from Linnaeus (1753) is as follows:

The male flowers of the hazel are a conspicuous feature of the countryside from late December onwards.  They are borne on long thin catkins; because they bear male flowers only they are termed staminate catkins.  These have a short stalk and then a central rachis to which the individual flowers are attached spirally.  Each flower has a hooded pale creamy-green bract with the stamens underneath.  The bract is pointed in the middle and is fringed with short, pale, curly hairs.  With magnification it can be seen that there are two further structures under the bracts – these are the bracteoles and they are attached to the outer half of the bracts; they are also fringed with curly hairs.  The stamens are attached to the underside of the bract at about the same point as the bracteoles.  The bracts taper to the rachis so it is difficult to decide whether the whole catkin is a raceme (each flower with a stalk) or a spike (each without a stalk).  There are six stamens in the upper part of the bract and two arising from the narrowed part of the bract, nearer its point of attachment to the rachis.  The six upper stamens form three pairs and one or more of these may have their filaments joined near the base.  In all the flowers inspected the basal pair of stamens have their filaments joined.

The number of flowers under each bracts as well as the number of

The female flowers are inside a bud-like structures which are further back on the branches than the male ones.  The bud scales towards the base are green and fringed with pale hairs.  The length of the fringe hairs is longer on the scales further inside and these also have an increasing amount of silvery hair on their outer surface.  From the tip of the bud there are a number of stigmas.  Early on these are bright red and then they fade to a purple-brown.  On the ones I inspected there were 18-20 stigmas.  Hayward (1987) in his New Key to Wild Flowers, page 138 wrongly states “the female flowers look like tiny buds with 2 red styles”

  

If the outer scales are removed, a central section is isolated, consisting of narrow hairy bracts and the stigmas.  If prised apart and carefully examined the stigmas are in pairs and each bract has two pairs of stigmas associated with it and a pair of smaller unequally-sized bracteoles.  It is very difficult to see this as the flowers are so closely associated.  If the bracteoles have dried a bit, some can be seen as having an uneven outline as two triangular lobes.  At the base of the stigmas is a small ovary which is divided into two ovules.  Each flower therefore has four stigmas (two per ovule) and thus the whole structure contains five flowers (= 20 stigmas).  There is no clear distinction between stigma and style.  The surface of the style is minutely papillose (uneven with microscopic rounded bumps).  The Flora of China (1999) volume 4 interprets the structures as each ovule having a single style which is divided to the base.

The Flora Europaea makes reference to a small irregularly-lobed perianth in addition to the bracts and bracteoles as does Stace (2011) but I could see no further structures apart from the silky hairs on top of the ovary.

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.

Salix caprea (the goat willow) Introducing the willow family (Salicaceae)

In a previous entry I described dog’s mercury which is a dioecious plant.  The willows are the same with the trees being either male (bearing the “pussy willow” catkins) or female.  These are a conspicuous feature of the English countryside in spring.

The genus name Salix is the old Latin name for willow; the species name caprea is the Latin for goat, its foliage browsed by them on the Continent.  The earliest reference to this is in the herbal by Hieronymus Bock which shows the species being eaten by a goat. 

The leaves taper to the base and the tip and may be broadest at half their length (middle photo - such leaves are described as the elliptic series) or broader beyond half way (left photo - the obovate series).  The ratio of length to breadth is about 2:1 in the obovate leaf illustrated and this means the leaf is described as narrowly obovate.  The elliptic leaf is between the descriptors for elliptic (ratio 2:1) and narrowly elliptic (ratio 3:1).  The margin of the leaf is crisped so it won’t lie flat.  The base of the leaf tapers onto the petiole and this is called cuneate (wedge-shaped).  The underside is more grey-green due to a dense covering of curly hairs.  Each of the veins curves round at its tip to meet the next vein forward as shown on the right hand photo.  This arrangement is termed brochidodromous.  

The male and female flowers are on separate trees.  The flowers are very densely arranged in elongate inflorescences. The male inflorescences have a fluffier appearance and the female inflorescences are narrower.  The individual flowers do not have stalks and are attached very closely together on a central axis (rachis). This arrangement is called a spike.  The flowers are also very simple.  This type of inflorescence is called a catkin (i.e. a dense spike of simplified flowers).

