In Yews more prostrate form, easily confused with :
Juniper [a bush with similar height, jizz and leaves, but the leaves of Juniper are sharp at the tip]
Easily confused with :
Irish Yew (Taxus baccata 'Fastigiata'),
Pacific Yew aka
Western Yew (Taxus brevifolia),
Chinese Yew (Taxus chinensis) and
Japanese Yew (Taxus cuspidata), neither of which are native but still widely planted. There are other Yew trees;
Walking Yew sheds branches at its periphery and these then root on the ground where they have fallen, creating a slowly extending ring of smaller Yew Trees around a central mature specimen. Other species of Yew include
Ground Hemlock (Taxus canadensis), and
Western Yew (Taxus brevifolia) which does not seem to contain the alkaloid Taxine.
Not relation to
Prince Albert's Yew (Saxegothaea conspicua) which looks similar to a Yew tree but belongs instead to the Podocarpaceae family. The Genus Saxegothaea is monotypic, which means there is only one species within the genus.
The Yew tree is evergreen and dioecious, having separate male and female trees each producing the corresponding male or female flower. Male trees produce cones whist female trees produce the red arils containing a single seed.
Skin contact with the sharply pointed Yew leaves (or with the wood) can cause erythema and skin irritation in some individuals, sometimes leading to blisters or even to excessive burns. This is a hypersensitivity response.
ANTIQUITY OF YEW
Yew is widely planted in parks and especially in churchyards and in the grounds of other very old buildings, going back centuries. The Yews in churchyards are usually very old, often pre-dating the present church on the site, for churches were often re-built on previously existing sacred sites. The trunks of old Yews are seldom visible; hidden by a dense mass of branches bearing profuse foliage. They are slow growing, as are most trees that last several centuries. The slow-growing nature lends itself to artistic pruning; many yew trees in church grounds have been shaped by man. With
Box trees man is able to take this one level higher; they are able to be pruned into almost any imaginable shape. A Yew in Perthsire is recently suspected of being at least 5000 years old, and possibly up to 9000 years old, which makes it the oldest tree known in the UK, and probably in Europe, and maybe even in the whole World.
Dating Yew trees is particularly difficult as most are hollow in the middle, and therefore counting tree rings will yield only a minimum possible age for the tree, because their annual growth rings rot away in the centre. But sawing a Yew tree down to enable a count of its tree rings is not a viable option. Instead a special drill is use to bore into the centre of the tree which measures the mechanical resistance to boring as it proceeds. Since each years growth contains hard and softer wood, the drill bit encounters a periodic variation in mechanical resistance, and these variations can be counted to yield a minimum age (that is, until the drill bit encounters the hollow centre). It is possible to estimate a Yew trees age by measuring its girth, but this is complicated by the fact that the Yew tree grows slower with age.
It is not possible to use the size of lichens on the bark as an indication of Yew tree age, since neither lichens nor mosses seem able to colonize the bark of Yew trees. This is possibly due to the inhibitory nature of the extensive poisons within most parts of the Yew Tree (apart from the fleshy part of the red arils).
Many old trees become hollow inside, losing the heart-wood, and are better able to withstand the ravages of storms as a result. Since the sap of a tree rises just underneath the bark, the tree really has no desperate need for the inner mart of the wood, and many that have rotted away actually survive longer than those that have not. This must be because, structurally, the stiffness of any object depends more upon its outer circumference than its inner. Getting rid of ~90% of the weight of the tree whilst retaining ~90% of the stiffness seems a good strategy, which structural engineers apply to modern constructions.
In the case of Yew Trees, the wood is permeated by toxic diterpenoids which are mostly anti-fungal. The Author surmises that, over millenia, these diterpenoids are either changed into less toxic substances or otherwise dissipate, allowing fungi to invade the inner wood.
Yew is a very hard wood due to its slow-growing nature. It has been used to make durable or hard-wearing utilities since antiquity; the oldest known is of a spear found in Clacton-on-Sea which is about 50,000 years old. In the more recent archaeological past it has been used to make yew bows, yew knives and yew bowls. In more modern times it has been used in the industrial revolution to make shuttles for weaving, wooden gears and cogs, pulley wheels and pivots for rotating machinery as well as for lute bodies, combs, pegs, tool handles, wood veneers and religious drinking cups.
The red arils are not berries, but fleshy non-poisonous receptacles for the hard female seed cone within them, which is intensely poisonous. The tree uses the aril to attract birds which feed on the pulpy red surround, depositing the in-edible seed cone elsewhere to propagate the species. The red arils are edible by humans too, but the hard and very poisonous seed cone must not be eaten! Besides the female cones within the arils, there are also male cones nestled near the outgrowth of the leaves on smaller branches; about the size of a black peppercorm these are inconspicuous and look like miniature brussel sprouts. The seeds take about two years to germinate, and the seedling requires deep shade and shelter. The seedling grows slowly so is vulnerable to disturbance and grazing.
