When thinking of conifers, one might feel a bit ‘schizophrenic’.  Perhaps picturing a Leylandii encroaching on your garden, whilst also remembering your Christmas tree.  Maybe the typical image of conifers is that of a tree with dark green foliage all year round.  However, this would be something of a disservice to the Conifer family - the Pinophyta, which contains an amazing variety of trees, many of which are at risk of extinction.  This group includes cedars, firs, cypresses, junipers, larches, pines, hemlocks, redwoods, spruces, and yews.

Leaves on the branchlets of Dawn Redwood

Conifers are important because

• They dominate vast areas of land, particularly in the Northern Hemisphere,  forming the boreal forests or taiga.
• Softwood from conifers accounts for approximately 45% of global timber production. Pine, spruce and larch are often grown specifically for softwood production. The wood is also used in the paper production^[.and, to a lesser extent, in making plastic from chemically treated wood pulp]. 
• Some species produce edible seeds , such as pine nuts provide foods such as pine nuts for humans and wildlife and juniper berries, which are used to flavour gin.  The Monkey Puzzle tree, (also known as the Pehuen Pine, native to Chile and Argentina) produces seeds known as piñones; traditionally harvested by indigenous communities. 

Monkey puzzle tree

To see the diversity of the Conifer family one could visit the Bedgebury Pinetum.  This is home to one of the world’s most important conifer collections.  Bedgebury was established in 1925 by Kew Gardens and the Forestry Commission.  The curator at Kew had observed that the conifers there were ‘being choked by London Smogs’.  The site at Bedgebury, situated on the Southern Kentish weald, was ideal.  It offered an escape from the pollution of London and it had wet and free draining areas, plus varied soils so it a range of conifer could be grown.
The land already had some conifers that had been planted by Viscount Beresford - an evergreen enthusiast.  In 1925, some 315 trees were planted.  This year, to celebrate reaching a century, some 89 of the original trees are marked with special yellow labels.  For  its first twenty years, the pinetum was managed by William Dallimore. His diaries record in some detail the trees he planted, and the challenges faced in establishing the pinetumIf you visit, then you might walk through through Dallimore Valley, and view his legacy. Bedgebury soon became a centre for the scientific interest in conifers, their conservation, and landscape planning.  The current curator is Dan Luscombe.

Apart from seeing a range of conifers, the pinetum offers a variety of activities, e.g. family cycling, mountain biking and walking, There is also the play trail or you can explore the canopy on a Go Ape tree-top adventure or challenge.  It is rumoured that the Gruffalow lurks within the grounds of the  Pinetum. There is also a cafe, serving a range of drinks, plus  breakfast and lunch options.

The pinetum is open from from 8 AM to 8 PM (March 2025 to 26 October 2025), and there are charges for car parking.

The tallest tree in England, a Douglas fir, is found on Exmoor.  The Douglas Fir was introduced into cultivation in 1827 by David Douglas.  Douglas was a gardener and botanist, who spent his early career in Scotland including time at the Botanical Gardens of Glasgow University.  He was recommended to the Royal Horticultural Society by William J Hooker, the first director of Kew Gardens.  Douglas made three trips to North America and was ultimately responsible for the introduction of many different species.  Some of the plants introduced during Victorian times include Sitka Spruce, Sugar Pine, Western White Pine, Ponderosa Pine, Lodgepole Pine, and Monterey Pine. These have transformed our landscape and the timber industry. Sadly, David Douglas died at an early age on an expedition to Hawaii, in suspicious circumstances.

The Douglas Fir, despite its other common names, Douglas Spruce, Oregon Pine - is not a true fir , spruce, or pine. It is also not a hemlock. Its genus name is Pseudotsuga, means false hemlock.  The trees can grow to heights of nearly 100 metres, especially in their native coastal regions and may have a life span of 500 years.

The Exmoor tree, planted 150 years ago, has reached a height of 63 metres (over 200 feet). However, as a relative youngster it still has some way to go — and it has competition.

A recent survey* revealed there are a half million redwood trees in the UK.  Three species exist:

• the coastal redwood (Sequoia sempervirens ),
• the giant sequoia / redwood (Sequoiadendron giganteum), and
• the dawn redwood (Metasequoia glyptostroboides)

They are among the largest trees on earth.  The coastal redwoods are the tallest of the three species.  The largest was measured to have a height of some 379 feet (115.5M).  