  

The male flowers consist of two stamens comprising a long filament and small yellow anther.  These give the catkins their fluffy appearance.  At the base of the stamens is a bract which is white towards the base and brown at the tip.  The bract is covered with long white downy hair.  The photo on the left shows the flower taken as it were from the axis.  At the base is a small golden yellow tube which is a nectary.  Although the pollen is said to be wind-dispersed in all the literature I've consulted, the presence of a nectary would suggest that insects are also involved in pollen transfer.  The flowers of willows do attract a number of species of flies (Diptera).

The female flowers are similarly simple.  They too have a hairy bract at the base and a small nectary on the inside surface.  The ovary is has a short stalk (i.e. the ovary is stipitate).  It is hairy and asymmetrical at the base and contains two cavities, each containing several ovules.  At the tip of the ovary are four stigmas which are shriveled and brown in the photos.

The floral formula is

This shows that the sexes are separate.  There is a single bract and the calyx and corolla are absent.  The male flowers have two anthers and the female flowers two carpels which are fused together into a single ovary.  The ovary is superior.

Viola odorata - in introduction to family Violaceae

The genus name Viola is derived from the Greek word ion, meaning purple, which became the Latin viola.  The species name odorata refers to the fact that this species is fragrant. 

The leaves all arise from the top of the root (in a basal whorl), along with a cluster of bracts (stipules).  The stipules are mostly transparent with a few green patches. 

The sides of the stipules have some teeth along one side which have got a round swelling at the tip - this is a gland and thus they are termed gland-tipped teeth.  The stipules taper into a narrow point (= acuminate) and the tip is curved to the side (= falcate).

The leaves are described as broadly ovate (this has a mathematical definition - the length divided by the width is about 1.2).  The base of the leaf is notched (= cordate) and the tip is bluntly pointed (= obtuse).  The margin of the leaf is roundly toothed (= crenate).  There are several main veins that start at the end of the leaf stalk; these curve towards the tip of the leaf and then repeatedly divide (= actinodromous-reticulate venation).  The upper and lower surfaces are sparsely covered with short transparent bristle-like hairs which are denser on the veins and on the margin and become denser towards the base.  The leaf stalk (petiole) is quite densely bristly hairy, concave on the top surface.  

The flowers are borne on long stalks (pedicels) which have two small pointed bracts part way along.  The pedicels are bristly hairy.  There are five sepals which are dark green with a narrow transparent edge.  They are attached just below the ovary and extend back as a rounded lobe from this point of attachment.  They are similar in size and are more or less free from one another.  The five petals are rather different in shape from one another.  If this is the case the flower is said to be zygomorphic.  The lower petal is extended backwards forming a rounded purple swelling (spur).  There are two side petals and then the two upper petals are curved around backwards.  The

petals are all free from one another at the base.  In the centre of the flower by eye there appears to an orange triangular projection.  With a lens this can be seen as five anthers which are all closely attached and point to the centre (they are said to be connivent).  The stigma can be seen in the centre of the anthers and it is hooked.  There are several varieties of Viola odorata that occur in Britain.  This variety is dumetorum characterised by the violet-purple spur, white petals with the side petals with a cluster of hairs at the base next to the stamens (probably visible on this photograph).

If the petals are removed it is found that three of the stamens are on very short stalks while the other two have a large green appendage which points back into the spur of the lower petal.  This appendage contains nectar.  The anthers split open towards the centre of the flower and this means they are introrse. 

If all the stamens are removed along with the petals and most of the sepals, the ovary can be seen.  This has three green ridges which are hairy with purple lower areas in between.  The style is narrow at its point of contact with the ovary.  It then broadens and ends with the hooked stigma.  If the ovary is sectioned it will be found to contain a single hollow containing many ovules.  There are ridges on the inner surface which match the hairy ridges on the outside.  The ovules are attached to these ridges.  The ridges on the inside are called placentas.  The term given to this arrangement where the ovules are attached to the outer wall of the ovary is parietal placentation.

 

The floral formula indicates that the calyx is formed of five sepals (Ca), five petals form the corolla (Co) and these are dissimilar (Z for zygomorphic), five anthers and a superior gynoecium with three placentas (superior denoted by the underlining).

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.

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.