The leaves and stems are poisonous to most (if not all) mammals, including humans, but not the fleshy red arils (although the seed cone contained within it is very poisonous). Yew contains a varied cocktail of toxic diterpene Taxanes and Baccatins, the Toxanes block Na+ and Ca2+ channels in heart cells, whilst Taxol and Decetaxel are spindle poisons, inhibiting cell division by preventing microtubules from de-polymerising. Symptoms of poisoning include mydriasis, nausea, vomiting, dizziness, tachycardia, diarrhoea, kidney damage. At first breathing is stimulated and the victim may hyperventilate, leading to acidosis; later breathing is suppressed. Pulmonary spasms follows, then coma and death from respiratory and circulatory failure in just 2 to 24 hours. Treatment is possible. The sawdust produced by sawing or sanding Yew wood should not be inhaled for it too contains toxins. Suicide by ingesting Yew tree leaves used to be a common occurrence. In antiquity Yew was known as the tree of death, and has been used to poison arrow tips. The dried leaves are more toxic than when fresh and green.
The Fortingall Yew growing in Perthshire, which is thought to be between 2000 to 5000 years old and is thought to be Britains oldest Yew. It is male (because it produces pollen), but has just recently suddenly started producing berries on one of its upper branches, which means that that part of the tree is now female. The rest of the tree is still producing pollen. Such sex-changes are not unknown, but are certainly rare.
[The Author speculates:]
This might be viewed as a last ditch strategy to propagate. The tree might be under more stress of late (after all, it is likely that the environment in which it first grew up in thousands of years ago has altered considerably in the intervening millennia and it might think itself now in danger of dying, and thus at risk losing its genetic make-up forever). This stress may well have encouraged some once-male flowers to change into female flowers. Stress can alter the expression of DNA by epigenetic changes to the DNA (by methylation and/or acetylation) and this may be one mechanism by which flowers can change sex. There are probably many other ways too. When the flowers are mated sexually, some of those methylated genes may find themselves in the seeds (and ultimately the offspring. However, not all epi-genetic changes are heritable (passed on to offspring); many changes are reset at fertilisation. But by implication, some are not reset. And these flowers may impart some extra feature to the seedling enabling it to better tolerate the changes that have occurred in its local environment over the last millennia. Although Yew trees do readily propagate vegetatively those seedlings will be clones of the parent. By reproducing sexually any modified germ-cell genes can become incorportated into the offspring (especially if the sex-change was due to genetic or epigenetic changes) may confer a better ability to tolerate changes in environment.
Taxol, or Paclitaxel, was first discovered in the bark of the rare Pacific Yew Tree (which is not native to the UK) from where it was commercially extracted until a method of laboratory synthesis was devised. It is also present in our Yew. It was found to be a very effective treatment for some cancers, but not all; cancer is not one disease, but many. Taxol stabilises microtubules against disassembly, inhibiting cell division (in both normal growing, and in cancer). It is now marketed under the Generic name Taxol and brand name Paclitaxel. Taxol consists of three condensed rings, one of 4, five of 6 and the other of 8 members. The eight-membered ring, oxetane (shown in blue), is crucial to the drugs activity, as is the benzoyl group (shown in red).
Taxol is just one of possibly seven Taxines and Taxoids (which include Taxine B, Isotaxine B and Cephalomannine) which are the main toxic pseudo-alkaloids and deadly poisonous constituents of Yew trees.
10-deacetyl Baccatin III was later discovered in the leaves of both the rare Pacific Yew Tree and the much more common Yew. As can be seen, it is more of a basic sub-block, still containing the requisite 8-membered ring (in blue) and the O-benzoyl group (in red). It is now used as a starting compound (from Taxus Baccata only) in the manufacture of both Paclitaxel (Taxol) and Taxotere (Docetaxel), thus saving the rare Pacific Yew tree from assured extinction by over-exploitation.
Cephalomannine, aka Taxol B, is almost identical to Paclitaxel, but part of a phenyl group is missing, shown in green. Cephalomannin was probably first discovered in Cephalotaxus fortunei a species of Yew Tree which in the UK is found only in Pembrokeshire.
Taxine does not have an 8-membered ring as do Taxol, Docetaxel, Baccatin derivatives and Cephalomannine, but a 9-membered ring, depicted in magenta. Also, the long side-chain containing the nitrogen atom has swapped allegiance to the opposite side of the multi-membered ring. Several other features are absent too.