Not only are these trees large but they are also impressive because of their longevity, living for a thousand years is not unusual.  Dendrochronology has aged one specimen at over 2500 years.   The coastal and the giant redwoods are found naturally on the pacific coast of California and Oregon.  Many of the UK redwoods, like the Douglas Fir, were introduced in Victorian times.  They were often planted on the estates of the wealthy and landed gentry. There are now probably more redwoods in the UK than in their native Pacific Coast range. Recent hot and dry weather has stressed the American trees, exposing them to intense wildfires.

A recent study of some 5000 redwoods investigated how well the trees were ‘performing’ when compared to their American counterparts.  The tree were scanned with lasers  [a non invasive technique] to determine their height and volume (= biomass),  They were found to be growing just as well as those in the Sierra Nevada, this is probably due to our relatively mild but wet climate.

The tallest redwood tree in the UK stands at 58 metres at Longleat, so somewhat behind the Douglas fir. However, whilst the Douglas Fir may have a life span of 500 years, a redwood can survive for 2000+ years, giving it time to catch up and overtake.

* see  https://www.bbc.co.uk/news/science-environment-68518623

and https://royalsocietypublishing.org/doi/10.1098/rsos.230603

]]> https://www.woodlands.co.uk/blog/flora-and-fauna/trees-come-in-all-shapes-and-sizes/ Fri, 10 Nov 2023 10:13:00 +0000 https://www.woodlands.co.uk/?p=40494

Trees come in many shapes and sizes.  Some are tall and thin, like Poplars, others have a ‘rounded’ canopy, like oak and horse chestnut.  Sometimes we ‘persuade’ trees to assume a particular shape or form, perhaps through pollarding or coppicing - or something more extreme - like topiary or bonsai.  However, sometimes nature itself has unusual or dramatic effects on trees.  Wind can leave trees on cliff tops or exposed places distorted and growing almost horizontally along the direction of the prevailing wind.

Occasionally, something very strange is seen.  For example, at Gryfino in Western Poland, there is a forest with some very weird looking trees. There are about 400 trees that are bent at the base.  At first, the trunk lies more or less parallel with the ground, then it bends upwards and the stem is erect.  Consequently each trunk of these pines trees has a pronounced bend in it (see photo below). 

The rest of the trees in this forest are quite normal, growing upright and straight - like most pines.It is thought that the pines were planted back in the 1930’s though the local town was forsaken by the residents during the second world war (and only repopulated in relatively recent times). The trees are sometime referred to as the Crooked Forest. There has been much speculation as to how the trees came to be so mis-shapen.  The theories run from

• The landing of alien space craft! This crushed / flattened the trees when young and tender
• The trees were damaged  by German tanks during the war (but why only a select number of trees?)
• Genetic mutation(s) which resulted in abnormal growth
• Fungal infection(s) which resulted in abnormal growth
• The young trees were flattened by a heavy fall of snow, which perhaps persisted for some time.  The trees were able to right themselves in the Spring, through a normal geotropic response.
• The trees were part of plantation / forest, in which some were deliberately cut at a young / sapling stage.  The area was a tree farm, where some of the pines were cut / bent for later use in furniture or frames.

By bending a young tree down to the ground in this manner (for some time), compression wood is formed. Such wood has higher lignin and lower cellulose content and it is stronger than wood that is bent after a straight tree is felled (for example, by a steaming process).  Indeed, English ‘hedgerow oak’ was known to be the best for the curved timbers needed to internally strengthen a sailing ship.  Trees were even deliberately bent in certain ways so as to " grow" a required set of curved timbers.  Such curved timbers were known as “compass timbers”.  In Gryfino, it is likely that the war interrupted the activities of local foresters / woodworkers, they left and these trees were left to grow on in their rather unusual form.

Thanks to Kalasancjusz at Pixabay for the image of the crooked forest.

The tree can grown to a height of 20 to 30 metres, if the conditions are right - and the growth can be fast.  Young trees can grow at a rate of one metre a year.   It sends down quite deep roots and develops strong laterals (which may extend beyond the diameter of the crown).  This means that they are ‘windfirm’ and relatively drought tolerant.  The leaves bear some similarity to those of maple.  In Spring, the tree produces bunches of creamy white flowers, which provide pollen and nectar for insects. The flowers when fertilised produce brown berries.  Like Medlars, these may be bletted - allowed to become over-ripe and have been used to make jams, drinks.  

However, many of the fruits fall to the ground where they may be eaten by birds or small mammals. If eaten by birds, then the seeds may be dispersed over a wide area but natural regeneration from seeds is uncommon (though suckering from roots is a possibility).  Young plants / saplings may be targeted by voles and mice, they are also sensitive to competition (especially from brambles).

