Easily confused with : Irish Yew 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.

The Yew tree is evergreen and dioicious, 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.

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 YEWS
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. 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).

USES
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.

PROPAGATION
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.

TOXICITY
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.

TAXOL 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).

TAXOIDS 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.

DOCETAXOL 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 generation process with bits 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 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 haphazardly 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 to taxanes to enable them to colonise the bark of Yew Trees. 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.

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