Docetaxel, which does not occur naturally, is related to Taxol which does. The only difference between Taxol and Docetaxel is depicted in green, where some further atoms are also missing. Again it has the 8-membered ring rather than the 9-membered ring of Taxine. It is manufactured by chemically esterifying 10-deacetyl baccatin III, which is obtained from the leaves of the much more abundant Taxus Baccata (Yew), shown on this page. Docetaxel is marketed under the brand name of Taxotere as a drug to treat breast, prostrate, ovarian, melanomas and lung cancers. The modus operandus is much the same as for Paclitaxel (Taxol) although it appears to be more effective, increasing survival periods by several more months. Thus neither appear to actually cure cancer.
TOXINS and PATHOGENS|
- the never-ending war
From these alkaloids, it is immediately apparent that synthesis of toxins by plants is not always a well targeted affair, for not only are several intermediate compounds fabricated (as would be expected anyway) but a whole plethora of 'wrongly' assembled molecules are generated in, what must be, a stochastic construction process whereby rings are broken apparently haphazardly or bits are tacked on here and there and other bits chopped off willy nilly. A scattergun approach.
Whilst this may seem counter-productive to us, to the plant trying to protect itself from invading pathogens and hungry beasts it matters not - as long as all or most of these compounds are poisonous. In fact, it can be of huge benefit to the plant not to selectively manufacture just one toxin, but instead fabricate a bizarre profusion. The attacking microorganisms or famished creature could, by natural selection, develop resistance to any one specific toxin, but is most unlikely to develop resistance to a huge arsenal of randomly assembled toxins.
This is a strategy which seems to be working, at least in the case of Yew trees: so far no moss or lichen has managed to develop sufficient resistance against taxanes to enable them to colonize the bark of Yew Trees. Although the bark of Yew trees is constantly cracking and is thus vulnerable to attack by fungi waiting to devour the nutritious wood, the Yew tree is able to hold at bay all fungi by means of the toxins it produces. All fungi, that is, bar one... [Read 'A Fungal Infection' below]. Even after millions of years of mammalian evolution, the leaves (and many other parts) of Yew are still toxic to many birds and animals, including humans. Except, of course, the red arils, which the Yew has decided not suffuse with poison but to leave toxin-free for the birds to eat and thence to disseminate its un-digestible seed cargo far and wide in their droppings.
A FUNGAL INFECTION
The Taxol produced by Yew trees is active against a wide-range of fungi that attack wood, apart from one endophytic fungus which has been able to penetrate the toxic alkaloid defences of the Yew tree with impunity. What is even more intriguing is that this fungus also produces Taxol. At first botanists thought that it might be the fungus which supplies Yew trees with their cargo of Taxol, but this was found to be not so; Yew trees lacking the fungus also produce Taxol. This fungus (species of Paraconiothyrium) produces the same toxic metabolite (Taxol) as does its host plant! However, this fungus is actually poisoned by Taxol, and not only produces it itself, but has found a way of secreting it safely away within hydrophobic bodies of the hyphae. This fungus only produces Taxol in response to any other fungus trying to gain entry into the Yew tree, at other times it sequesters the taxol which the Yew tree is producing safely away inside these hydrophobic bodies. The rapidly dividing yew cells on new shoots which have not yet started to protect themselves with Taxol are protected from external fungal attack by this Paraconiothyrium fungus instead. Thus there is a symbiotic relationship between this fungus and the Yew tree which may have been extant for a very long time.
Yew is not the only tree which has a symbiotic relationship with Taxol-producing fungi - some ancient gymnosperm species such as the long-lived Gingo and Wollemia pines which also have this symbiotic relationship. These species have been extant for 100My.
Your Author is still, however, very puzzled by this apparent coincidence of Taxol production. He thinks that one or other (Yew or Paraconiothyrium) instigated the synthesis of Taxol, which the other then copied by some means unknown involving evolution and transfer of genes over the aeons available to them.
Rhodoxanthin is a keto-carotenoid and symmetrical dimer that is coloured deep-red to purple and which is found in small amounts in several plants, including the Yew tree. Under the e-number E-1611 it is used as a food colouring in some parts of the world such as Australia and New Zealand, but not including either the European Union nor the USA. It has two cyclo-hexenone groups at each end of a conjugated chain. It is induced by the action of very strong sunlight on the leaves of Aloe arborescens, which turn a deep red in response. The leaves of several Gymnosperms have a similar reaction to strong sunlight generating Rhodoxanthin.
Other sources say that it is an yellow-orange pigment found in Autumn leaves and in the seeds of this species of Yew. So there you have it: purple is orange-yellow...
It is also present as a colouring in the feathers of some birds, namely Pin-tailed Manakins.
Not to be confused with Rubixanthin, another carotenoid.