The tree is not grown for timber production in the UK, but abroad the timber is valued as one of the hardest, native European woods.  The colour is yellow to light red, though old trees can have a deeper colour.  Because of its hardness it was used in the past to make mill cogs, mangle wheels, parts in textile mills and wine presses.  Now, the wood is used in furniture making and for wind instruments (flutes, recorders, bag pipes).  Wild service trees, grown for timber, are ‘raised’ in ‘coppice with standards”, where the service trees are the standards.

Thanks to Ruth for images.

According to some interpretations of the bible, the Magi or ‘wise men’ travelled from afar bearing gifts of gold, frankincense and myrrh to present to the infant Jesus. The meaning and significance of these gifts has been debated over the years.  One things is clear - all were valuable materials as might have been presented to a king or deity.  Together with spices, frankincense and myrrh moved through ancient trade routes for thousands of years.

Gold is a relatively rare element and as such is a precious metal that has been used for jewellery and coinage throughout recorded history.  It is a noble metal, that is it is relatively unreactive, resisting attack by most acids - with the exception of aqua regia, a mixture of nitric and hydrochloric acids.

But what of frankincense and myrrh?   Their origins are not geological as both are plant products.  They come from a group of plants known as the Torchwood family or the Burseraceae.  These are trees or shrubs that have prominent resin ducts / canals.  The resin ducts are tubes, surrounded by cells which produce and secrete a resin into the canals / ducts.  The resin is viscous, ’antiseptic’ and aromatic (often smelling of almonds) and helps to prevents microbial attack (and may deter wood boring insects).

Frankincense comes from trees of the genus Boswellia.  Nearly all species of this genus have a bark that produces an aromatic sap but it is B. sacra, (also known as the olibanum tree) that is the main source of frankincense (from its papery, peeling bark).  It is found in Somalia, Yemen and Oman, often growing in relatively inhospitable places. 

To obtain the resin, the bark of the tree is cut and resin seeps out and is collected, rather like the tapping of a rubber tree.  The trees do not produce resin until they have reached a certain maturity and over-exploitation of the trees can lead to their death.  The seeds from heavily tapped trees are less likely to germinate than those from trees that have not been ‘drained’ of resin.  All Boswellia species are threatened by habitat loss, over-exploitation, and damage by long horn beetles.

What is Frankincense used for?  The word derives from the Old French ‘franc encens’ meaning high quality incense.  Many tonnes of frankincense are traded each year and are used in religious ceremonies, and in the making of perfumes and natural medicines.  In ancient times, the Egyptians used it in the process of mummification, it was added to the body cavities together with natron (a mixture of sodium salts).  The resin has also been used in traditional Chinese medicine for its antibacterial properties and ‘blood moving’ properties.

Like Frankincense, myrrh is a resin harvested by wounding the bark of a tree. The bark is a silvery grey, and the twigs are quite spiny (see image).  The resin that exudes is ‘waxy’ and quickly congeals becoming hard and glossy, darkening as it ages. The tree in question is Commiphora myrrha.  It is found in north east Africa - Somalia, Yemen, Eritrea and parts of Saudi Arabia.   The related Commiphora gileadensis, native to Israel, Palestine and Jordan, is also accepted as an alternate source of myrrh. 

Myrrh has been used as an antiseptic in mouthwashes, and as a constituent in salve / ointment for skin abrasions, bruises and sprains.  It  has also been used in perfumes and  as a special flavouring for wine.  Like frankincense, it was used in making incense^❋ and in the preparation of bodies for mummification / embalming. 

In Exodus [30:22-24], God said to Moses to take the best spices and liquid myrrh to make a holy anointing oil.  Anointing oil is still used in certain ceremonies / rituals of both Eastern Orthodox and Western Churches.  In some cultures, it can be used to ‘fumigate’ or refresh a house, giving a warm, earthy and balsamic odour.  It is also said that myrrh is a powerful detoxifier, can lower cholesterol and stabilise blood sugar levels. 

Both frankincense and myrrh were extensively traded in ancient and more recent historic times, along with various spices (such as cinnamon, ginger and nutmeg) across the Mediterranean and Arabian peninsula, through to India.  Interestingly, in Ancient Rome, myrrh was priced at five times the cost of frankincense.

^❋ Incense can be made from various aromatic plant materials that produce a scent. The actual ingredients used vary by region / area. Apart from frankincense and myrrh, incense may contain

• cinnamon
• musk
• patchouli (from a plant of the mint family)
• sandalwood.

Many thanks to Pixabay for images of frankincense and myrrh  (Leo_65, xbqs42  et al))

.

Veteran trees may be defined by a number of features:

• age
• size;
• condition;
• history;
• position.

Neither age nor size in themselves define veteran status. These features have to be viewed in relation to typical values for each tree species.  Thus, a one hundred year old birch or willow might be ‘deemed’ a veteran but a one hundred year old oak or yew would be a youngster. 

To be termed a veteran, a tree should show some of the following features

• the trunk should be large (for the species)
• decay holes in parts of the trunk
• the trunk may show signs of damage and/or bark loss
• dead wood in the canopy
• fungal fruiting bodies often present (from heart rot fungi)
• epiphytes, such as mosses and lichens are present
• the tree supports a rich variety of different species
• the shape or position of the tree is of interest
• the tree may have cultural or historical interest, some  were used as gallows!

Some veterans achieve their status through the management of the tree, such as pollarding or coppicing.  There are thousands of ancient trees in the UK and the Ancient Tree Inventory not only offers a way of finding ancient trees across the country but also you can add details of trees).

Veteran trees can be found in many countries, though the may go under different names. In Australia, veteran or ancient trees are often connected with the social, cultural, and legal practices of the aboriginal peoples.  In Italy, an Albero Monumentale (‘a monumental tree’) is defined under National Law [number 10, 14^th January 2013].

In Sweden, the oldest oak (Quercus robur) is the Rumskulla Oak , also known as the Kvill Oak. It is found in Kalmar County, Småland.  The name Rumskulla derives from its older form Romfarakulla ( = Rome + travel + hill); the area was a resting place for pilgrims that  to made the journey to Rome.

It is one of the largest trees in Scandinavia, being some 14 metres (46 feet) high and with a trunk circumference of 13 metres (43 feet). Its girth is still increasing.  In the severe winter of 1708-09, the crown was was damaged and much lost. The tree is over a thousand years old and was first described in 1772.  The tree is now supported by iron bands and wire.  Like many veterans, its centre is hollowed out and it is covered with mosses.   There are many holes, cracks and crevices.   The Rumskulla Oak is registered as a national natural object of interest, with the Swedish Heritage Board.

Thanks to Fredrika for the photos.

The redwoods are special trees because of their size and age.  There are three species :

• the coastal redwood (Sequoia sempervirens ),
• the giant sequoia / redwood (Sequoiadendron giganteum), and
• the dawn redwood (Metasequoia glyptostroboides)

They are the most massive trees on earth.  The coastal redwoods are the tallest of the three species.  The largest was measured to have a height of some 379 feet (115.5M).  Not only are these trees large but they are also impressive because of their longevity, a thousand years is not unusual but dendrochronology has aged one specimen at over 2500 years.   The coastal and the giant redwoods are found naturally on the pacific coast regions of California and Oregon, though specimens trees are found in the U.K. and many other countries [and plantations have been established in New Zealand].  

Redwood timber was highly prized due to its fire resistance and this lead to the extensive felling of redwood stands from the early nineteenth century onwards.  In consequence, their distribution is much reduced and limited to ‘pockets’ of old growth (in National Parks) along the coast. Recent years have seen episodes of drought (thought to be associated with climate change), these have exposed the trees to water stress which has weakened their resistance to pests and pathogens.

The supply of water to the treetop leaves of a coastal redwood is a major factor in determining the maximum height of a tree.  The distance from roots to leaves is a major problem in terms of water transport. The treetop leaves operate under some degree of water stress (due to higher light intensities, drier air and higher temperatures). These factors constrain certain physiological functions, such as cell elongation and photosynthesis, which in turn limit growth in height.  The maximal attainable height has been calculated to be 122M.

It is thought that the trees can reduce some of the water stress by the absorption of moisture through the leaf surfaces.  Summer fogs occur in the coastal regions and it is noteworthy that in drier regions of their distribution the trees are shorter.  It is also interesting that there is a difference between the leaves at the top of the tree and those lower down.  Those at the top are small, thick, and fused to the vertical stem axis; this fusion of leaf and stem creates a relatively large volume of tissue and intercellular space that can store water. The leaves in the lower part of the crown by comparison are large, flat and horizontal to the stem axis.

The giant sequoias are shorter than the coastal redwoods but what they ‘lack’ in height, they make up for in mass.  They can outweigh giant redwoods substantially.  General Sherman, a giant sequoia, can claim the title ‘largest tree in the world’ on the grounds of mass / volume.  It stands some 274 feet tall and its circumference at its base is 100 feet plus.   The species was originally named (in 1853) Wellingtonia gigantea, but the name Wellingtonia had already been used for a completely unrelated plant.  It was renamed several times, and finally called Sequoiadendron in 1939.

The dawn redwood is native to China.  Like the Coastal Redwood and the Giant Sequoia, it belongs to the Sequoioideae, in the Cypress family.  It is the only living member of its genus Metasequoia, though three fossil species are known. It is sometimes referred to as a ‘living fossil’ as it appears to be basically unchanged for millions of years.

Below left, a recently germinated redwood seedling, and right seedlings about six month post germination

There are two main species of walnut tree - Black Walnut and English (or Persian) Walnut.  Walnut trees are often grown in the U.K. as an ornamental or specimen tree in parks or gardens.  However, there are areas where the trees are grown for their timber or the nut crop.  The Black Walnut (Juglans nigra) is native to eastern North America; Missouri is one of the top producers of the nuts.  Whilst the nuts have an intense flavour, their tough coats represent a challenge. 

Black Walnuts have had a number of uses over the years, some of them associated with the U.S. military.

• the wood was used for gunstocks, indeed the phrase ‘shouldering walnut’ was at one time equivalent to saying doing military service
• walnut husks were used to create a dye for confederate uniforms (and make ink)
• powdered shells were mixed with nitroglycerine to make explosives
• walnut wood was favoured in the making of aircraft parts in World War 1 - particularly propellors.
• black walnut has also been much prized for furniture making and for veneers.
• there are various industrial uses - detailed here

The Black Walnut (Juglans nigra) has been grown for its timber since being introduced to Europe from North America probably late in the seventeenth century. It is faster growing and more resilient to honey fungus than English Walnut. It is possible that with climate warming the range of suitable suites for it in England and Wales will extend.

The English Walnut (Juglans regia) originated in central Asia, it was probably introduced by the Romans. It is still to be found in significant numbers in Tien Shan in Kyrgyzstan, where it provides food, timber and fuel.   The actual nuts of the tree are rich in polyunsaturated fatty acids (particularly the essential fatty acids : linoleic and linolenic acids), manganese and B vitamins.  It is important that walnuts (like peanuts) are stored properly so that the fungus (Aspergillus) does not grow on them - otherwise the toxin AFLATOXIN is formed which is a potent carcinogen.  

The actual nuts look something like a small brain - with its various folds and fissures. In times when the Doctrine of Signatures was in vogue, it was thought that eating walnuts was good for the brain,  especially so, as the nut is encased in its own ‘cranium’ - the outer shell.

Walnut trees are interesting in that their roots secrete chemicals that inhibit the growth of other species - a phenomenon known as allelopathy.  That having been said, they may be grown alongside Alders (Alnus sp).  Alders are capable of fixing atmospheric nitrogen by means of their root nodules, and they seem to encourage the growth of the Walnuts.

Walnut Nut Image by Pezibear from Pixabay

Figs can grow as rather straggly bushes or tall trees; they are native to the Middle East and parts of Asia. Evidence of fig ‘cultivation’ has been found in a neolithic settlement in the Jordan Valley - many thousands of years ago. In Ancient Greece, Aristotle described in some detail the biology and cultivation of the fig. Figs were said to be responsible for the death of Augustus (first emperor of Rome), as his wife Livia smeared the figs in his garden with poison - allegedly. Figs were cultivated in Pakistan through to the Mediterranean and Portugal; they were later introduced to the “New World” and have been grown extensively in areas like California since the late 1800’s, forming a valuable part of the local economy.

Figs belong to the same family as the Mulberries (Morus alba & nigra) and the breadfruit (Artocarpus altilis) - the Moraceae.   The fig can grow to a height of some 10 metres and it is deciduous.  Its large, deeply lobed leaves are shed in autumn - leaving noticeable leaf scars on the stems. The surface of the leaves has a ‘rough’ texture.  The bark is a dark grey, slate or brown colour. If the young shoots / leaves or developing figs are cut, they may exude a milky sap (latex) - which can cause skin irritation.

The trees / shrubs have been grown since ancient times - as a source of their fruit - the fig.  Its roots penetrate to considerable depths (often down to the water table), it is a phreatophyte. The root  system enables the plant to grow in inhospitable, dry and even rocky places.

The fruit - the fig,  is not a ‘straightforward’ fruit.  It is a complex structure called a synconium, a hollow-ended stem structure containing many unisexual flowers. The flowers are not seen outside this structure.  The structure / synconium also has a small hole - an ostiole, which allows a tiny wasp (Blastophaga psenes) to enter and pollinate the flowers, which then develop seeds. (details of the complex role of the wasps in pollination can be seen in a video here or see p336-338, Mabberley’s Plant Book).  The synconium matures and contains a number of one seeded fruits (druplets).  The mature fig is one to two inches long; it starts with a green ‘skin’ that usually ripens to a purple or brown colour.  

Figs are rich in natural sugars, minerals and soluble fibre. The minerals include potassium, calcium, magnesium, iron and copper and figs are a good source of antioxidant vitamins A and K that contribute to health and wellbeing.  Figs are a good way to add fibre into your diet, dried figs particularly can provide a significant amount  of fibre,   Syrup of figs used to be sold as a natural remedy for constipation.

The most common form of Fig grown in the UK is probably Ficus carina ‘Brown Turkey’, often seen on sale in garden centres.  This has all female flowers that do not need to be pollinated for fruits to form; the fruit develops through parthenocarpy, ie. fruit production without the involvement of pollen grains.  Parthenocarpy is often a desirable characteristic in fruit crops that might be difficult to pollinate or fertilise, such as tomatoes and summer squash ; interestingly it is thought that the oldest known cultivated plant was a parthenocarpic fig, (as mentioned above), that was grown in the Jordan Valley.

Interior of a fig

The olive tree [Olea europaea] , is an evergreen tree native to the mediterranean area.  Indeed, it is often cited as an ‘indicator plant’ for the area. It is of major agricultural importance throughout the mediterranean basin as it is the source of olive oil. 

Generally, the outline of the tree is somewhat short and squat^†, its height varies between 8 –15 M. The oblong/spear-shaped leaves are silvery green; the upper surface is green, whereas the lower surface is more silvery. The leaves measure between 4–10 cm in length and are between 1–3 cm in width. Olive trees can be very long lived and, as the tree ages, the trunk frequently becomes gnarled and twisted.  The tree produces small, white, ‘feathery’ flowers on the previous year's growth.

The olive fruit is a small stone fruit , i.e. a drupe between 1–2.5 cm long.  The fruit is smaller in wild plants than in the cultivars seen in the olive groves. Many other fruits (the almond, apricot, cherry, damson, nectarine, and plum) are also drupes.   

Olives are harvested when green or purple and then are pressed or ground up to extract the oil.  The oil has many uses - apart from the culinary.  For example, it is used in lamps in religious ceremonies, in skin care / cosmetic products, and as a lubricant. 

The cultivation of the olive dates back many thousands of years.  As far back as 3000 BC, olives were grown commercially in Crete where, some have suggested, they might have been the source of the wealth of the Minoan Civilisation.   Olive oil has been traded throughout the Mediterranean throughout historic times.  Wreaths of olive branches was used in the burial of Tutankhamen.

Interestingly, these funereal wreathes have been used in studies to investigate the effect of increasing carbon dioxide on stomatal density.  Stomates are pores involved in gas exchange (the uptake of carbon dioxide for photosynthesis).  They also help regulate water loss via transpiration.  Stomatal pores are surrounded by guard cells (see image of a stomate) that change their shape, allowing the pore to open or close as required for the uptake of carbon dioxide. 

Some of the Tutankhamen wreathes, [which date from 1327 BC] are held by Kew Gardens, and were used by researchers (Beerling and Chaloner*) to compare the number of stomates on the undersides of the leaves with more modern olives (and some herbarium specimens from the nineteenth century).

The number of stomates (per unit leaf area) on the more modern olive leaves is approximately 530 stomates per mm² .This is significantly lower than the average density on the ancient Egyptian (or indeed nineteenth century) olive leaves of 790 stomates per mm² in 1372 BC. This is in line with studies in controlled environments which showed that as CO₂ levels increase so stomatal density falls.  As of January this year, the level of CO₂ in the atmosphere is 413.4 ppm (as measured at Mauna Loa, Hawaii ). The last two centuries have seen the extensive extraction and burning of fossil fuels (oil, coal & gas). Prior to this, in pre-industrial times, the level of carbon dioxide was circa 280 ppm, as estimated during the ‘Middle Ages’.   

*Beerling and Chaloner, Annals of Botany, vol 71, p431-435.

 † An exception to this general form is the variety / cultivar ‘Pisciottana', found in the Campania region of southern Italy.